Welcome to the Twisted documentation!¶
Contents:
Installing Twisted¶
Installing Optional Dependencies¶
This document describes the optional dependencies that Twisted supports. The dependencies are python packages that Twisted’s developers have found useful either for developing Twisted itself or for developing Twisted applications.
The intended audience of this document is someone who is familiar with installing optional dependencies using pip.
If you are unfamiliar with the installation of optional dependencies, the python packaging tutorial can show you how. For a deeper explanation of what optional dependencies are and how they are declared, please see the setuptools documentation.
The following optional dependencies are supported:
- dev - packages that aid in the development of Twisted itself.
- tls - packages that are needed to work with TLS.
- conch - packages for working with conch/SSH.
soap - the SOAPpy package to work with SOAP.
serial - the pyserial package to work with serial data.
all_non_platform - installs tls, conch, soap, and serial options.
osx_platform - all_non_platform options and pyobjc to work with Objective-C apis.
windows_platform - all_non_platform options and pypiwin32 to work with Windows’s apis.
- Installing Optional Dependencies: documentation on how to install Twisted’s optional dependencies.
Twisted Core¶
Developer Guides¶
The Vision For Twisted¶
Many other documents in this repository are dedicated to defining what Twisted is. Here, I will attempt to explain not what Twisted is, but what it should be, once I’ve met my goals with it.
First, Twisted should be fun. It began as a game, it is being used commercially in games, and it will be, I hope, an interactive and entertaining experience for the end-user.
Twisted is a platform for developing internet applications. While Python by itself is a very powerful language, there are many facilities it lacks which other languages have spent great attention to adding. It can do this now; Twisted is a good (if somewhat idiosyncratic) pure-python framework or library, depending on how you treat it, and it continues to improve.
As a platform, Twisted should be focused on integration. Ideally, all functionality will be accessible through all protocols. Failing that, all functionality should be configurable through at least one protocol, with a seamless and consistent user-interface. The next phase of development will be focusing strongly on a configuration system which will unify many disparate pieces of the current infrastructure, and allow them to be tacked together by a non-programmer.
Writing Servers¶
Overview¶
This document explains how you can use Twisted to implement network protocol parsing and handling for TCP servers (the same code can be reused for SSL and Unix socket servers). There is a separate document covering UDP.
Your protocol handling class will usually subclass twisted.internet.protocol.Protocol.
Most protocol handlers inherit either from this class or from one of its convenience children.
An instance of the protocol class is instantiated per-connection, on demand, and will go away when the connection is finished.
This means that persistent configuration is not saved in the Protocol.
The persistent configuration is kept in a Factory class, which usually inherits from twisted.internet.protocol.Factory.
The buildProtocol method of the Factory is used to create a Protocol for each new connection.
It is usually useful to be able to offer the same service on multiple ports or network addresses.
This is why the Factory does not listen to connections, and in fact does not know anything about the network.
See the endpoints documentation for more information, or IReactorTCP.listenTCP and the other IReactor*.listen* APIs for the lower level APIs that endpoints are based on.
This document will explain each step of the way.
Protocols¶
As mentioned above, this, along with auxiliary classes and functions, is where most of the code is. A Twisted protocol handles data in an asynchronous manner. The protocol responds to events as they arrive from the network and the events arrive as calls to methods on the protocol.
Here is a simple example:
from twisted.internet.protocol import Protocol
class Echo(Protocol):
def dataReceived(self, data):
self.transport.write(data)
This is one of the simplest protocols. It simply writes back whatever is written to it, and does not respond to all events. Here is an example of a Protocol responding to another event:
from twisted.internet.protocol import Protocol
class QOTD(Protocol):
def connectionMade(self):
self.transport.write("An apple a day keeps the doctor away\r\n")
self.transport.loseConnection()
This protocol responds to the initial connection with a well known quote, and then terminates the connection.
The connectionMade event is usually where setup of the connection object happens, as well as any initial greetings (as in the QOTD protocol above, which is actually based on RFC 865).
The connectionLost event is where tearing down of any connection-specific objects is done.
Here is an example:
from twisted.internet.protocol import Protocol
class Echo(Protocol):
def __init__(self, factory):
self.factory = factory
def connectionMade(self):
self.factory.numProtocols = self.factory.numProtocols + 1
self.transport.write(
"Welcome! There are currently %d open connections.\n" %
(self.factory.numProtocols,))
def connectionLost(self, reason):
self.factory.numProtocols = self.factory.numProtocols - 1
def dataReceived(self, data):
self.transport.write(data)
Here connectionMade and connectionLost cooperate to keep a count of the active protocols in a shared object, the factory.
The factory must be passed to Echo.__init__ when creating a new instance.
The factory is used to share state that exists beyond the lifetime of any given connection.
You will see why this object is called a “factory” in the next section.
loseConnection() and abortConnection()¶
In the code above, loseConnection is called immediately after writing to the transport.
The loseConnection call will close the connection only when all the data has been written by Twisted out to the operating system, so it is safe to use in this case without worrying about transport writes being lost.
If a producer is being used with the transport, loseConnection will only close the connection once the producer is unregistered.
In some cases, waiting until all the data is written out is not what we want.
Due to network failures, or bugs or maliciousness in the other side of the connection, data written to the transport may not be deliverable, and so even though loseConnection was called the connection will not be lost.
In these cases, abortConnection can be used: it closes the connection immediately, regardless of buffered data that is still unwritten in the transport, or producers that are still registered.
Note that abortConnection is only available in Twisted 11.1 and newer.
Using the Protocol¶
In this section, you will learn how to run a server which uses your Protocol.
Here is code that will run the QOTD server discussed earlier:
from twisted.internet.protocol import Factory
from twisted.internet.endpoints import TCP4ServerEndpoint
from twisted.internet import reactor
class QOTDFactory(Factory):
def buildProtocol(self, addr):
return QOTD()
# 8007 is the port you want to run under. Choose something >1024
endpoint = TCP4ServerEndpoint(reactor, 8007)
endpoint.listen(QOTDFactory())
reactor.run()
In this example, I create a protocol Factory.
I want to tell this factory that its job is to build QOTD protocol instances, so I set its buildProtocol method to return instances of the QOTD class.
Then, I want to listen on a TCP port, so I make a TCP4ServerEndpoint to identify the port that I want to bind to, and then pass the factory I just created to its listen method.
endpoint.listen() tells the reactor to handle connections to the endpoint’s address using a particular protocol, but the reactor needs to be running in order for it to do anything.
reactor.run() starts the reactor and then waits forever for connections to arrive on the port you’ve specified.
You can stop the reactor by hitting Control-C in a terminal or calling reactor.stop().
For more information on different ways you can listen for incoming connections, see the documentation for the endpoints API. For more information on using the reactor, see the reactor overview.
Helper Protocols¶
Many protocols build upon similar lower-level abstractions.
For example, many popular internet protocols are line-based, containing text data terminated by line breaks (commonly CR-LF), rather than containing straight raw data. However, quite a few protocols are mixed - they have line-based sections and then raw data sections. Examples include HTTP/1.1 and the Freenet protocol.
For those cases, there is the LineReceiver protocol.
This protocol dispatches to two different event handlers – lineReceived and rawDataReceived.
By default, only lineReceived will be called, once for each line.
However, if setRawMode is called, the protocol will call rawDataReceived until setLineMode is called, which returns it to using lineReceived.
It also provides a method, sendLine, that writes data to the transport along with the delimiter the class uses to split lines (by default, \r\n).
Here is an example for a simple use of the line receiver:
from twisted.protocols.basic import LineReceiver
class Answer(LineReceiver):
answers = {'How are you?': 'Fine', None: "I don't know what you mean"}
def lineReceived(self, line):
if self.answers.has_key(line):
self.sendLine(self.answers[line])
else:
self.sendLine(self.answers[None])
Note that the delimiter is not part of the line.
Several other helpers exist, such as a netstring based protocol and prefixed-message-length protocols.
State Machines¶
Many Twisted protocol handlers need to write a state machine to record the state they are at. Here are some pieces of advice which help to write state machines:
- Don’t write big state machines. Prefer to write a state machine which deals with one level of abstraction at a time.
- Don’t mix application-specific code with Protocol handling code. When the protocol handler has to make an application-specific call, keep it as a method call.
Factories¶
Simpler Protocol Creation¶
For a factory which simply instantiates instances of a specific protocol class, there is a simpler way to implement the factory.
The default implementation of the buildProtocol method calls the protocol attribute of the factory to create a Protocol instance, and then sets an attribute on it called factory which points to the factory itself.
This lets every Protocol access, and possibly modify, the persistent configuration.
Here is an example that uses these features instead of overriding buildProtocol:
from twisted.internet.protocol import Factory, Protocol
from twisted.internet.endpoints import TCP4ServerEndpoint
from twisted.internet import reactor
class QOTD(Protocol):
def connectionMade(self):
# self.factory was set by the factory's default buildProtocol:
self.transport.write(self.factory.quote + '\r\n')
self.transport.loseConnection()
class QOTDFactory(Factory):
# This will be used by the default buildProtocol to create new protocols:
protocol = QOTD
def __init__(self, quote=None):
self.quote = quote or 'An apple a day keeps the doctor away'
endpoint = TCP4ServerEndpoint(reactor, 8007)
endpoint.listen(QOTDFactory("configurable quote"))
reactor.run()
If all you need is a simple factory that builds a protocol without any additional behavior, Twisted 13.1 added Factory.forProtocol, an even simpler approach.
Factory Startup and Shutdown¶
A Factory has two methods to perform application-specific building up and tearing down (since a Factory is frequently persisted, it is often not appropriate to do them in __init__ or __del__, and would frequently be too early or too late).
Here is an example of a factory which allows its Protocols to write to a special log-file:
from twisted.internet.protocol import Factory
from twisted.protocols.basic import LineReceiver
class LoggingProtocol(LineReceiver):
def lineReceived(self, line):
self.factory.fp.write(line + '\n')
class LogfileFactory(Factory):
protocol = LoggingProtocol
def __init__(self, fileName):
self.file = fileName
def startFactory(self):
self.fp = open(self.file, 'a')
def stopFactory(self):
self.fp.close()
Putting it All Together¶
As a final example, here’s a simple chat server that allows users to choose a username and then communicate with other users. It demonstrates the use of shared state in the factory, a state machine for each individual protocol, and communication between different protocols.
from twisted.internet.protocol import Factory
from twisted.protocols.basic import LineReceiver
from twisted.internet import reactor
class Chat(LineReceiver):
def __init__(self, users):
self.users = users
self.name = None
self.state = "GETNAME"
def connectionMade(self):
self.sendLine("What's your name?")
def connectionLost(self, reason):
if self.name in self.users:
del self.users[self.name]
def lineReceived(self, line):
if self.state == "GETNAME":
self.handle_GETNAME(line)
else:
self.handle_CHAT(line)
def handle_GETNAME(self, name):
if name in self.users:
self.sendLine("Name taken, please choose another.")
return
self.sendLine("Welcome, %s!" % (name,))
self.name = name
self.users[name] = self
self.state = "CHAT"
def handle_CHAT(self, message):
message = "<%s> %s" % (self.name, message)
for name, protocol in self.users.iteritems():
if protocol != self:
protocol.sendLine(message)
class ChatFactory(Factory):
def __init__(self):
self.users = {} # maps user names to Chat instances
def buildProtocol(self, addr):
return Chat(self.users)
reactor.listenTCP(8123, ChatFactory())
reactor.run()
The only API you might not be familiar with is listenTCP.
listenTCP is the method which connects a Factory to the network.
This is the lower-level API that endpoints wraps for you.
Here’s a sample transcript of a chat session (emphasised text is entered by the user):
$ telnet 127.0.0.1 8123
Trying 127.0.0.1...
Connected to 127.0.0.1.
Escape character is '^]'.
What's your name?
test
Name taken, please choose another.
bob
Welcome, bob!
hello
<alice> hi bob
twisted makes writing servers so easy!
<alice> I couldn't agree more
<carrol> yeah, it's great
Writing Clients¶
Overview¶
Twisted is a framework designed to be very flexible, and let you write powerful clients. The cost of this flexibility is a few layers in the way to writing your client. This document covers creating clients that can be used for TCP, SSL and Unix sockets. UDP is covered in a different document .
At the base, the place where you actually implement the protocol parsing
and handling, is the Protocol class. This class will usually be
descended
from twisted.internet.protocol.Protocol . Most
protocol handlers inherit either from this class or from one of its
convenience children. An instance of the protocol class will be instantiated
when you connect to the server and will go away when the connection is
finished. This means that persistent configuration is not saved in the
Protocol .
The persistent configuration is kept in a Factory class,
which usually inherits from twisted.internet.protocol.Factory
(or twisted.internet.protocol.ClientFactory : see below).
The default factory class just instantiates the Protocol and then sets the protocol’s factory attribute to point to itself (the factory).
This lets the Protocol access, and possibly modify, the persistent configuration.
Protocol¶
As mentioned above, this and auxiliary classes and functions are where most of the code is. A Twisted protocol handles data in an asynchronous manner. This means that the protocol never waits for an event, but rather responds to events as they arrive from the network.
Here is a simple example:
from twisted.internet.protocol import Protocol
from sys import stdout
class Echo(Protocol):
def dataReceived(self, data):
stdout.write(data)
This is one of the simplest protocols. It just writes whatever it reads
from the connection to standard output. There are many events it does not
respond to. Here is an example of a Protocol responding to
another event:
from twisted.internet.protocol import Protocol
class WelcomeMessage(Protocol):
def connectionMade(self):
self.transport.write("Hello server, I am the client!\r\n")
self.transport.loseConnection()
This protocol connects to the server, sends it a welcome message, and then terminates the connection.
The connectionMade event is
usually where set up of the Protocol object happens, as well as
any initial greetings (as in the
WelcomeMessage protocol above). Any tearing down of
Protocol -specific objects is done in connectionLost .
Simple, single-use clients¶
In many cases, the protocol only needs to connect to the server once, and the code just wants to get a connected instance of the protocol. In those cases twisted.internet.endpoints provides the appropriate API, and in particular connectProtocol which takes a protocol instance rather than a factory.
from twisted.internet import reactor
from twisted.internet.protocol import Protocol
from twisted.internet.endpoints import TCP4ClientEndpoint, connectProtocol
class Greeter(Protocol):
def sendMessage(self, msg):
self.transport.write("MESSAGE %s\n" % msg)
def gotProtocol(p):
p.sendMessage("Hello")
reactor.callLater(1, p.sendMessage, "This is sent in a second")
reactor.callLater(2, p.transport.loseConnection)
point = TCP4ClientEndpoint(reactor, "localhost", 1234)
d = connectProtocol(point, Greeter())
d.addCallback(gotProtocol)
reactor.run()
Regardless of the type of client endpoint, the way to set up a new
connection is simply pass it to connectProtocol along with a
protocol instance. This means it’s easy to change the mechanism you’re
using to connect, without changing the rest of your program. For example,
to run the greeter example over SSL, the only change required is to
instantiate an
SSL4ClientEndpoint instead of a
TCP4ClientEndpoint . To take advantage of this, functions and
methods which initiates a new connection should generally accept an
endpoint as an argument and let the caller construct it, rather than taking
arguments like ‘host’ and ‘port’ and constructing its own.
For more information on different ways you can make outgoing connections to different types of endpoints, as well as parsing strings into endpoints, see the documentation for the endpoints API .
You may come across code using ClientCreator , an older API which is not as flexible as
the endpoint API. Rather than calling connect on an endpoint,
such code will look like this:
from twisted.internet.protocol import ClientCreator
...
creator = ClientCreator(reactor, Greeter)
d = creator.connectTCP("localhost", 1234)
d.addCallback(gotProtocol)
reactor.run()
In general, the endpoint API should be preferred in new code, as it lets the caller select the method of connecting.
ClientFactory¶
Still, there’s plenty of code out there that uses lower-level APIs, and a few features (such as automatic reconnection) have not been re-implemented with endpoints yet, so in some cases they may be more convenient to use.
To use the lower-level connection APIs, you will need to call one of the reactor.connect* methods directly.
For these cases, you need a ClientFactory .
The ClientFactory is in charge of creating the Protocol and also receives events relating to the connection state.
This allows it to do things like reconnect in the event of a connection error.
Here is an example of a simple ClientFactory that uses the Echo protocol (above) and also prints what state the connection is in.
from twisted.internet.protocol import Protocol, ClientFactory
from sys import stdout
class Echo(Protocol):
def dataReceived(self, data):
stdout.write(data)
class EchoClientFactory(ClientFactory):
def startedConnecting(self, connector):
print 'Started to connect.'
def buildProtocol(self, addr):
print 'Connected.'
return Echo()
def clientConnectionLost(self, connector, reason):
print 'Lost connection. Reason:', reason
def clientConnectionFailed(self, connector, reason):
print 'Connection failed. Reason:', reason
To connect this EchoClientFactory to a server, you could use
this code:
from twisted.internet import reactor
reactor.connectTCP(host, port, EchoClientFactory())
reactor.run()
Note that clientConnectionFailed is called when a connection could not be established, and that clientConnectionLost is called when a connection was made and then disconnected.
Reactor Client APIs¶
IReactorTCP.connectTCP provides support for IPv4 and IPv6 TCP clients.
The host argument it accepts can be either a hostname or an IP address literal.
In the case of a hostname, the reactor will automatically resolve the name to an IP address before attempting the connection.
This means that for a hostname with multiple address records, reconnection attempts may not always go to the same server (see below).
It also means that there is name resolution overhead for each connection attempt.
If you are creating many short-lived connections (typically around hundreds or thousands per second) then you may want to resolve the hostname to an address first and then pass the address to connectTCP instead.
Reconnection¶
Often, the connection of a client will be lost unintentionally due to
network problems. One way to reconnect after a disconnection would be to
call connector.connect() when the connection is lost:
from twisted.internet.protocol import ClientFactory
class EchoClientFactory(ClientFactory):
def clientConnectionLost(self, connector, reason):
connector.connect()
The connector passed as the first argument is the interface between a
connection and a protocol. When the connection fails and the factory
receives the clientConnectionLost event, the factory can
call connector.connect() to start the connection over again
from scratch.
However, most programs that want this functionality should implement ReconnectingClientFactory instead, which tries to reconnect if a connection is lost or fails and which exponentially delays repeated reconnect attempts.
Here is the Echo protocol implemented with
a ReconnectingClientFactory :
from twisted.internet.protocol import Protocol, ReconnectingClientFactory
from sys import stdout
class Echo(Protocol):
def dataReceived(self, data):
stdout.write(data)
class EchoClientFactory(ReconnectingClientFactory):
def startedConnecting(self, connector):
print 'Started to connect.'
def buildProtocol(self, addr):
print 'Connected.'
print 'Resetting reconnection delay'
self.resetDelay()
return Echo()
def clientConnectionLost(self, connector, reason):
print 'Lost connection. Reason:', reason
ReconnectingClientFactory.clientConnectionLost(self, connector, reason)
def clientConnectionFailed(self, connector, reason):
print 'Connection failed. Reason:', reason
ReconnectingClientFactory.clientConnectionFailed(self, connector,
reason)
A Higher-Level Example: ircLogBot¶
Overview of ircLogBot¶
The clients so far have been fairly simple.
A more complicated example comes with Twisted Words in the doc/words/examples directory.
# Copyright (c) Twisted Matrix Laboratories.
# See LICENSE for details.
"""
An example IRC log bot - logs a channel's events to a file.
If someone says the bot's name in the channel followed by a ':',
e.g.
<foo> logbot: hello!
the bot will reply:
<logbot> foo: I am a log bot
Run this script with two arguments, the channel name the bot should
connect to, and file to log to, e.g.:
$ python ircLogBot.py test test.log
will log channel #test to the file 'test.log'.
To run the script:
$ python ircLogBot.py <channel> <file>
"""
# twisted imports
from twisted.words.protocols import irc
from twisted.internet import reactor, protocol
from twisted.python import log
# system imports
import time, sys
class MessageLogger:
"""
An independent logger class (because separation of application
and protocol logic is a good thing).
"""
def __init__(self, file):
self.file = file
def log(self, message):
"""Write a message to the file."""
timestamp = time.strftime("[%H:%M:%S]", time.localtime(time.time()))
self.file.write('%s %s\n' % (timestamp, message))
self.file.flush()
def close(self):
self.file.close()
class LogBot(irc.IRCClient):
"""A logging IRC bot."""
nickname = "twistedbot"
def connectionMade(self):
irc.IRCClient.connectionMade(self)
self.logger = MessageLogger(open(self.factory.filename, "a"))
self.logger.log("[connected at %s]" %
time.asctime(time.localtime(time.time())))
def connectionLost(self, reason):
irc.IRCClient.connectionLost(self, reason)
self.logger.log("[disconnected at %s]" %
time.asctime(time.localtime(time.time())))
self.logger.close()
# callbacks for events
def signedOn(self):
"""Called when bot has succesfully signed on to server."""
self.join(self.factory.channel)
def joined(self, channel):
"""This will get called when the bot joins the channel."""
self.logger.log("[I have joined %s]" % channel)
def privmsg(self, user, channel, msg):
"""This will get called when the bot receives a message."""
user = user.split('!', 1)[0]
self.logger.log("<%s> %s" % (user, msg))
# Check to see if they're sending me a private message
if channel == self.nickname:
msg = "It isn't nice to whisper! Play nice with the group."
self.msg(user, msg)
return
# Otherwise check to see if it is a message directed at me
if msg.startswith(self.nickname + ":"):
msg = "%s: I am a log bot" % user
self.msg(channel, msg)
self.logger.log("<%s> %s" % (self.nickname, msg))
def action(self, user, channel, msg):
"""This will get called when the bot sees someone do an action."""
user = user.split('!', 1)[0]
self.logger.log("* %s %s" % (user, msg))
# irc callbacks
def irc_NICK(self, prefix, params):
"""Called when an IRC user changes their nickname."""
old_nick = prefix.split('!')[0]
new_nick = params[0]
self.logger.log("%s is now known as %s" % (old_nick, new_nick))
# For fun, override the method that determines how a nickname is changed on
# collisions. The default method appends an underscore.
def alterCollidedNick(self, nickname):
"""
Generate an altered version of a nickname that caused a collision in an
effort to create an unused related name for subsequent registration.
"""
return nickname + '^'
class LogBotFactory(protocol.ClientFactory):
"""A factory for LogBots.
A new protocol instance will be created each time we connect to the server.
"""
def __init__(self, channel, filename):
self.channel = channel
self.filename = filename
def buildProtocol(self, addr):
p = LogBot()
p.factory = self
return p
def clientConnectionLost(self, connector, reason):
"""If we get disconnected, reconnect to server."""
connector.connect()
def clientConnectionFailed(self, connector, reason):
print "connection failed:", reason
reactor.stop()
if __name__ == '__main__':
# initialize logging
log.startLogging(sys.stdout)
# create factory protocol and application
f = LogBotFactory(sys.argv[1], sys.argv[2])
# connect factory to this host and port
reactor.connectTCP("irc.freenode.net", 6667, f)
# run bot
reactor.run()
ircLogBot.py connects to an IRC server, joins a channel, and
logs all traffic on it to a file. It demonstrates some of the
connection-level logic of reconnecting on a lost connection, as well as
storing persistent data in the Factory .
Persistent Data in the Factory¶
Since the Protocol instance is recreated each time the
connection is made, the client needs some way to keep track of data that
should be persisted. In the case of the logging bot, it needs to know which
channel it is logging, and where to log it.
from twisted.words.protocols import irc
from twisted.internet import protocol
class LogBot(irc.IRCClient):
def connectionMade(self):
irc.IRCClient.connectionMade(self)
self.logger = MessageLogger(open(self.factory.filename, "a"))
self.logger.log("[connected at %s]" %
time.asctime(time.localtime(time.time())))
def signedOn(self):
self.join(self.factory.channel)
class LogBotFactory(protocol.ClientFactory):
def __init__(self, channel, filename):
self.channel = channel
self.filename = filename
def buildProtocol(self, addr):
p = LogBot()
p.factory = self
return p
When the protocol is created, it gets a reference to the factory as
self.factory . It can then access attributes of the factory in
its logic. In the case of LogBot , it opens the file and
connects to the channel stored in the factory.
Factories have a default implementation of buildProtocol.
It does the same thing the example above does using the protocol attribute of the factory to create the protocol instance.
In the example above, the factory could be rewritten to look like this:
class LogBotFactory(protocol.ClientFactory):
protocol = LogBot
def __init__(self, channel, filename):
self.channel = channel
self.filename = filename
Further Reading¶
The Protocol class used throughout this document is a base implementation of IProtocol used in most Twisted applications for convenience.
To learn about the complete IProtocol interface, see the API documentation for IProtocol .
The transport attribute used in some examples in this
document provides the ITCPTransport interface. To learn
about the complete interface, see the API documentation
for ITCPTransport .
Interface classes are a way of specifying what methods and attributes an object has and how they behave. See the Components: Interfaces and Adapters document for more information on using interfaces in Twisted.
Test-driven development with Twisted¶
Writing good code is hard, or at least it can be. A major challenge is to ensure that your code remains correct as you add new functionality.
Unit testing is a modern, light-weight testing methodology in widespread use in many programming languages. Development that relies on unit tests is often referred to as Test-Driven Development (TDD ). Most Twisted code is tested using TDD.
To gain a solid understanding of unit testing in Python, you should read the unittest –Unit testing framework chapter of the Python LibraryReference . There is a lot of information available online and in books.
Introductory example of Python unit testing¶
This document is principally a guide to Trial, Twisted’s unit testing framework. Trial is based on Python’s unit testing framework. While we do not aim to give a comprehensive guide to general Python unit testing, it will be helpful to consider a simple non-networked example before expanding to cover networking code that requires the special capabilities of Trial. If you are already familiar with unit test in Python, jump straight to the section specific to testing Twisted code .
Note
In what follows we will make a series of refinements
to some simple classes. In order to keep the examples and source code links
complete and to allow you to run Trial on the intermediate results at every
stage, I add _N (where the N are successive
integers) to file names to keep them separate. This is a minor visual
distraction that should be ignored.
Creating an API and writing tests¶
We’ll create a library for arithmetic calculation. First, create a
project structure with a directory called calculus containing an empty __init__.py file.
Then put the following simple class definition API into calculus/base_1.py :
# -*- test-case-name: calculus.test.test_base_1 -*-
class Calculation(object):
def add(self, a, b):
pass
def subtract(self, a, b):
pass
def multiply(self, a, b):
pass
def divide(self, a, b):
pass
(Ignore the test-case-name comment for
now. You’ll see why that’s useful below .)
We’ve written the interface, but not the code. Now we’ll write a set of tests. At this point of development, we’ll be expecting all tests to fail. Don’t worry, that’s part of the point. Once we have a test framework functioning, and we have some decent tests written (and failing!), we’ll go and do the actual development of our calculation API. This is the preferred way to work for many people using TDD - write tests first, make sure they fail, then do development. Others are not so strict and write tests after doing the development.
Create a test directory beneath calculus , with an empty __init__.py file. In a calculus/test/test_base_1.py , put the
following:
from calculus.base_1 import Calculation
from twisted.trial import unittest
class CalculationTestCase(unittest.TestCase):
def test_add(self):
calc = Calculation()
result = calc.add(3, 8)
self.assertEqual(result, 11)
def test_subtract(self):
calc = Calculation()
result = calc.subtract(7, 3)
self.assertEqual(result, 4)
def test_multiply(self):
calc = Calculation()
result = calc.multiply(12, 5)
self.assertEqual(result, 60)
def test_divide(self):
calc = Calculation()
result = calc.divide(12, 5)
self.assertEqual(result, 2)
You should now have the following 4 files:
calculus/__init__.py
calculus/base_1.py
calculus/test/__init__.py
calculus/test/test_base_1.py
To run the tests, there are two things you must set up. Make sure you get both done - nothing below will work unless you do.
First, make sure that the directory that contains your
calculus directory is in your Python load path. If you’re
using the Bash shell on some form of unix (e.g., Linux, Mac OS X), run
PYTHONPATH="$PYTHONPATH:`pwd`/.." at
the command line in the calculus directory. Once you have your
Python path set up correctly, you should be able to run Python from the
command line and import calculus without seeing
an import error.
Second, make sure you can run the trial
command. That is, make sure the directory containing the trial
program on you system is in your shell’s PATH . The easiest way to check if you have this is to
try running trial --help at the command line. If
you see a list of invocation options, you’re in business. If your shell
reports something like trial: command not found ,
make sure you have Twisted installed properly, and that the Twisted
bin directory is in your PATH . If
you don’t know how to do this, get some local help, or figure it out by
searching online for information on setting and changing environment
variables for you operating system.
With those (one-time) preliminary steps out of the way, let’s perform
the tests. Run trial calculus.test.test_base_1 from the
command line when you are in the directory containing the calculus
directory.
You should see the following output (though your files are probably not in
/tmp ):
$ trial calculus.test.test_base_1
calculus.test.test_base_1
CalculationTestCase
test_add ... [FAIL]
test_divide ... [FAIL]
test_multiply ... [FAIL]
test_subtract ... [FAIL]
===============================================================================
[FAIL]
Traceback (most recent call last):
File "/tmp/calculus/test/test_base_1.py", line 8, in test_add
self.assertEqual(result, 11)
twisted.trial.unittest.FailTest: not equal:
a = None
b = 11
calculus.test.test_base_1.CalculationTestCase.test_add
===============================================================================
[FAIL]
Traceback (most recent call last):
File "/tmp/calculus/test/test_base_1.py", line 23, in test_divide
self.assertEqual(result, 2)
twisted.trial.unittest.FailTest: not equal:
a = None
b = 2
calculus.test.test_base_1.CalculationTestCase.test_divide
===============================================================================
[FAIL]
Traceback (most recent call last):
File "/tmp/calculus/test/test_base_1.py", line 18, in test_multiply
self.assertEqual(result, 60)
twisted.trial.unittest.FailTest: not equal:
a = None
b = 60
calculus.test.test_base_1.CalculationTestCase.test_multiply
===============================================================================
[FAIL]
Traceback (most recent call last):
File "/tmp/calculus/test/test_base_1.py", line 13, in test_subtract
self.assertEqual(result, 4)
twisted.trial.unittest.FailTest: not equal:
a = None
b = 4
calculus.test.test_base_1.CalculationTestCase.test_subtract
-------------------------------------------------------------------------------
Ran 4 tests in 0.042s
FAILED (failures=4)
How to interpret this output? You get a list of the individual tests, each
followed by its result. By default, failures are printed at the end, but this
can be changed with the -e (or --rterrors ) option.
One very useful thing in this output is the fully-qualified name of the
failed tests. This appears at the bottom of each =-delimited area of the
output. This allows you to copy and paste it to just run a single test you’re
interested in. In our example, you could run trial calculus.test.test_base_1.CalculationTestCase.test_subtract from the
shell.
Note that trial can use different reporters to modify its output. Run
trial --help-reporters to see a list of
reporters.
The tests can be run by trial in multiple ways:
trial calculus: run all the tests for the calculus package.trial calculus.test: run using Python’simportnotation.trial calculus.test.test_base_1: as above, for a specific test module. You can follow that logic by putting your class name and even a method name to only run those specific tests.trial --testmodule=calculus/base_1.py: use thetest-case-namecomment in the first line ofcalculus/base_1.pyto find the tests.trial calculus/test: run all the tests in the test directory (not recommended).trial calculus/test/test_base_1.py: run a specific test file (not recommended).
The first 3 versions using full qualified names are strongly encouraged: they are much more reliable and they allow you to easily be more selective in your test runs.
You’ll notice that Trial creates a _trial_temp directory in
the directory where you run the tests. This has a file called
test.log which contains the log output of the tests (created
using log.msg or log.err functions). Examine this file if you add
logging to your tests.
Making the tests pass¶
Now that we have a working test framework in place, and our tests are
failing (as expected) we can go and try to implement the correct API. We’ll do
that in a new version of the above base_1
module, calculus/base_2.py :
# -*- test-case-name: calculus.test.test_base_2 -*-
class Calculation(object):
def add(self, a, b):
return a + b
def subtract(self, a, b):
return a - b
def multiply(self, a, b):
return a * b
def divide(self, a, b):
return a / b
We’ll also create a new version of test_base_1 which imports and tests this
new implementation,
in calculus/test_base_2.py :
from calculus.base_2 import Calculation
from twisted.trial import unittest
class CalculationTestCase(unittest.TestCase):
def test_add(self):
calc = Calculation()
result = calc.add(3, 8)
self.assertEqual(result, 11)
def test_subtract(self):
calc = Calculation()
result = calc.subtract(7, 3)
self.assertEqual(result, 4)
def test_multiply(self):
calc = Calculation()
result = calc.multiply(12, 5)
self.assertEqual(result, 60)
def test_divide(self):
calc = Calculation()
result = calc.divide(12, 5)
self.assertEqual(result, 2)
is a copy of test_base_1, but with the import changed. Run trial again as above, and your tests should now pass:
$ trial calculus.test.test_base_2
Running 4 tests.
calculus.test.test_base
CalculationTestCase
test_add ... [OK]
test_divide ... [OK]
test_multiply ... [OK]
test_subtract ... [OK]
-------------------------------------------------------------------------------
Ran 4 tests in 0.067s
PASSED (successes=4)
Factoring out common test logic¶
You’ll notice that our test file contains redundant code. Let’s get rid
of that. Python’s unit testing framework allows your test class to define a
setUp method that is called before
each test method in the class. This allows you to add attributes
to self that can be used in test
methods. We’ll also add a parameterized test method to further simplify the
code.
Note that a test class may also provide the counterpart of setUp , named tearDown ,
which will be called after each test (whether successful or
not). tearDown is mainly used for post-test
cleanup purposes. We will not use tearDown
until later.
Create calculus/test/test_base_2b.py as
follows:
from calculus.base_2 import Calculation
from twisted.trial import unittest
class CalculationTestCase(unittest.TestCase):
def setUp(self):
self.calc = Calculation()
def _test(self, operation, a, b, expected):
result = operation(a, b)
self.assertEqual(result, expected)
def test_add(self):
self._test(self.calc.add, 3, 8, 11)
def test_subtract(self):
self._test(self.calc.subtract, 7, 3, 4)
def test_multiply(self):
self._test(self.calc.multiply, 6, 9, 54)
def test_divide(self):
self._test(self.calc.divide, 12, 5, 2)
Much cleaner, isn’t it?
We’ll now add some additional error tests. Testing just for successful
use of the API is generally not enough, especially if you expect your code
to be used by others. Let’s make sure the Calculation class raises exceptions if someone tries
to call its methods with arguments that cannot be converted to
integers.
We arrive at calculus/test/test_base_3.py :
from calculus.base_3 import Calculation
from twisted.trial import unittest
class CalculationTestCase(unittest.TestCase):
def setUp(self):
self.calc = Calculation()
def _test(self, operation, a, b, expected):
result = operation(a, b)
self.assertEqual(result, expected)
def _test_error(self, operation):
self.assertRaises(TypeError, operation, "foo", 2)
self.assertRaises(TypeError, operation, "bar", "egg")
self.assertRaises(TypeError, operation, [3], [8, 2])
self.assertRaises(TypeError, operation, {"e": 3}, {"r": "t"})
def test_add(self):
self._test(self.calc.add, 3, 8, 11)
def test_subtract(self):
self._test(self.calc.subtract, 7, 3, 4)
def test_multiply(self):
self._test(self.calc.multiply, 6, 9, 54)
def test_divide(self):
self._test(self.calc.divide, 12, 5, 2)
def test_errorAdd(self):
self._test_error(self.calc.add)
def test_errorSubtract(self):
self._test_error(self.calc.subtract)
def test_errorMultiply(self):
self._test_error(self.calc.multiply)
def test_errorDivide(self):
self._test_error(self.calc.divide)
We’ve added four new tests and one general-purpose function, _test_error . This function uses the assertRaises method, which takes an exception class,
a function to run and its arguments, and checks that calling the function
on the arguments does indeed raise the given exception.
If you run the above, you’ll see that not all tests fail. In Python it’s
often valid to add and multiply objects of different and even differing
types, so the code in the add and multiply tests does not raise an exception
and those tests therefore fail. So let’s add explicit type conversion to
our API class. This brings us to calculus/base_3.py :
# -*- test-case-name: calculus.test.test_base_3 -*-
class Calculation(object):
def _make_ints(self, *args):
try:
return map(int, args)
except ValueError:
raise TypeError("Couldn't coerce arguments to integers: %s" % args)
def add(self, a, b):
a, b = self._make_ints(a, b)
return a + b
def subtract(self, a, b):
a, b = self._make_ints(a, b)
return a - b
def multiply(self, a, b):
a, b = self._make_ints(a, b)
return a * b
def divide(self, a, b):
a, b = self._make_ints(a, b)
return a / b
Here the _make_ints helper function tries to
convert a list into a list of equivalent integers, and raises a TypeError in case the conversion goes wrong.
Note
The int conversion can also raise a TypeError if passed something of
the wrong type, such as a list. We’ll just let that exception go by, as
TypeError is already what we want in case something goes wrong.
Twisted specific testing¶
Up to this point we’ve been doing fairly standard Python unit testing.
With only a few cosmetic changes (most importantly, directly importing
unittest instead of using Twisted’s unittest version) we could make the
above tests run using Python’s standard library unit testing framework.
Here we will assume a basic familiarity with Twisted’s network I/O, timing, and Deferred APIs. If you haven’t already read them, you should read the documentation on Writing Servers , Writing Clients , and Deferreds .
Now we’ll get to the real point of this tutorial and take advantage of Trial to test Twisted code.
Testing a protocol¶
We’ll now create a custom protocol to invoke our class from a telnet-like session. We’ll remotely call commands with arguments and read back the response. The goal will be to test our network code without creating sockets.
Creating and testing the server¶
First we’ll write the tests, and then explain what they do. The first version of the remote test code is:
from calculus.remote_1 import RemoteCalculationFactory
from twisted.trial import unittest
from twisted.test import proto_helpers
class RemoteCalculationTestCase(unittest.TestCase):
def setUp(self):
factory = RemoteCalculationFactory()
self.proto = factory.buildProtocol(('127.0.0.1', 0))
self.tr = proto_helpers.StringTransport()
self.proto.makeConnection(self.tr)
def _test(self, operation, a, b, expected):
self.proto.dataReceived('%s %d %d\r\n' % (operation, a, b))
self.assertEqual(int(self.tr.value()), expected)
def test_add(self):
return self._test('add', 7, 6, 13)
def test_subtract(self):
return self._test('subtract', 82, 78, 4)
def test_multiply(self):
return self._test('multiply', 2, 8, 16)
def test_divide(self):
return self._test('divide', 14, 3, 4)
To fully understand this client, it helps a lot to be comfortable with the Factory/Protocol/Transport pattern used in Twisted.
We first create a protocol factory object. Note that we have yet to see
the RemoteCalculationFactory class. It is in
calculus/remote_1.py below. We
call buildProtocol to ask the factory to build us a
protocol object that knows how to talk to our server. We then make a fake
network transport, an instance of twisted.test.proto_helpers.StringTransport
class (note that test packages are generally not part of Twisted’s public API;``twisted.test.proto_helpers`` is an exception). This fake
transport is the key to the communications. It is used to emulate a network
connection without a network. The address and port passed to buildProtocol
are typically used by the factory to choose to immediately deny remote connections; since we’re using a fake transport, we can choose any value that will be acceptable to the factory. In this case the factory just ignores the address, so we don’t need to pick anything in particular.
Testing protocols without the use of real network connections is both simple and recommended when testing Twisted code. Even though there are many tests in Twisted that use the network, most good tests don’t. The problem with unit tests and networking is that networks aren’t reliable. We cannot know that they will exhibit reasonable behavior all the time. This creates intermittent test failures due to network vagaries. Right now we’re trying to test our Twisted code, not network reliability. By setting up and using a fake transport, we can write 100% reliable tests. We can also test network failures in a deterministic manner, another important part of your complete test suite.
The final key to understanding this client code is the _test method. The call to dataReceived simulates data arriving on the network
transport. But where does it arrive? It’s handed to the lineReceived method of the protocol instance (in
calculus/remote_1.py below). So the client
is essentially tricking the server into thinking it has received the
operation and the arguments over the network. The server (once again, see
below) hands over the work to its CalculationProxy object which in turn hands it to its
Calculation instance. The result is written
back via sendLine (into the fake string
transport object), and is then immediately available to the client, who
fetches it with tr.value() and checks that it
has the expected value. So there’s quite a lot going on behind the scenes
in the two-line _test method above.
Finally , let’s see the implementation of this protocol. Put the
following into calculus/remote_1.py :
# -*- test-case-name: calculus.test.test_remote_1 -*-
from twisted.protocols import basic
from twisted.internet import protocol
from calculus.base_3 import Calculation
class CalculationProxy(object):
def __init__(self):
self.calc = Calculation()
for m in ['add', 'subtract', 'multiply', 'divide']:
setattr(self, 'remote_%s' % m, getattr(self.calc, m))
class RemoteCalculationProtocol(basic.LineReceiver):
def __init__(self):
self.proxy = CalculationProxy()
def lineReceived(self, line):
op, a, b = line.split()
a = int(a)
b = int(b)
op = getattr(self.proxy, 'remote_%s' % (op,))
result = op(a, b)
self.sendLine(str(result))
class RemoteCalculationFactory(protocol.Factory):
protocol = RemoteCalculationProtocol
def main():
from twisted.internet import reactor
from twisted.python import log
import sys
log.startLogging(sys.stdout)
reactor.listenTCP(0, RemoteCalculationFactory())
reactor.run()
if __name__ == "__main__":
main()
As mentioned, this server creates a protocol that inherits from basic.LineReceiver , and then a
factory that uses it as protocol. The only trick is the CalculationProxy object, which calls Calculation methods through remote_* methods. This pattern is used frequently in
Twisted, because it is very explicit about what methods you are making
accessible.
If you run this test (trial calculus.test.test_remote_1 ), everything should be fine. You can also
run a server to test it with a telnet client. To do that, call python calculus/remote_1.py . You should have the following output:
2008-04-25 10:53:27+0200 [-] Log opened.
2008-04-25 10:53:27+0200 [-] __main__.RemoteCalculationFactory starting on 46194
2008-04-25 10:53:27+0200 [-] Starting factory <__main__.RemoteCalculationFactory instance at 0x846a0cc>
46194 is replaced by a random port. You can then call telnet on it:
$ telnet localhost 46194
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
add 4123 9423
13546
It works!
Creating and testing the client¶
Of course, what we build is not particularly useful for now: we’ll now build a client for our server, to be able to use it inside a Python program. And it will serve our next purpose.
Create calculus/test/test_client_1.py :
from calculus.client_1 import RemoteCalculationClient
from twisted.trial import unittest
from twisted.test import proto_helpers
class ClientCalculationTestCase(unittest.TestCase):
def setUp(self):
self.tr = proto_helpers.StringTransport()
self.proto = RemoteCalculationClient()
self.proto.makeConnection(self.tr)
def _test(self, operation, a, b, expected):
d = getattr(self.proto, operation)(a, b)
self.assertEqual(self.tr.value(), '%s %d %d\r\n' % (operation, a, b))
self.tr.clear()
d.addCallback(self.assertEqual, expected)
self.proto.dataReceived("%d\r\n" % (expected,))
return d
def test_add(self):
return self._test('add', 7, 6, 13)
def test_subtract(self):
return self._test('subtract', 82, 78, 4)
def test_multiply(self):
return self._test('multiply', 2, 8, 16)
def test_divide(self):
return self._test('divide', 14, 3, 4)
It’s really symmetric to the server test cases. The only tricky part is that we don’t use a client factory. We’re lazy, and it’s not very useful in the client part, so we instantiate the protocol directly.
Incidentally, we have introduced a very important concept here: the tests now return a Deferred object, and the assertion is done in a callback. When a test returns a Deferred, the reactor is run until the Deferred fires and its callbacks run. The important thing to do here is to not forget to return the Deferred . If you do, your tests will pass even if nothing is asserted. That’s also why it’s important to make tests fail first: if your tests pass whereas you know they shouldn’t, there is a problem in your tests.
We’ll now add the remote client class to produce calculus/client_1.py :
# -*- test-case-name: calculus.test.test_client_1 -*-
from twisted.protocols import basic
from twisted.internet import defer
class RemoteCalculationClient(basic.LineReceiver):
def __init__(self):
self.results = []
def lineReceived(self, line):
d = self.results.pop(0)
d.callback(int(line))
def _sendOperation(self, op, a, b):
d = defer.Deferred()
self.results.append(d)
line = "%s %d %d" % (op, a, b)
self.sendLine(line)
return d
def add(self, a, b):
return self._sendOperation("add", a, b)
def subtract(self, a, b):
return self._sendOperation("subtract", a, b)
def multiply(self, a, b):
return self._sendOperation("multiply", a, b)
def divide(self, a, b):
return self._sendOperation("divide", a, b)
More good practices¶
Testing scheduling¶
When testing code that involves the passage of time, waiting e.g. for a two hour timeout to occur in a test is not very realistic. Twisted provides a solution to this, the Clock class that allows one to simulate the passage of time.
As an example we’ll test the code for client request timeout: since our client uses TCP it can hang for a long time (firewall, connectivity problems, etc...). So generally we need to implement timeouts on the client side. Basically it’s just that we send a request, don’t receive a response and expect a timeout error to be triggered after a certain duration.
from calculus.client_2 import RemoteCalculationClient, ClientTimeoutError
from twisted.internet import task
from twisted.trial import unittest
from twisted.test import proto_helpers
class ClientCalculationTestCase(unittest.TestCase):
def setUp(self):
self.tr = proto_helpers.StringTransportWithDisconnection()
self.clock = task.Clock()
self.proto = RemoteCalculationClient()
self.tr.protocol = self.proto
self.proto.callLater = self.clock.callLater
self.proto.makeConnection(self.tr)
def _test(self, operation, a, b, expected):
d = getattr(self.proto, operation)(a, b)
self.assertEqual(self.tr.value(), '%s %d %d\r\n' % (operation, a, b))
self.tr.clear()
d.addCallback(self.assertEqual, expected)
self.proto.dataReceived("%d\r\n" % (expected,))
return d
def test_add(self):
return self._test('add', 7, 6, 13)
def test_subtract(self):
return self._test('subtract', 82, 78, 4)
def test_multiply(self):
return self._test('multiply', 2, 8, 16)
def test_divide(self):
return self._test('divide', 14, 3, 4)
def test_timeout(self):
d = self.proto.add(9, 4)
self.assertEqual(self.tr.value(), 'add 9 4\r\n')
self.clock.advance(self.proto.timeOut)
return self.assertFailure(d, ClientTimeoutError)
What happens here? We instantiate our protocol as usual, the only trick
is to create the clock, and assign proto.callLater to
clock.callLater . Thus, every callLater
call in the protocol will finish before clock.advance() returns.
In the new test (test_timeout), we call clock.advance , that simulates an advance in time
(logically it’s similar to a time.sleep call). And
we just have to verify that our Deferred got a timeout error.
Let’s implement that in our code.
# -*- test-case-name: calculus.test.test_client_2 -*-
from twisted.protocols import basic
from twisted.internet import defer, reactor
class ClientTimeoutError(Exception):
pass
class RemoteCalculationClient(basic.LineReceiver):
callLater = reactor.callLater
timeOut = 60
def __init__(self):
self.results = []
def lineReceived(self, line):
d, callID = self.results.pop(0)
callID.cancel()
d.callback(int(line))
def _cancel(self, d):
d.errback(ClientTimeoutError())
def _sendOperation(self, op, a, b):
d = defer.Deferred()
callID = self.callLater(self.timeOut, self._cancel, d)
self.results.append((d, callID))
line = "%s %d %d" % (op, a, b)
self.sendLine(line)
return d
def add(self, a, b):
return self._sendOperation("add", a, b)
def subtract(self, a, b):
return self._sendOperation("subtract", a, b)
def multiply(self, a, b):
return self._sendOperation("multiply", a, b)
def divide(self, a, b):
return self._sendOperation("divide", a, b)
If everything completed successfully,
it is important to remember to cancel the
DelayedCall
returned by callLater .
Cleaning up after tests¶
This chapter is mainly intended for people who want to have sockets or processes created in their tests. If it’s still not obvious, you must try to avoid using them, because it ends up with a lot of problems, one of them being intermittent failures. And intermittent failures are the plague of automated tests.
To actually test that, we’ll launch a server with our protocol.
from calculus.remote_1 import RemoteCalculationFactory
from calculus.client_2 import RemoteCalculationClient
from twisted.trial import unittest
from twisted.internet import reactor, protocol
class RemoteRunCalculationTestCase(unittest.TestCase):
def setUp(self):
factory = RemoteCalculationFactory()
self.port = reactor.listenTCP(0, factory, interface="127.0.0.1")
self.client = None
def tearDown(self):
if self.client is not None:
self.client.transport.loseConnection()
return self.port.stopListening()
def _test(self, op, a, b, expected):
creator = protocol.ClientCreator(reactor, RemoteCalculationClient)
def cb(client):
self.client = client
return getattr(self.client, op)(a, b
).addCallback(self.assertEqual, expected)
return creator.connectTCP('127.0.0.1', self.port.getHost().port
).addCallback(cb)
def test_add(self):
return self._test("add", 5, 9, 14)
def test_subtract(self):
return self._test("subtract", 47, 13, 34)
def test_multiply(self):
return self._test("multiply", 7, 3, 21)
def test_divide(self):
return self._test("divide", 84, 10, 8)
Recent versions of trial will fail loudly if you remove the
stopListening call, which is good.
Also, you should be aware that tearDown will
be called in any case, after success or failure. So don’t expect every
object you created in the test method to be present, because your tests may
have failed in the middle.
Trial also has a addCleanup method, which makes
these kind of cleanups easy and removes the need for tearDown . For example, you could remove the code in _test
this way:
def setUp(self):
factory = RemoteCalculationFactory()
self.port = reactor.listenTCP(0, factory, interface="127.0.0.1")
self.addCleanup(self.port.stopListening)
def _test(self, op, a, b, expected):
creator = protocol.ClientCreator(reactor, RemoteCalculationClient)
def cb(client):
self.addCleanup(self.client.transport.loseConnection)
return getattr(client, op)(a, b).addCallback(self.assertEqual, expected)
return creator.connectTCP('127.0.0.1', self.port.getHost().port).addCallback(cb)
This removes the need of a tearDown method, and you don’t have to check for
the value of self.client: you only call addCleanup when the client is
created.
Handling logged errors¶
Currently, if you send an invalid command or invalid arguments to our server, it logs an exception and closes the connection. This is a perfectly valid behavior, but for the sake of this tutorial, we want to return an error to the user if they send invalid operators, and log any errors on server side. So we’ll want a test like this:
def test_invalidParameters(self):
self.proto.dataReceived('add foo bar\r\n')
self.assertEqual(self.tr.value(), "error\r\n")
# -*- test-case-name: calculus.test.test_remote_1 -*-
from twisted.protocols import basic
from twisted.internet import protocol
from twisted.python import log
from calculus.base_3 import Calculation
class CalculationProxy(object):
def __init__(self):
self.calc = Calculation()
for m in ['add', 'subtract', 'multiply', 'divide']:
setattr(self, 'remote_%s' % m, getattr(self.calc, m))
class RemoteCalculationProtocol(basic.LineReceiver):
def __init__(self):
self.proxy = CalculationProxy()
def lineReceived(self, line):
op, a, b = line.split()
op = getattr(self.proxy, 'remote_%s' % (op,))
try:
result = op(a, b)
except TypeError:
log.err()
self.sendLine("error")
else:
self.sendLine(str(result))
class RemoteCalculationFactory(protocol.Factory):
protocol = RemoteCalculationProtocol
def main():
from twisted.internet import reactor
from twisted.python import log
import sys
log.startLogging(sys.stdout)
reactor.listenTCP(0, RemoteCalculationFactory())
reactor.run()
if __name__ == "__main__":
main()
If you try something like that, it will not work. Here is the output you should have:
trial calculus.test.test_remote_3.RemoteCalculationTestCase.test_invalidParameters
calculus.test.test_remote_3
RemoteCalculationTestCase
test_invalidParameters ... [ERROR]
===============================================================================
[ERROR]: calculus.test.test_remote_3.RemoteCalculationTestCase.test_invalidParameters
Traceback (most recent call last):
File "/tmp/calculus/remote_2.py", line 27, in lineReceived
result = op(a, b)
File "/tmp/calculus/base_3.py", line 11, in add
a, b = self._make_ints(a, b)
File "/tmp/calculus/base_3.py", line 8, in _make_ints
raise TypeError
exceptions.TypeError:
-------------------------------------------------------------------------------
Ran 1 tests in 0.004s
FAILED (errors=1)
At first, you could think there is a problem, because you catch this
exception. But in fact trial doesn’t let you do that without controlling it:
you must expect logged errors and clean them. To do that, you have to use the
flushLoggedErrors method. You call it with the
exception you expect, and it returns the list of exceptions logged since the
start of the test. Generally, you’ll want to check that this list has the
expected length, or possibly that each exception has an expected message. We do
the former in our test:
from calculus.remote_2 import RemoteCalculationFactory
from twisted.trial import unittest
from twisted.test import proto_helpers
class RemoteCalculationTestCase(unittest.TestCase):
def setUp(self):
factory = RemoteCalculationFactory()
self.proto = factory.buildProtocol(('127.0.0.1', 0))
self.tr = proto_helpers.StringTransport()
self.proto.makeConnection(self.tr)
def _test(self, operation, a, b, expected):
self.proto.dataReceived('%s %d %d\r\n' % (operation, a, b))
self.assertEqual(int(self.tr.value()), expected)
def test_add(self):
return self._test('add', 7, 6, 13)
def test_subtract(self):
return self._test('subtract', 82, 78, 4)
def test_multiply(self):
return self._test('multiply', 2, 8, 16)
def test_divide(self):
return self._test('divide', 14, 3, 4)
def test_invalidParameters(self):
self.proto.dataReceived('add foo bar\r\n')
self.assertEqual(self.tr.value(), "error\r\n")
errors = self.flushLoggedErrors(TypeError)
self.assertEqual(len(errors), 1)
Resolve a bug¶
A bug was left over during the development of the timeout (probably several bugs, but that’s not the point), concerning the reuse of the protocol when you got a timeout: the connection is not dropped, so you can get timeout forever. Generally a user will come to you saying “I have this strange problem on my crappy network. It seems you could solve it with doing XXX at YYY.”
Actually, this bug can be corrected several ways. But if you correct it without adding tests, one day you’ll face a big problem: regression. So the first step is adding a failing test.
from calculus.client_3 import RemoteCalculationClient, ClientTimeoutError
from twisted.internet import task
from twisted.trial import unittest
from twisted.test import proto_helpers
class ClientCalculationTestCase(unittest.TestCase):
def setUp(self):
self.tr = proto_helpers.StringTransportWithDisconnection()
self.clock = task.Clock()
self.proto = RemoteCalculationClient()
self.tr.protocol = self.proto
self.proto.callLater = self.clock.callLater
self.proto.makeConnection(self.tr)
def _test(self, operation, a, b, expected):
d = getattr(self.proto, operation)(a, b)
self.assertEqual(self.tr.value(), '%s %d %d\r\n' % (operation, a, b))
self.tr.clear()
d.addCallback(self.assertEqual, expected)
self.proto.dataReceived("%d\r\n" % (expected,))
return d
def test_add(self):
return self._test('add', 7, 6, 13)
def test_subtract(self):
return self._test('subtract', 82, 78, 4)
def test_multiply(self):
return self._test('multiply', 2, 8, 16)
def test_divide(self):
return self._test('divide', 14, 3, 4)
def test_timeout(self):
d = self.proto.add(9, 4)
self.assertEqual(self.tr.value(), 'add 9 4\r\n')
self.clock.advance(self.proto.timeOut)
return self.assertFailure(d, ClientTimeoutError)
def test_timeoutConnectionLost(self):
called = []
def lost(arg):
called.append(True)
self.proto.connectionLost = lost
d = self.proto.add(9, 4)
self.assertEqual(self.tr.value(), 'add 9 4\r\n')
self.clock.advance(self.proto.timeOut)
def check(ignore):
self.assertEqual(called, [True])
return self.assertFailure(d, ClientTimeoutError).addCallback(check)
What have we done here ?
- We switched to StringTransportWithDisconnection. This transport manages
loseConnectionand forwards it to its protocol. - We assign the protocol to the transport via the
protocolattribute. - We check that after a timeout our connection has closed.
For doing that, we then use the TimeoutMixin
class, that does almost everything we want. The great thing is that it almost
changes nothing to our class.
# -*- test-case-name: calculus.test.test_client -*-
from twisted.protocols import basic, policies
from twisted.internet import defer
class ClientTimeoutError(Exception):
pass
class RemoteCalculationClient(object, basic.LineReceiver, policies.TimeoutMixin):
def __init__(self):
self.results = []
self._timeOut = 60
def lineReceived(self, line):
self.setTimeout(None)
d = self.results.pop(0)
d.callback(int(line))
def timeoutConnection(self):
for d in self.results:
d.errback(ClientTimeoutError())
self.transport.loseConnection()
def _sendOperation(self, op, a, b):
d = defer.Deferred()
self.results.append(d)
line = "%s %d %d" % (op, a, b)
self.sendLine(line)
self.setTimeout(self._timeOut)
return d
def add(self, a, b):
return self._sendOperation("add", a, b)
def subtract(self, a, b):
return self._sendOperation("subtract", a, b)
def multiply(self, a, b):
return self._sendOperation("multiply", a, b)
def divide(self, a, b):
return self._sendOperation("divide", a, b)
Testing Deferreds without the reactor¶
Above we learned about returning Deferreds from test methods in order to make
assertions about their results, or side-effects that only happen after they
fire. This can be useful, but we don’t actually need the feature in this
example. Because we were careful to use Clock , we
don’t need the global reactor to run in our tests. Instead of returning the
Deferred with a callback attached to it which performs the necessary assertions,
we can use a testing helper,
successResultOf (and
the corresponding error-case helper
failureResultOf ), to
extract its result and make assertions against it directly. Compared to
returning a Deferred, this avoids the problem of forgetting to return the
Deferred, improves the stack trace reported when the assertion fails, and avoids
the complexity of using global reactor (which, for example, may then require
cleanup).
from calculus.client_3 import RemoteCalculationClient, ClientTimeoutError
from twisted.internet import task
from twisted.trial import unittest
from twisted.test import proto_helpers
class ClientCalculationTestCase(unittest.TestCase):
def setUp(self):
self.tr = proto_helpers.StringTransportWithDisconnection()
self.clock = task.Clock()
self.proto = RemoteCalculationClient()
self.tr.protocol = self.proto
self.proto.callLater = self.clock.callLater
self.proto.makeConnection(self.tr)
def _test(self, operation, a, b, expected):
d = getattr(self.proto, operation)(a, b)
self.assertEqual(self.tr.value(), '%s %d %d\r\n' % (operation, a, b))
self.tr.clear()
self.proto.dataReceived("%d\r\n" % (expected,))
self.assertEqual(expected, self.successResultOf(d))
def test_add(self):
self._test('add', 7, 6, 13)
def test_subtract(self):
self._test('subtract', 82, 78, 4)
def test_multiply(self):
self._test('multiply', 2, 8, 16)
def test_divide(self):
self._test('divide', 14, 3, 4)
def test_timeout(self):
d = self.proto.add(9, 4)
self.assertEqual(self.tr.value(), 'add 9 4\r\n')
self.clock.advance(self.proto.timeOut)
self.failureResultOf(d).trap(ClientTimeoutError)
def test_timeoutConnectionLost(self):
called = []
def lost(arg):
called.append(True)
self.proto.connectionLost = lost
d = self.proto.add(9, 4)
self.assertEqual(self.tr.value(), 'add 9 4\r\n')
self.clock.advance(self.proto.timeOut)
def check(ignore):
self.assertEqual(called, [True])
self.failureResultOf(d).trap(ClientTimeoutError)
self.assertEqual(called, [True])
This version of the code makes the same assertions, but no longer returns any
Deferreds from any test methods. Instead of making assertions about the result
of the Deferred in a callback, it makes the assertions as soon as
it knows the Deferred is supposed to have a result (in
the _test method and in test_timeout
and test_timeoutConnectionLost ). The possibility
of knowing exactly when a Deferred is supposed to have a test is what
makes successResultOf useful in unit testing, but prevents it from being
applicable to non-testing purposes.
successResultOf will raise an exception (failing the test) if
the Deferred passed to it does not have a result, or has a failure
result. Similarly, failureResultOf will raise an exception (also
failing the test) if the Deferred passed to it does not have a
result, or has a success result. There is a third helper method for testing the
final case,
assertNoResult ,
which only raises an exception (failing the test) if the Deferred passed
to it has a result (either success or failure).
Dropping into a debugger¶
In the course of writing and running your tests, it is often helpful to
employ the use of a debugger. This can be particularly helpful in tracking down
where the source of a troublesome bug is in your code. Python’s standard library
includes a debugger in the form of the pdb module.
Running your tests with pdb is as simple as invoking
twisted with the --debug option, which will start pdb at the beginning of the execution of your test
suite.
Trial also provides a --debugger option which can
run your test suite using another debugger instead. To specify a debugger other
than pdb , pass in the fully-qualified name of an
object that provides the same interface as pdb .
Most third-party debuggers tend to implement an interface similar to pdb , or at least provide a wrapper object that
does. For example, invoking trial with the line trial --debug --debugger pudb will open the PuDB debugger instead, provided
it is properly installed.
Code coverage¶
Code coverage is one of the aspects of software testing that shows how much your tests cross (cover) the code of your program. There are different kinds of measures: path coverage, condition coverage, statement coverage... We’ll only consider statement coverage here, whether a line has been executed or not.
Trial has an option to generate the statement coverage of your tests. This option is –coverage. It creates a coverage directory in _trial_temp, with a file .cover for every module used during the tests. The ones interesting for us are calculus.base.cover and calculus.remote.cover. Each line starts with a counter showing how many times the line was executed during the tests, or the marker ‘>>>>>>’ if the line was not covered. If you went through the whole tutorial to this point, you should have complete coverage :).
Again, this is only another useful pointer, but it doesn’t mean your code is perfect: your tests should consider every possible input and output, to get full coverage (condition, path, etc.) as well .
Conclusion¶
So what did you learn in this document?
- How to use the trial command-line tool to run your tests
- How to use string transports to test individual clients and servers without creating sockets
- If you really want to create sockets, how to cleanly do it so that it doesn’t have bad side effects
- And some small tips you can’t live without.
If one of the topics still looks cloudy to you, please give us your feedback! You can file tickets to improve this document - learn how to contribute on the Twisted web site.
Twisted from Scratch, or The Evolution of Finger¶
The Evolution of Finger: building a simple finger service¶
Introduction¶
This is the first part of the Twisted tutorial Twisted from Scratch, or The Evolution of Finger .
If you’re not familiar with ‘finger’ it’s probably because it’s not used as
much nowadays as it used to be. Basically, if you run finger nail
or finger nail@example.com the target computer spits out some
information about the user named nail . For instance:
Login: nail Name: Nail Sharp
Directory: /home/nail Shell: /usr/bin/sh
Last login Wed Mar 31 18:32 2004 (PST)
New mail received Thu Apr 1 10:50 2004 (PST)
Unread since Thu Apr 1 10:50 2004 (PST)
No Plan.
If the target computer does not have
the fingerd daemon
running you’ll get a “Connection Refused” error. Paranoid sysadmins
keep fingerd off or limit the output to hinder crackers
and harassers. The above format is the standard fingerd
default, but an alternate implementation can output anything it wants,
such as automated responsibility status for everyone in an
organization. You can also define pseudo “users”, which are
essentially keywords.
This portion of the tutorial makes use of factories and protocols as introduced in the Writing a TCP Server howto and deferreds as introduced in Using Deferreds and Generating Deferreds . Services and applications are discussed in Using the Twisted Application Framework .
By the end of this section of the tutorial, our finger server will answer TCP finger requests on port 1079, and will read data from the web.
Refuse Connections¶
from twisted.internet import reactor
reactor.run()
This example only runs the reactor. It will consume almost no CPU
resources. As it is not listening on any port, it can’t respond to network
requests — nothing at all will happen until we interrupt the program. At
this point if you run finger nail or telnet localhost 1079 , you’ll get a “Connection refused” error since there’s no daemon
running to respond. Not very useful, perhaps — but this is the skeleton
inside which the Twisted program will grow.
As implied above, at various points in this tutorial you’ll want to
observe the behavior of the server being developed. Unless you have a
finger program which can use an alternate port, the easiest way to do this
is with a telnet client. telnet localhost 1079 will connect to
the local host on port 1079, where a finger server will eventually be
listening.
You don’t call Twisted, Twisted calls you. The reactor is Twisted’s main event loop, similar to the main loop in other toolkits available in Python (Qt, wx, and Gtk). There is exactly one reactor in any running Twisted application. Once started it loops over and over again, responding to network events and making scheduled calls to code.
Note that there are actually several different reactors to choose
from; from twisted.internet import reactor returns the
current reactor. If you haven’t chosen a reactor class yet, it
automatically chooses the default. See
the Reactor Basics HOWTO for
more information.
Do Nothing¶
from twisted.internet import protocol, reactor
class FingerProtocol(protocol.Protocol):
pass
class FingerFactory(protocol.ServerFactory):
protocol = FingerProtocol
reactor.listenTCP(1079, FingerFactory())
reactor.run()
Here, reactor.listenTCP opens port 1079. (The number 1079 is a
reminder that eventually we want to run on port 79, the standard port for
finger servers.) The specified factory, FingerFactory , is used to
handle incoming requests on that port. Specifically, for each request, the
reactor calls the factory’s buildProtocol method, which in this
case causes FingerProtocol to be instantiated. Since the protocol
defined here does not actually respond to any events, connections to 1079 will
be accepted, but the input ignored.
A Factory is the proper place for data that you want to make available to the protocol instances, since the protocol instances are garbage collected when the connection is closed.
Drop Connections¶
from twisted.internet import protocol, reactor
class FingerProtocol(protocol.Protocol):
def connectionMade(self):
self.transport.loseConnection()
class FingerFactory(protocol.ServerFactory):
protocol = FingerProtocol
reactor.listenTCP(1079, FingerFactory())
reactor.run()
Here we add to the protocol the ability to respond to the event of beginning
a connection — by terminating it. Perhaps not an interesting behavior,
but it is already close to behaving according to the letter of the standard
finger protocol. After all, there is no requirement to send any data to the
remote connection in the standard. The only problem, as far as the standard is
concerned, is that we terminate the connection too soon. A client which is slow
enough will see his send() of the username result in an error.
Read Username, Drop Connections¶
from twisted.internet import protocol, reactor
from twisted.protocols import basic
class FingerProtocol(basic.LineReceiver):
def lineReceived(self, user):
self.transport.loseConnection()
class FingerFactory(protocol.ServerFactory):
protocol = FingerProtocol
reactor.listenTCP(1079, FingerFactory())
reactor.run()
Here we make FingerProtocol inherit from LineReceiver , so that we get data-based
events on a line-by-line basis. We respond to the event of receiving the line
with shutting down the connection.
If you use a telnet client to interact with this server, the result will look something like this:
$ telnet localhost 1079
Trying 127.0.0.1...
Connected to localhost.localdomain.
alice
Connection closed by foreign host.
Congratulations, this is the first standard-compliant version of the code. However, usually people actually expect some data about users to be transmitted.
Read Username, Output Error, Drop Connections¶
from twisted.internet import protocol, reactor
from twisted.protocols import basic
class FingerProtocol(basic.LineReceiver):
def lineReceived(self, user):
self.transport.write("No such user\r\n")
self.transport.loseConnection()
class FingerFactory(protocol.ServerFactory):
protocol = FingerProtocol
reactor.listenTCP(1079, FingerFactory())
reactor.run()
Finally, a useful version. Granted, the usefulness is somewhat limited by the fact that this version only prints out a “No such user” message. It could be used for devastating effect in honey-pots (decoy servers), of course.
Output From Empty Factory¶
# Read username, output from empty factory, drop connections
from twisted.internet import protocol, reactor
from twisted.protocols import basic
class FingerProtocol(basic.LineReceiver):
def lineReceived(self, user):
self.transport.write(self.factory.getUser(user)+"\r\n")
self.transport.loseConnection()
class FingerFactory(protocol.ServerFactory):
protocol = FingerProtocol
def getUser(self, user):
return "No such user"
reactor.listenTCP(1079, FingerFactory())
reactor.run()
The same behavior, but finally we see what usefulness the factory has: as something that does not get constructed for every connection, it can be in charge of the user database. In particular, we won’t have to change the protocol if the user database back-end changes.
Output from Non-empty Factory¶
# Read username, output from non-empty factory, drop connections
from twisted.internet import protocol, reactor
from twisted.protocols import basic
class FingerProtocol(basic.LineReceiver):
def lineReceived(self, user):
self.transport.write(self.factory.getUser(user)+"\r\n")
self.transport.loseConnection()
class FingerFactory(protocol.ServerFactory):
protocol = FingerProtocol
def __init__(self, **kwargs):
self.users = kwargs
def getUser(self, user):
return self.users.get(user, "No such user")
reactor.listenTCP(1079, FingerFactory(moshez='Happy and well'))
reactor.run()
Finally, a really useful finger database. While it does not supply information about logged in users, it could be used to distribute things like office locations and internal office numbers. As hinted above, the factory is in charge of keeping the user database: note that the protocol instance has not changed. This is starting to look good: we really won’t have to keep tweaking our protocol.
Use Deferreds¶
# Read username, output from non-empty factory, drop connections
# Use deferreds, to minimize synchronicity assumptions
from twisted.internet import protocol, reactor, defer
from twisted.protocols import basic
class FingerProtocol(basic.LineReceiver):
def lineReceived(self, user):
d = self.factory.getUser(user)
def onError(err):
return 'Internal error in server'
d.addErrback(onError)
def writeResponse(message):
self.transport.write(message + '\r\n')
self.transport.loseConnection()
d.addCallback(writeResponse)
class FingerFactory(protocol.ServerFactory):
protocol = FingerProtocol
def __init__(self, **kwargs):
self.users = kwargs
def getUser(self, user):
return defer.succeed(self.users.get(user, "No such user"))
reactor.listenTCP(1079, FingerFactory(moshez='Happy and well'))
reactor.run()
But, here we tweak it just for the hell of it. Yes, while the previous version worked, it did assume the result of getUser is always immediately available. But what if instead of an in-memory database, we would have to fetch the result from a remote Oracle server? By allowing getUser to return a Deferred, we make it easier for the data to be retrieved asynchronously so that the CPU can be used for other tasks in the meanwhile.
As described in the Deferred HOWTO , Deferreds allow a program to be driven by events. For instance, if one task in a program is waiting on data, rather than have the CPU (and the program!) idly waiting for that data (a process normally called ‘blocking’), the program can perform other operations in the meantime, and waits for some signal that data is ready to be processed before returning to that process.
In brief, the code in FingerFactory above creates a
Deferred, to which we start to attach callbacks . The
deferred action in FingerFactory is actually a
fast-running expression consisting of one dictionary
method, get . Since this action can execute without
delay, FingerFactory.getUser
uses defer.succeed to create a Deferred which already has
a result, meaning its return value will be passed immediately to the
first callback function, which turns out to
be FingerProtocol.writeResponse . We’ve also defined
an errback (appropriately
named FingerProtocol.onError ) that will be called instead
of writeResponse if something goes wrong.
Run ‘finger’ Locally¶
# Read username, output from factory interfacing to OS, drop connections
from twisted.internet import protocol, reactor, defer, utils
from twisted.protocols import basic
class FingerProtocol(basic.LineReceiver):
def lineReceived(self, user):
d = self.factory.getUser(user)
def onError(err):
return 'Internal error in server'
d.addErrback(onError)
def writeResponse(message):
self.transport.write(message + '\r\n')
self.transport.loseConnection()
d.addCallback(writeResponse)
class FingerFactory(protocol.ServerFactory):
protocol = FingerProtocol
def getUser(self, user):
return utils.getProcessOutput("finger", [user])
reactor.listenTCP(1079, FingerFactory())
reactor.run()
This example also makes use of a
Deferred. twisted.internet.utils.getProcessOutput is a
non-blocking version of Python’s commands.getoutput : it
runs a shell command (finger , in this case) and captures
its standard output. However, getProcessOutput returns a
Deferred instead of the output itself.
Since FingerProtocol.lineReceived is already expecting a
Deferred to be returned by getUser , it doesn’t need to be
changed, and it returns the standard output as the finger result.
Note that in this case the shell’s built-in finger command is
simply run with whatever arguments it is given. This is probably insecure, so
you probably don’t want a real server to do this without a lot more validation
of the user input. This will do exactly what the standard version of the finger
server does.
Read Status from the Web¶
The web. That invention which has infiltrated homes around the
world finally gets through to our invention. In this case we use the
built-in Twisted web client
via twisted.web.client.getPage , a non-blocking version of
Python’s urllib2.urlopen(URL).read .
Like getProcessOutput it returns a Deferred which will be
called back with a string, and can thus be used as a drop-in
replacement.
Thus, we have examples of three different database back-ends, none of which change the protocol class. In fact, we will not have to change the protocol again until the end of this tutorial: we have achieved, here, one truly usable class.
# Read username, output from factory interfacing to web, drop connections
from twisted.internet import protocol, reactor, defer, utils
from twisted.protocols import basic
from twisted.web import client
class FingerProtocol(basic.LineReceiver):
def lineReceived(self, user):
d = self.factory.getUser(user)
def onError(err):
return 'Internal error in server'
d.addErrback(onError)
def writeResponse(message):
self.transport.write(message + '\r\n')
self.transport.loseConnection()
d.addCallback(writeResponse)
class FingerFactory(protocol.ServerFactory):
protocol = FingerProtocol
def __init__(self, prefix):
self.prefix=prefix
def getUser(self, user):
return client.getPage(self.prefix+user)
reactor.listenTCP(1079, FingerFactory(prefix='http://livejournal.com/~'))
reactor.run()
Use Application¶
Up until now, we faked. We kept using port 1079, because really, who wants to
run a finger server with root privileges? Well, the common solution
is “privilege shedding” : after binding to the network, become a different,
less privileged user. We could have done it ourselves, but Twisted has a
built-in way to do it. We will create a snippet as above, but now we will define
an application object. That object will have uid
and gid attributes. When running it (later we will see how) it will
bind to ports, shed privileges and then run.
Read on to find out how to run this code using the twistd utility.
twistd¶
This is how to run “Twisted Applications” — files which define an ‘application’. A daemon is expected to adhere to certain behavioral standards so that standard tools can stop/start/query them. If a Twisted application is run via twistd, the TWISTed Daemonizer, all this behavioral stuff will be handled for you. twistd does everything a daemon can be expected to — shuts down stdin/stdout/stderr, disconnects from the terminal and can even change runtime directory, or even the root filesystems. In short, it does everything so the Twisted application developer can concentrate on writing his networking code.
root% twistd -ny finger11.tac # just like before
root% twistd -y finger11.tac # daemonize, keep pid in twistd.pid
root% twistd -y finger11.tac --pidfile=finger.pid
root% twistd -y finger11.tac --rundir=/
root% twistd -y finger11.tac --chroot=/var
root% twistd -y finger11.tac -l /var/log/finger.log
root% twistd -y finger11.tac --syslog # just log to syslog
root% twistd -y finger11.tac --syslog --prefix=twistedfinger # use given prefix
There are several ways to tell twistd where your application is; here we
show how it is done using the application global variable in a
Python source file (a Twisted Application
Configuration file).
# Read username, output from non-empty factory, drop connections
# Use deferreds, to minimize synchronicity assumptions
# Write application. Save in 'finger.tpy'
from twisted.application import internet, service
from twisted.internet import protocol, reactor, defer
from twisted.protocols import basic
class FingerProtocol(basic.LineReceiver):
def lineReceived(self, user):
d = self.factory.getUser(user)
def onError(err):
return 'Internal error in server'
d.addErrback(onError)
def writeResponse(message):
self.transport.write(message + '\r\n')
self.transport.loseConnection()
d.addCallback(writeResponse)
class FingerFactory(protocol.ServerFactory):
protocol = FingerProtocol
def __init__(self, **kwargs):
self.users = kwargs
def getUser(self, user):
return defer.succeed(self.users.get(user, "No such user"))
application = service.Application('finger', uid=1, gid=1)
factory = FingerFactory(moshez='Happy and well')
internet.TCPServer(79, factory).setServiceParent(
service.IServiceCollection(application))
Instead of using reactor.listenTCP as in the above
examples, here we are using its application-aware
counterpart, internet.TCPServer . Notice that when it is
instantiated, the application object itself does not reference either
the protocol or the factory. Any services (such as TCPServer) which
have the application as their parent will be started when the
application is started by twistd. The application object is more
useful for returning an object that supports the IService , IServiceCollection , IProcess ,
and sob.IPersistable
interfaces with the given parameters; we’ll be seeing these in the
next part of the tutorial. As the parent of the TCPServer we opened,
the application lets us manage the TCPServer.
With the daemon running on the standard finger port, you can test it with
the standard finger command: finger moshez .
The Evolution of Finger: adding features to the finger service¶
Introduction¶
This is the second part of the Twisted tutorial Twisted from Scratch, or The Evolution of Finger .
In this section of the tutorial, our finger server will continue to sprout features: the ability for users to set finger announces, and using our finger service to send those announcements on the web, on IRC and over XML-RPC. Resources and XML-RPC are introduced in the Web Applications portion of the Twisted Web howto . More examples using twisted.words.protocols.irc can be found in Writing a TCP Client and the Twisted Words examples .
Setting Message By Local Users¶
Now that port 1079 is free, maybe we can use it with a different server, one which will let people set their messages. It does no access control, so anyone who can login to the machine can set any message. We assume this is the desired behavior in our case. Testing it can be done by simply:
% nc localhost 1079 # or telnet localhost 1079
moshez
Giving a tutorial now, sorry!
^D
# But let's try and fix setting away messages, shall we?
from twisted.application import internet, service
from twisted.internet import protocol, reactor, defer
from twisted.protocols import basic
class FingerProtocol(basic.LineReceiver):
def lineReceived(self, user):
d = self.factory.getUser(user)
def onError(err):
return 'Internal error in server'
d.addErrback(onError)
def writeResponse(message):
self.transport.write(message + '\r\n')
self.transport.loseConnection()
d.addCallback(writeResponse)
class FingerFactory(protocol.ServerFactory):
protocol = FingerProtocol
def __init__(self, **kwargs):
self.users = kwargs
def getUser(self, user):
return defer.succeed(self.users.get(user, "No such user"))
class FingerSetterProtocol(basic.LineReceiver):
def connectionMade(self):
self.lines = []
def lineReceived(self, line):
self.lines.append(line)
def connectionLost(self, reason):
user = self.lines[0]
status = self.lines[1]
self.factory.setUser(user, status)
class FingerSetterFactory(protocol.ServerFactory):
protocol = FingerSetterProtocol
def __init__(self, fingerFactory):
self.fingerFactory = fingerFactory
def setUser(self, user, status):
self.fingerFactory.users[user] = status
ff = FingerFactory(moshez='Happy and well')
fsf = FingerSetterFactory(ff)
application = service.Application('finger', uid=1, gid=1)
serviceCollection = service.IServiceCollection(application)
internet.TCPServer(79,ff).setServiceParent(serviceCollection)
internet.TCPServer(1079,fsf).setServiceParent(serviceCollection)
This program has two protocol-factory-TCPServer pairs, which are
both child services of the application. Specifically,
the setServiceParent
method is used to define the two TCPServer services as children
of application , which implements IServiceCollection . Both
services are thus started with the application.
Use Services to Make Dependencies Sane¶
The previous version had the setter poke at the innards of the finger factory. This strategy is usually not a good idea: this version makes both factories symmetric by making them both look at a single object. Services are useful for when an object is needed which is not related to a specific network server. Here, we define a common service class with methods that will create factories on the fly. The service also contains methods the factories will depend on.
The factory-creation methods, getFingerFactory
and getFingerSetterFactory , follow this pattern:
- Instantiate a generic server
factory,
twisted.internet.protocol.ServerFactory. - Set the protocol class, just like our factory class would have.
- Copy a service method to the factory as a function attribute. The
function won’t have access to the factory’s
self, but that’s OK because as a bound method it has access to the service’sself, which is what it needs. ForgetUser, a custom method defined in the service gets copied. ForsetUser, a standard method of theusersdictionary is copied.
Thus, we stopped subclassing: the service simply puts useful methods and attributes inside the factories. We are getting better at protocol design: none of our protocol classes had to be changed, and neither will have to change until the end of the tutorial.
As an application service, this new finger service implements the IService interface and can be started and stopped in a standardized manner. We’ll make use of this in the next example.
# Fix asymmetry
from twisted.application import internet, service
from twisted.internet import protocol, reactor, defer
from twisted.protocols import basic
class FingerProtocol(basic.LineReceiver):
def lineReceived(self, user):
d = self.factory.getUser(user)
def onError(err):
return 'Internal error in server'
d.addErrback(onError)
def writeResponse(message):
self.transport.write(message + '\r\n')
self.transport.loseConnection()
d.addCallback(writeResponse)
class FingerSetterProtocol(basic.LineReceiver):
def connectionMade(self):
self.lines = []
def lineReceived(self, line):
self.lines.append(line)
def connectionLost(self,reason):
user = self.lines[0]
status = self.lines[1]
self.factory.setUser(user, status)
class FingerService(service.Service):
def __init__(self, **kwargs):
self.users = kwargs
def getUser(self, user):
return defer.succeed(self.users.get(user, "No such user"))
def setUser(self, user, status):
self.users[user] = status
def getFingerFactory(self):
f = protocol.ServerFactory()
f.protocol = FingerProtocol
f.getUser = self.getUser
return f
def getFingerSetterFactory(self):
f = protocol.ServerFactory()
f.protocol = FingerSetterProtocol
f.setUser = self.setUser
return f
application = service.Application('finger', uid=1, gid=1)
f = FingerService(moshez='Happy and well')
serviceCollection = service.IServiceCollection(application)
internet.TCPServer(79,f.getFingerFactory()
).setServiceParent(serviceCollection)
internet.TCPServer(1079,f.getFingerSetterFactory()
).setServiceParent(serviceCollection)
Most application services will want to use the Service base class, which implements
all the generic IService behavior.
Read Status File¶
This version shows how, instead of just letting users set their messages, we can read those from a centrally managed file. We cache results, and every 30 seconds we refresh it. Services are useful for such scheduled tasks.
listings/finger/etc.users
# Read from file
from twisted.application import internet, service
from twisted.internet import protocol, reactor, defer
from twisted.protocols import basic
class FingerProtocol(basic.LineReceiver):
def lineReceived(self, user):
d = self.factory.getUser(user)
def onError(err):
return 'Internal error in server'
d.addErrback(onError)
def writeResponse(message):
self.transport.write(message + '\r\n')
self.transport.loseConnection()
d.addCallback(writeResponse)
class FingerService(service.Service):
def __init__(self, filename):
self.users = {}
self.filename = filename
def _read(self):
for line in file(self.filename):
user, status = line.split(':', 1)
user = user.strip()
status = status.strip()
self.users[user] = status
self.call = reactor.callLater(30, self._read)
def startService(self):
self._read()
service.Service.startService(self)
def stopService(self):
service.Service.stopService(self)
self.call.cancel()
def getUser(self, user):
return defer.succeed(self.users.get(user, "No such user"))
def getFingerFactory(self):
f = protocol.ServerFactory()
f.protocol = FingerProtocol
f.getUser = self.getUser
return f
application = service.Application('finger', uid=1, gid=1)
f = FingerService('/etc/users')
finger = internet.TCPServer(79, f.getFingerFactory())
finger.setServiceParent(service.IServiceCollection(application))
f.setServiceParent(service.IServiceCollection(application))
Since this version is reading data from a file (and refreshing the data
every 30 seconds), there is no FingerSetterFactory and thus
nothing listening on port 1079.
Here we override the standard startService
and stopService hooks in
the Finger service, which is set up as a child service of the
application in the last line of the code. startService
calls _read , the function responsible for reading the
data; reactor.callLater is then used to schedule it to
run again after thirty seconds every time it is
called. reactor.callLater returns an object that lets us
cancel the scheduled run in stopService using
its cancel method.
Announce on Web, Too¶
The same kind of service can also produce things useful for other
protocols. For example, in twisted.web, the factory itself
(Site ) is almost
never subclassed — instead, it is given a resource, which
represents the tree of resources available via URLs. That hierarchy is
navigated by Site
and overriding it dynamically is possible with getChild .
To integrate this into the Finger application (just because we can), we set
up a new TCPServer that calls the Site factory and retrieves resources via a
new function of FingerService named getResource .
This function specifically returns a Resource object with an overridden getChild method.
# Read from file, announce on the web!
from twisted.application import internet, service
from twisted.internet import protocol, reactor, defer
from twisted.protocols import basic
from twisted.web import resource, server, static
import cgi
class FingerProtocol(basic.LineReceiver):
def lineReceived(self, user):
d = self.factory.getUser(user)
def onError(err):
return 'Internal error in server'
d.addErrback(onError)
def writeResponse(message):
self.transport.write(message + '\r\n')
self.transport.loseConnection()
d.addCallback(writeResponse)
class FingerResource(resource.Resource):
def __init__(self, users):
self.users = users
resource.Resource.__init__(self)
# we treat the path as the username
def getChild(self, username, request):
"""
'username' is a string.
'request' is a 'twisted.web.server.Request'.
"""
messagevalue = self.users.get(username)
username = cgi.escape(username)
if messagevalue is not None:
messagevalue = cgi.escape(messagevalue)
text = '<h1>%s</h1><p>%s</p>' % (username,messagevalue)
else:
text = '<h1>%s</h1><p>No such user</p>' % username
return static.Data(text, 'text/html')
class FingerService(service.Service):
def __init__(self, filename):
self.filename = filename
self.users = {}
def _read(self):
self.users.clear()
for line in file(self.filename):
user, status = line.split(':', 1)
user = user.strip()
status = status.strip()
self.users[user] = status
self.call = reactor.callLater(30, self._read)
def getUser(self, user):
return defer.succeed(self.users.get(user, "No such user"))
def getFingerFactory(self):
f = protocol.ServerFactory()
f.protocol = FingerProtocol
f.getUser = self.getUser
return f
def getResource(self):
r = FingerResource(self.users)
return r
def startService(self):
self._read()
service.Service.startService(self)
def stopService(self):
service.Service.stopService(self)
self.call.cancel()
application = service.Application('finger', uid=1, gid=1)
f = FingerService('/etc/users')
serviceCollection = service.IServiceCollection(application)
f.setServiceParent(serviceCollection)
internet.TCPServer(79, f.getFingerFactory()
).setServiceParent(serviceCollection)
internet.TCPServer(8000, server.Site(f.getResource())
).setServiceParent(serviceCollection)
Announce on IRC, Too¶
This is the first time there is client code. IRC clients often act a lot like
servers: responding to events from the network. The reconnecting client factory
will make sure that severed links will get re-established, with intelligent
tweaked exponential back-off algorithms. The IRC client itself is simple: the
only real hack is getting the nickname from the factory
in connectionMade .
# Read from file, announce on the web, irc
from twisted.application import internet, service
from twisted.internet import protocol, reactor, defer
from twisted.words.protocols import irc
from twisted.protocols import basic
from twisted.web import resource, server, static
import cgi
class FingerProtocol(basic.LineReceiver):
def lineReceived(self, user):
d = self.factory.getUser(user)
def onError(err):
return 'Internal error in server'
d.addErrback(onError)
def writeResponse(message):
self.transport.write(message + '\r\n')
self.transport.loseConnection()
d.addCallback(writeResponse)
class IRCReplyBot(irc.IRCClient):
def connectionMade(self):
self.nickname = self.factory.nickname
irc.IRCClient.connectionMade(self)
def privmsg(self, user, channel, msg):
user = user.split('!')[0]
if self.nickname.lower() == channel.lower():
d = self.factory.getUser(msg)
def onError(err):
return 'Internal error in server'
d.addErrback(onError)
def writeResponse(message):
irc.IRCClient.msg(self, user, msg+': '+message)
d.addCallback(writeResponse)
class FingerService(service.Service):
def __init__(self, filename):
self.filename = filename
self.users = {}
def _read(self):
self.users.clear()
for line in file(self.filename):
user, status = line.split(':', 1)
user = user.strip()
status = status.strip()
self.users[user] = status
self.call = reactor.callLater(30, self._read)
def getUser(self, user):
return defer.succeed(self.users.get(user, "No such user"))
def getFingerFactory(self):
f = protocol.ServerFactory()
f.protocol = FingerProtocol
f.getUser = self.getUser
return f
def getResource(self):
r = resource.Resource()
r.getChild = (lambda path, request:
static.Data('<h1>%s</h1><p>%s</p>' %
tuple(map(cgi.escape,
[path,self.users.get(path,
"No such user <p/> usage: site/user")])),
'text/html'))
return r
def getIRCBot(self, nickname):
f = protocol.ReconnectingClientFactory()
f.protocol = IRCReplyBot
f.nickname = nickname
f.getUser = self.getUser
return f
def startService(self):
self._read()
service.Service.startService(self)
def stopService(self):
service.Service.stopService(self)
self.call.cancel()
application = service.Application('finger', uid=1, gid=1)
f = FingerService('/etc/users')
serviceCollection = service.IServiceCollection(application)
f.setServiceParent(serviceCollection)
internet.TCPServer(79, f.getFingerFactory()
).setServiceParent(serviceCollection)
internet.TCPServer(8000, server.Site(f.getResource())
).setServiceParent(serviceCollection)
internet.TCPClient('irc.freenode.org', 6667, f.getIRCBot('fingerbot')
).setServiceParent(serviceCollection)
FingerService now has another new
function, getIRCbot , which returns
the ReconnectingClientFactory . This factory in turn will
instantiate the IRCReplyBot protocol. The IRCBot is
configured in the last line to connect
to irc.freenode.org with a nickname
of fingerbot .
By
overriding irc.IRCClient.connectionMade , IRCReplyBot
can access the nickname attribute of the factory that
instantiated it.
Add XML-RPC Support¶
In Twisted, XML-RPC support is handled just as though it was another resource. That resource will still support GET calls normally through render(), but that is usually left unimplemented. Note that it is possible to return deferreds from XML-RPC methods. The client, of course, will not get the answer until the deferred is triggered.
# Read from file, announce on the web, irc, xml-rpc
from twisted.application import internet, service
from twisted.internet import protocol, reactor, defer
from twisted.words.protocols import irc
from twisted.protocols import basic
from twisted.web import resource, server, static, xmlrpc
import cgi
class FingerProtocol(basic.LineReceiver):
def lineReceived(self, user):
d = self.factory.getUser(user)
def onError(err):
return 'Internal error in server'
d.addErrback(onError)
def writeResponse(message):
self.transport.write(message + '\r\n')
self.transport.loseConnection()
d.addCallback(writeResponse)
class IRCReplyBot(irc.IRCClient):
def connectionMade(self):
self.nickname = self.factory.nickname
irc.IRCClient.connectionMade(self)
def privmsg(self, user, channel, msg):
user = user.split('!')[0]
if self.nickname.lower() == channel.lower():
d = self.factory.getUser(msg)
def onError(err):
return 'Internal error in server'
d.addErrback(onError)
def writeResponse(message):
irc.IRCClient.msg(self, user, msg+': '+message)
d.addCallback(writeResponse)
class FingerService(service.Service):
def __init__(self, filename):
self.filename = filename
self.users = {}
def _read(self):
self.users.clear()
for line in file(self.filename):
user, status = line.split(':', 1)
user = user.strip()
status = status.strip()
self.users[user] = status
self.call = reactor.callLater(30, self._read)
def getUser(self, user):
return defer.succeed(self.users.get(user, "No such user"))
def getFingerFactory(self):
f = protocol.ServerFactory()
f.protocol = FingerProtocol
f.getUser = self.getUser
return f
def getResource(self):
r = resource.Resource()
r.getChild = (lambda path, request:
static.Data('<h1>%s</h1><p>%s</p>' %
tuple(map(cgi.escape,
[path,self.users.get(path, "No such user")])),
'text/html'))
x = xmlrpc.XMLRPC()
x.xmlrpc_getUser = self.getUser
r.putChild('RPC2', x)
return r
def getIRCBot(self, nickname):
f = protocol.ReconnectingClientFactory()
f.protocol = IRCReplyBot
f.nickname = nickname
f.getUser = self.getUser
return f
def startService(self):
self._read()
service.Service.startService(self)
def stopService(self):
service.Service.stopService(self)
self.call.cancel()
application = service.Application('finger', uid=1, gid=1)
f = FingerService('/etc/users')
serviceCollection = service.IServiceCollection(application)
f.setServiceParent(serviceCollection)
internet.TCPServer(79, f.getFingerFactory()
).setServiceParent(serviceCollection)
internet.TCPServer(8000, server.Site(f.getResource())
).setServiceParent(serviceCollection)
internet.TCPClient('irc.freenode.org', 6667, f.getIRCBot('fingerbot')
).setServiceParent(serviceCollection)
Instead of a web browser, we can test the XMLRPC finger using a simple
client based on Python’s built-in xmlrpclib , which will access
the resource we’ve made available at localhost/RPC2 .
# testing xmlrpc finger
import xmlrpclib
server = xmlrpclib.Server('http://127.0.0.1:8000/RPC2')
print server.getUser('moshez')
The Evolution of Finger: cleaning up the finger code¶
Introduction¶
This is the third part of the Twisted tutorial Twisted from Scratch, or The Evolution of Finger .
In this section of the tutorial, we’ll clean up our code so that it is closer to a readable and extensible style.
Write Readable Code¶
The last version of the application had a lot of hacks. We avoided sub-classing, didn’t support things like user listings over the web, and removed all blank lines – all in the interest of code which is shorter. Here we take a step back, subclass what is more naturally a subclass, make things which should take multiple lines take them, etc. This shows a much better style of developing Twisted applications, though the hacks in the previous stages are sometimes used in throw-away prototypes.
# Do everything properly
from twisted.application import internet, service
from twisted.internet import protocol, reactor, defer
from twisted.words.protocols import irc
from twisted.protocols import basic
from twisted.web import resource, server, static, xmlrpc
import cgi
def catchError(err):
return "Internal error in server"
class FingerProtocol(basic.LineReceiver):
def lineReceived(self, user):
d = self.factory.getUser(user)
d.addErrback(catchError)
def writeValue(value):
self.transport.write(value+'\r\n')
self.transport.loseConnection()
d.addCallback(writeValue)
class IRCReplyBot(irc.IRCClient):
def connectionMade(self):
self.nickname = self.factory.nickname
irc.IRCClient.connectionMade(self)
def privmsg(self, user, channel, msg):
user = user.split('!')[0]
if self.nickname.lower() == channel.lower():
d = self.factory.getUser(msg)
d.addErrback(catchError)
d.addCallback(lambda m: "Status of %s: %s" % (msg, m))
d.addCallback(lambda m: self.msg(user, m))
class UserStatusTree(resource.Resource):
def __init__(self, service):
resource.Resource.__init__(self)
self.service = service
def render_GET(self, request):
d = self.service.getUsers()
def formatUsers(users):
l = ['<li><a href="%s">%s</a></li>' % (user, user)
for user in users]
return '<ul>'+''.join(l)+'</ul>'
d.addCallback(formatUsers)
d.addCallback(request.write)
d.addCallback(lambda _: request.finish())
return server.NOT_DONE_YET
def getChild(self, path, request):
if path=="":
return UserStatusTree(self.service)
else:
return UserStatus(path, self.service)
class UserStatus(resource.Resource):
def __init__(self, user, service):
resource.Resource.__init__(self)
self.user = user
self.service = service
def render_GET(self, request):
d = self.service.getUser(self.user)
d.addCallback(cgi.escape)
d.addCallback(lambda m:
'<h1>%s</h1>'%self.user+'<p>%s</p>'%m)
d.addCallback(request.write)
d.addCallback(lambda _: request.finish())
return server.NOT_DONE_YET
class UserStatusXR(xmlrpc.XMLRPC):
def __init__(self, service):
xmlrpc.XMLRPC.__init__(self)
self.service = service
def xmlrpc_getUser(self, user):
return self.service.getUser(user)
class FingerService(service.Service):
def __init__(self, filename):
self.filename = filename
self.users = {}
def _read(self):
self.users.clear()
for line in file(self.filename):
user, status = line.split(':', 1)
user = user.strip()
status = status.strip()
self.users[user] = status
self.call = reactor.callLater(30, self._read)
def getUser(self, user):
return defer.succeed(self.users.get(user, "No such user"))
def getUsers(self):
return defer.succeed(self.users.keys())
def getFingerFactory(self):
f = protocol.ServerFactory()
f.protocol = FingerProtocol
f.getUser = self.getUser
return f
def getResource(self):
r = UserStatusTree(self)
x = UserStatusXR(self)
r.putChild('RPC2', x)
return r
def getIRCBot(self, nickname):
f = protocol.ReconnectingClientFactory()
f.protocol = IRCReplyBot
f.nickname = nickname
f.getUser = self.getUser
return f
def startService(self):
self._read()
service.Service.startService(self)
def stopService(self):
service.Service.stopService(self)
self.call.cancel()
application = service.Application('finger', uid=1, gid=1)
f = FingerService('/etc/users')
serviceCollection = service.IServiceCollection(application)
f.setServiceParent(serviceCollection)
internet.TCPServer(79, f.getFingerFactory()
).setServiceParent(serviceCollection)
internet.TCPServer(8000, server.Site(f.getResource())
).setServiceParent(serviceCollection)
internet.TCPClient('irc.freenode.org', 6667, f.getIRCBot('fingerbot')
).setServiceParent(serviceCollection)
The Evolution of Finger: moving to a component based architecture¶
Introduction¶
This is the fourth part of the Twisted tutorial Twisted from Scratch, or The Evolution of Finger .
In this section of the tutorial, we’ll move our code to a component architecture so that adding new features is trivial. See Interfaces and Adapters for a more complete discussion of components.
Write Maintainable Code¶
In the last version, the service class was three times longer than any other
class, and was hard to understand. This was because it turned out to have
multiple responsibilities. It had to know how to access user information, by
rereading the file every half minute, but also how to display itself in a myriad
of protocols. Here, we used the component-based architecture that Twisted
provides to achieve a separation of concerns. All the service is responsible
for, now, is supporting getUser /getUsers . It declares
its support via a call to zope.interface.implements . Then, adapters
are used to make this service look like an appropriate class for various things:
for supplying a finger factory to TCPServer , for supplying a
resource to site’s constructor, and to provide an IRC client factory
for TCPClient . All the adapters use are the methods
in FingerService they are declared to use:getUser /getUsers . We could, of course, skip the
interfaces and let the configuration code use things
like FingerFactoryFromService(f) directly. However, using
interfaces provides the same flexibility inheritance gives: future subclasses
can override the adapters.
# Do everything properly, and componentize
from twisted.application import internet, service
from twisted.internet import protocol, reactor, defer
from twisted.words.protocols import irc
from twisted.protocols import basic
from twisted.python import components
from twisted.web import resource, server, static, xmlrpc
from zope.interface import Interface, implements
import cgi
class IFingerService(Interface):
def getUser(user):
"""
Return a deferred returning a string.
"""
def getUsers():
"""
Return a deferred returning a list of strings.
"""
class IFingerSetterService(Interface):
def setUser(user, status):
"""
Set the user's status to something.
"""
def catchError(err):
return "Internal error in server"
class FingerProtocol(basic.LineReceiver):
def lineReceived(self, user):
d = self.factory.getUser(user)
d.addErrback(catchError)
def writeValue(value):
self.transport.write(value+'\r\n')
self.transport.loseConnection()
d.addCallback(writeValue)
class IFingerFactory(Interface):
def getUser(user):
"""
Return a deferred returning a string.
"""
def buildProtocol(addr):
"""
Return a protocol returning a string.
"""
class FingerFactoryFromService(protocol.ServerFactory):
implements(IFingerFactory)
protocol = FingerProtocol
def __init__(self, service):
self.service = service
def getUser(self, user):
return self.service.getUser(user)
components.registerAdapter(FingerFactoryFromService,
IFingerService,
IFingerFactory)
class FingerSetterProtocol(basic.LineReceiver):
def connectionMade(self):
self.lines = []
def lineReceived(self, line):
self.lines.append(line)
def connectionLost(self, reason):
if len(self.lines) == 2:
self.factory.setUser(*self.lines)
class IFingerSetterFactory(Interface):
def setUser(user, status):
"""
Return a deferred returning a string.
"""
def buildProtocol(addr):
"""
Return a protocol returning a string.
"""
class FingerSetterFactoryFromService(protocol.ServerFactory):
implements(IFingerSetterFactory)
protocol = FingerSetterProtocol
def __init__(self, service):
self.service = service
def setUser(self, user, status):
self.service.setUser(user, status)
components.registerAdapter(FingerSetterFactoryFromService,
IFingerSetterService,
IFingerSetterFactory)
class IRCReplyBot(irc.IRCClient):
def connectionMade(self):
self.nickname = self.factory.nickname
irc.IRCClient.connectionMade(self)
def privmsg(self, user, channel, msg):
user = user.split('!')[0]
if self.nickname.lower() == channel.lower():
d = self.factory.getUser(msg)
d.addErrback(catchError)
d.addCallback(lambda m: "Status of %s: %s" % (msg, m))
d.addCallback(lambda m: self.msg(user, m))
class IIRCClientFactory(Interface):
"""
@ivar nickname
"""
def getUser(user):
"""
Return a deferred returning a string.
"""
def buildProtocol(addr):
"""
Return a protocol.
"""
class IRCClientFactoryFromService(protocol.ClientFactory):
implements(IIRCClientFactory)
protocol = IRCReplyBot
nickname = None
def __init__(self, service):
self.service = service
def getUser(self, user):
return self.service.getUser(user)
components.registerAdapter(IRCClientFactoryFromService,
IFingerService,
IIRCClientFactory)
class UserStatusTree(resource.Resource):
implements(resource.IResource)
def __init__(self, service):
resource.Resource.__init__(self)
self.service = service
self.putChild('RPC2', UserStatusXR(self.service))
def render_GET(self, request):
d = self.service.getUsers()
def formatUsers(users):
l = ['<li><a href="%s">%s</a></li>' % (user, user)
for user in users]
return '<ul>'+''.join(l)+'</ul>'
d.addCallback(formatUsers)
d.addCallback(request.write)
d.addCallback(lambda _: request.finish())
return server.NOT_DONE_YET
def getChild(self, path, request):
if path=="":
return UserStatusTree(self.service)
else:
return UserStatus(path, self.service)
components.registerAdapter(UserStatusTree, IFingerService,
resource.IResource)
class UserStatus(resource.Resource):
def __init__(self, user, service):
resource.Resource.__init__(self)
self.user = user
self.service = service
def render_GET(self, request):
d = self.service.getUser(self.user)
d.addCallback(cgi.escape)
d.addCallback(lambda m:
'<h1>%s</h1>'%self.user+'<p>%s</p>'%m)
d.addCallback(request.write)
d.addCallback(lambda _: request.finish())
return server.NOT_DONE_YET
class UserStatusXR(xmlrpc.XMLRPC):
def __init__(self, service):
xmlrpc.XMLRPC.__init__(self)
self.service = service
def xmlrpc_getUser(self, user):
return self.service.getUser(user)
class FingerService(service.Service):
implements(IFingerService)
def __init__(self, filename):
self.filename = filename
self.users = {}
def _read(self):
self.users.clear()
for line in file(self.filename):
user, status = line.split(':', 1)
user = user.strip()
status = status.strip()
self.users[user] = status
self.call = reactor.callLater(30, self._read)
def getUser(self, user):
return defer.succeed(self.users.get(user, "No such user"))
def getUsers(self):
return defer.succeed(self.users.keys())
def startService(self):
self._read()
service.Service.startService(self)
def stopService(self):
service.Service.stopService(self)
self.call.cancel()
application = service.Application('finger', uid=1, gid=1)
f = FingerService('/etc/users')
serviceCollection = service.IServiceCollection(application)
f.setServiceParent(serviceCollection)
internet.TCPServer(79, IFingerFactory(f)
).setServiceParent(serviceCollection)
internet.TCPServer(8000, server.Site(resource.IResource(f))
).setServiceParent(serviceCollection)
i = IIRCClientFactory(f)
i.nickname = 'fingerbot'
internet.TCPClient('irc.freenode.org', 6667, i
).setServiceParent(serviceCollection)
Advantages of Latest Version¶
- Readable – each class is short
- Maintainable – each class knows only about interfaces
- Dependencies between code parts are minimized
- Example: writing a new
IFingerServiceis easy
class IFingerSetterService(Interface):
def setUser(user, status):
"""Set the user's status to something"""
# Advantages of latest version
class MemoryFingerService(service.Service):
implements([IFingerService, IFingerSetterService])
def __init__(self, **kwargs):
self.users = kwargs
def getUser(self, user):
return defer.succeed(self.users.get(user, "No such user"))
def getUsers(self):
return defer.succeed(self.users.keys())
def setUser(self, user, status):
self.users[user] = status
f = MemoryFingerService(moshez='Happy and well')
serviceCollection = service.IServiceCollection(application)
internet.TCPServer(1079, IFingerSetterFactory(f), interface='127.0.0.1'
).setServiceParent(serviceCollection)
Full source code here:
# Do everything properly, and componentize
from twisted.application import internet, service
from twisted.internet import protocol, reactor, defer
from twisted.words.protocols import irc
from twisted.protocols import basic
from twisted.python import components
from twisted.web import resource, server, static, xmlrpc
from zope.interface import Interface, implements
import cgi
class IFingerService(Interface):
def getUser(user):
"""Return a deferred returning a string"""
def getUsers():
"""Return a deferred returning a list of strings"""
class IFingerSetterService(Interface):
def setUser(user, status):
"""Set the user's status to something"""
def catchError(err):
return "Internal error in server"
class FingerProtocol(basic.LineReceiver):
def lineReceived(self, user):
d = self.factory.getUser(user)
d.addErrback(catchError)
def writeValue(value):
self.transport.write(value+'\r\n')
self.transport.loseConnection()
d.addCallback(writeValue)
class IFingerFactory(Interface):
def getUser(user):
"""Return a deferred returning a string"""
def buildProtocol(addr):
"""Return a protocol returning a string"""
class FingerFactoryFromService(protocol.ServerFactory):
implements(IFingerFactory)
protocol = FingerProtocol
def __init__(self, service):
self.service = service
def getUser(self, user):
return self.service.getUser(user)
components.registerAdapter(FingerFactoryFromService,
IFingerService,
IFingerFactory)
class FingerSetterProtocol(basic.LineReceiver):
def connectionMade(self):
self.lines = []
def lineReceived(self, line):
self.lines.append(line)
def connectionLost(self, reason):
if len(self.lines) == 2:
self.factory.setUser(*self.lines)
class IFingerSetterFactory(Interface):
def setUser(user, status):
"""Return a deferred returning a string"""
def buildProtocol(addr):
"""Return a protocol returning a string"""
class FingerSetterFactoryFromService(protocol.ServerFactory):
implements(IFingerSetterFactory)
protocol = FingerSetterProtocol
def __init__(self, service):
self.service = service
def setUser(self, user, status):
self.service.setUser(user, status)
components.registerAdapter(FingerSetterFactoryFromService,
IFingerSetterService,
IFingerSetterFactory)
class IRCReplyBot(irc.IRCClient):
def connectionMade(self):
self.nickname = self.factory.nickname
irc.IRCClient.connectionMade(self)
def privmsg(self, user, channel, msg):
user = user.split('!')[0]
if self.nickname.lower() == channel.lower():
d = self.factory.getUser(msg)
d.addErrback(catchError)
d.addCallback(lambda m: "Status of %s: %s" % (msg, m))
d.addCallback(lambda m: self.msg(user, m))
class IIRCClientFactory(Interface):
"""
@ivar nickname
"""
def getUser(user):
"""Return a deferred returning a string"""
def buildProtocol(addr):
"""Return a protocol"""
class IRCClientFactoryFromService(protocol.ClientFactory):
implements(IIRCClientFactory)
protocol = IRCReplyBot
nickname = None
def __init__(self, service):
self.service = service
def getUser(self, user):
return self.service.getUser(user)
components.registerAdapter(IRCClientFactoryFromService,
IFingerService,
IIRCClientFactory)
class UserStatusTree(resource.Resource):
implements(resource.IResource)
def __init__(self, service):
resource.Resource.__init__(self)
self.service = service
self.putChild('RPC2', UserStatusXR(self.service))
def render_GET(self, request):
d = self.service.getUsers()
def formatUsers(users):
l = ['<li><a href="%s">%s</a></li>' % (user, user)
for user in users]
return '<ul>'+''.join(l)+'</ul>'
d.addCallback(formatUsers)
d.addCallback(request.write)
d.addCallback(lambda _: request.finish())
return server.NOT_DONE_YET
def getChild(self, path, request):
if path=="":
return UserStatusTree(self.service)
else:
return UserStatus(path, self.service)
components.registerAdapter(UserStatusTree, IFingerService,
resource.IResource)
class UserStatus(resource.Resource):
def __init__(self, user, service):
resource.Resource.__init__(self)
self.user = user
self.service = service
def render_GET(self, request):
d = self.service.getUser(self.user)
d.addCallback(cgi.escape)
d.addCallback(lambda m:
'<h1>%s</h1>'%self.user+'<p>%s</p>'%m)
d.addCallback(request.write)
d.addCallback(lambda _: request.finish())
return server.NOT_DONE_YET
class UserStatusXR(xmlrpc.XMLRPC):
def __init__(self, service):
xmlrpc.XMLRPC.__init__(self)
self.service = service
def xmlrpc_getUser(self, user):
return self.service.getUser(user)
class MemoryFingerService(service.Service):
implements([IFingerService, IFingerSetterService])
def __init__(self, **kwargs):
self.users = kwargs
def getUser(self, user):
return defer.succeed(self.users.get(user, "No such user"))
def getUsers(self):
return defer.succeed(self.users.keys())
def setUser(self, user, status):
self.users[user] = status
application = service.Application('finger', uid=1, gid=1)
f = MemoryFingerService(moshez='Happy and well')
serviceCollection = service.IServiceCollection(application)
internet.TCPServer(79, IFingerFactory(f)
).setServiceParent(serviceCollection)
internet.TCPServer(8000, server.Site(resource.IResource(f))
).setServiceParent(serviceCollection)
i = IIRCClientFactory(f)
i.nickname = 'fingerbot'
internet.TCPClient('irc.freenode.org', 6667, i
).setServiceParent(serviceCollection)
internet.TCPServer(1079, IFingerSetterFactory(f), interface='127.0.0.1'
).setServiceParent(serviceCollection)
Aspect-Oriented Programming¶
At last, an example of aspect-oriented programming that isn’t about logging or timing. This code is actually useful! Watch how aspect-oriented programming helps you write less code and have fewer dependencies!
The Evolution of Finger: pluggable backends¶
Introduction¶
This is the fifth part of the Twisted tutorial Twisted from Scratch, or The Evolution of Finger .
In this part we will add new several new backends to our finger service using
the component-based architecture developed in The Evolution of Finger: moving to a component based architecture . This will
show just how convenient it is to implement new back-ends when we move to a
component based architecture. Note that here we also use an interface we
previously wrote, FingerSetterFactory , by supporting one single
method. We manage to preserve the service’s ignorance of the network.
Another Back-end¶
from twisted.internet import protocol, reactor, defer, utils
import pwd
# Another back-end
class LocalFingerService(service.Service):
implements(IFingerService)
def getUser(self, user):
# need a local finger daemon running for this to work
return utils.getProcessOutput("finger", [user])
def getUsers(self):
return defer.succeed([])
f = LocalFingerService()
Full source code here:
# Do everything properly, and componentize
from twisted.application import internet, service
from twisted.internet import protocol, reactor, defer, utils
from twisted.words.protocols import irc
from twisted.protocols import basic
from twisted.python import components
from twisted.web import resource, server, static, xmlrpc
from zope.interface import Interface, implements
import cgi
import pwd
class IFingerService(Interface):
def getUser(user):
"""
Return a deferred returning a string.
"""
def getUsers():
"""
Return a deferred returning a list of strings.
"""
class IFingerSetterService(Interface):
def setUser(user, status):
"""
Set the user's status to something.
"""
class IFingerSetterService(Interface):
def setUser(user, status):
"""
Set the user's status to something.
"""
def catchError(err):
return "Internal error in server"
class FingerProtocol(basic.LineReceiver):
def lineReceived(self, user):
d = self.factory.getUser(user)
d.addErrback(catchError)
def writeValue(value):
self.transport.write(value+'\r\n')
self.transport.loseConnection()
d.addCallback(writeValue)
class IFingerFactory(Interface):
def getUser(user):
"""
Return a deferred returning a string.
"""
def buildProtocol(addr):
"""
Return a protocol returning a string.
"""
class FingerFactoryFromService(protocol.ServerFactory):
implements(IFingerFactory)
protocol = FingerProtocol
def __init__(self, service):
self.service = service
def getUser(self, user):
return self.service.getUser(user)
components.registerAdapter(FingerFactoryFromService,
IFingerService,
IFingerFactory)
class FingerSetterProtocol(basic.LineReceiver):
def connectionMade(self):
self.lines = []
def lineReceived(self, line):
self.lines.append(line)
def connectionLost(self, reason):
if len(self.lines) == 2:
self.factory.setUser(*self.lines)
class IFingerSetterFactory(Interface):
def setUser(user, status):
"""
Return a deferred returning a string.
"""
def buildProtocol(addr):
"""
Return a protocol returning a string.
"""
class FingerSetterFactoryFromService(protocol.ServerFactory):
implements(IFingerSetterFactory)
protocol = FingerSetterProtocol
def __init__(self, service):
self.service = service
def setUser(self, user, status):
self.service.setUser(user, status)
components.registerAdapter(FingerSetterFactoryFromService,
IFingerSetterService,
IFingerSetterFactory)
class IRCReplyBot(irc.IRCClient):
def connectionMade(self):
self.nickname = self.factory.nickname
irc.IRCClient.connectionMade(self)
def privmsg(self, user, channel, msg):
user = user.split('!')[0]
if self.nickname.lower() == channel.lower():
d = self.factory.getUser(msg)
d.addErrback(catchError)
d.addCallback(lambda m: "Status of %s: %s" % (msg, m))
d.addCallback(lambda m: self.msg(user, m))
class IIRCClientFactory(Interface):
"""
@ivar nickname
"""
def getUser(user):
"""
Return a deferred returning a string.
"""
def buildProtocol(addr):
"""
Return a protocol.
"""
class IRCClientFactoryFromService(protocol.ClientFactory):
implements(IIRCClientFactory)
protocol = IRCReplyBot
nickname = None
def __init__(self, service):
self.service = service
def getUser(self, user):
return self.service.getUser(user)
components.registerAdapter(IRCClientFactoryFromService,
IFingerService,
IIRCClientFactory)
class UserStatusTree(resource.Resource):
implements(resource.IResource)
def __init__(self, service):
resource.Resource.__init__(self)
self.service = service
self.putChild('RPC2', UserStatusXR(self.service))
def render_GET(self, request):
d = self.service.getUsers()
def formatUsers(users):
l = ['<li><a href="%s">%s</a></li>' % (user, user)
for user in users]
return '<ul>'+''.join(l)+'</ul>'
d.addCallback(formatUsers)
d.addCallback(request.write)
d.addCallback(lambda _: request.finish())
return server.NOT_DONE_YET
def getChild(self, path, request):
if path=="":
return UserStatusTree(self.service)
else:
return UserStatus(path, self.service)
components.registerAdapter(UserStatusTree, IFingerService,
resource.IResource)
class UserStatus(resource.Resource):
def __init__(self, user, service):
resource.Resource.__init__(self)
self.user = user
self.service = service
def render_GET(self, request):
d = self.service.getUser(self.user)
d.addCallback(cgi.escape)
d.addCallback(lambda m:
'<h1>%s</h1>'%self.user+'<p>%s</p>'%m)
d.addCallback(request.write)
d.addCallback(lambda _: request.finish())
return server.NOT_DONE_YET
class UserStatusXR(xmlrpc.XMLRPC):
def __init__(self, service):
xmlrpc.XMLRPC.__init__(self)
self.service = service
def xmlrpc_getUser(self, user):
return self.service.getUser(user)
class FingerService(service.Service):
implements(IFingerService)
def __init__(self, filename):
self.filename = filename
self.users = {}
def _read(self):
self.users.clear()
for line in file(self.filename):
user, status = line.split(':', 1)
user = user.strip()
status = status.strip()
self.users[user] = status
self.call = reactor.callLater(30, self._read)
def getUser(self, user):
return defer.succeed(self.users.get(user, "No such user"))
def getUsers(self):
return defer.succeed(self.users.keys())
def startService(self):
self._read()
service.Service.startService(self)
def stopService(self):
service.Service.stopService(self)
self.call.cancel()
# Another back-end
class LocalFingerService(service.Service):
implements(IFingerService)
def getUser(self, user):
# need a local finger daemon running for this to work
return utils.getProcessOutput("finger", [user])
def getUsers(self):
return defer.succeed([])
application = service.Application('finger', uid=1, gid=1)
f = LocalFingerService()
serviceCollection = service.IServiceCollection(application)
internet.TCPServer(79, IFingerFactory(f)
).setServiceParent(serviceCollection)
internet.TCPServer(8000, server.Site(resource.IResource(f))
).setServiceParent(serviceCollection)
i = IIRCClientFactory(f)
i.nickname = 'fingerbot'
internet.TCPClient('irc.freenode.org', 6667, i
).setServiceParent(serviceCollection)
We’ve already written this, but now we get more for less work: the network code is completely separate from the back-end.
Yet Another Back-end: Doing the Standard Thing¶
from twisted.internet import protocol, reactor, defer, utils
import pwd
import os
# Yet another back-end
class LocalFingerService(service.Service):
implements(IFingerService)
def getUser(self, user):
user = user.strip()
try:
entry = pwd.getpwnam(user)
except KeyError:
return defer.succeed("No such user")
try:
f = file(os.path.join(entry[5],'.plan'))
except (IOError, OSError):
return defer.succeed("No such user")
data = f.read()
data = data.strip()
f.close()
return defer.succeed(data)
def getUsers(self):
return defer.succeed([])
f = LocalFingerService()
Full source code here:
# Do everything properly, and componentize
from twisted.application import internet, service
from twisted.internet import protocol, reactor, defer, utils
from twisted.words.protocols import irc
from twisted.protocols import basic
from twisted.python import components
from twisted.web import resource, server, static, xmlrpc
from zope.interface import Interface, implements
import cgi
import pwd
import os
class IFingerService(Interface):
def getUser(user):
"""
Return a deferred returning a string.
"""
def getUsers():
"""
Return a deferred returning a list of strings.
"""
class IFingerSetterService(Interface):
def setUser(user, status):
"""
Set the user's status to something.
"""
class IFingerSetterService(Interface):
def setUser(user, status):
"""
Set the user's status to something.
"""
def catchError(err):
return "Internal error in server"
class FingerProtocol(basic.LineReceiver):
def lineReceived(self, user):
d = self.factory.getUser(user)
d.addErrback(catchError)
def writeValue(value):
self.transport.write(value+'\r\n')
self.transport.loseConnection()
d.addCallback(writeValue)
class IFingerFactory(Interface):
def getUser(user):
"""
Return a deferred returning a string.
"""
def buildProtocol(addr):
"""
Return a protocol returning a string.
"""
class FingerFactoryFromService(protocol.ServerFactory):
implements(IFingerFactory)
protocol = FingerProtocol
def __init__(self, service):
self.service = service
def getUser(self, user):
return self.service.getUser(user)
components.registerAdapter(FingerFactoryFromService,
IFingerService,
IFingerFactory)
class FingerSetterProtocol(basic.LineReceiver):
def connectionMade(self):
self.lines = []
def lineReceived(self, line):
self.lines.append(line)
def connectionLost(self, reason):
if len(self.lines) == 2:
self.factory.setUser(*self.lines)
class IFingerSetterFactory(Interface):
def setUser(user, status):
"""
Return a deferred returning a string.
"""
def buildProtocol(addr):
"""
Return a protocol returning a string.
"""
class FingerSetterFactoryFromService(protocol.ServerFactory):
implements(IFingerSetterFactory)
protocol = FingerSetterProtocol
def __init__(self, service):
self.service = service
def setUser(self, user, status):
self.service.setUser(user, status)
components.registerAdapter(FingerSetterFactoryFromService,
IFingerSetterService,
IFingerSetterFactory)
class IRCReplyBot(irc.IRCClient):
def connectionMade():
self.nickname = self.factory.nickname
irc.IRCClient.connectionMade(self)
def privmsg(self, user, channel, msg):
user = user.split('!')[0]
if self.nickname.lower() == channel.lower():
d = self.factory.getUser(msg)
d.addErrback(catchError)
d.addCallback(lambda m: "Status of %s: %s" % (msg, m))
d.addCallback(lambda m: self.msg(user, m))
class IIRCClientFactory(Interface):
"""
@ivar nickname
"""
def getUser(user):
"""
Return a deferred returning a string.
"""
def buildProtocol(addr):
"""
Return a protocol.
"""
class IRCClientFactoryFromService(protocol.ClientFactory):
implements(IIRCClientFactory)
protocol = IRCReplyBot
nickname = None
def __init__(self, service):
self.service = service
def getUser(self, user):
return self.service.getUser(user)
components.registerAdapter(IRCClientFactoryFromService,
IFingerService,
IIRCClientFactory)
class UserStatusTree(resource.Resource):
implements(resource.IResource)
def __init__(self, service):
resource.Resource.__init__(self)
self.service = service
self.putChild('RPC2', UserStatusXR(self.service))
def render_GET(self, request):
d = self.service.getUsers()
def formatUsers(users):
l = ['<li><a href="%s">%s</a></li>' % (user, user)
for user in users]
return '<ul>'+''.join(l)+'</ul>'
d.addCallback(formatUsers)
d.addCallback(request.write)
d.addCallback(lambda _: request.finish())
return server.NOT_DONE_YET
def getChild(self, path, request):
if path=="":
return UserStatusTree(self.service)
else:
return UserStatus(path, self.service)
components.registerAdapter(UserStatusTree, IFingerService,
resource.IResource)
class UserStatus(resource.Resource):
def __init__(self, user, service):
resource.Resource.__init__(self)
self.user = user
self.service = service
def render_GET(self, request):
d = self.service.getUser(self.user)
d.addCallback(cgi.escape)
d.addCallback(lambda m:
'<h1>%s</h1>'%self.user+'<p>%s</p>'%m)
d.addCallback(request.write)
d.addCallback(lambda _: request.finish())
return server.NOT_DONE_YET
class UserStatusXR(xmlrpc.XMLRPC):
def __init__(self, service):
xmlrpc.XMLRPC.__init__(self)
self.service = service
def xmlrpc_getUser(self, user):
return self.service.getUser(user)
class FingerService(service.Service):
implements(IFingerService)
def __init__(self, filename):
self.filename = filename
self.users = {}
def _read(self):
self.users.clear()
for line in file(self.filename):
user, status = line.split(':', 1)
user = user.strip()
status = status.strip()
self.users[user] = status
self.call = reactor.callLater(30, self._read)
def getUser(self, user):
return defer.succeed(self.users.get(user, "No such user"))
def getUsers(self):
return defer.succeed(self.users.keys())
def startService(self):
self._read()
service.Service.startService(self)
def stopService(self):
service.Service.stopService(self)
self.call.cancel()
# Yet another back-end
class LocalFingerService(service.Service):
implements(IFingerService)
def getUser(self, user):
user = user.strip()
try:
entry = pwd.getpwnam(user)
except KeyError:
return defer.succeed("No such user")
try:
f = file(os.path.join(entry[5],'.plan'))
except (IOError, OSError):
return defer.succeed("No such user")
data = f.read()
data = data.strip()
f.close()
return defer.succeed(data)
def getUsers(self):
return defer.succeed([])
application = service.Application('finger', uid=1, gid=1)
f = LocalFingerService()
serviceCollection = service.IServiceCollection(application)
internet.TCPServer(79, IFingerFactory(f)
).setServiceParent(serviceCollection)
internet.TCPServer(8000, server.Site(resource.IResource(f))
).setServiceParent(serviceCollection)
i = IIRCClientFactory(f)
i.nickname = 'fingerbot'
internet.TCPClient('irc.freenode.org', 6667, i
).setServiceParent(serviceCollection)
Not much to say except that now we can be churn out backends like crazy. Feel like doing a back-end for Advogato , for example? Dig out the XML-RPC client support Twisted has, and get to work!
The Evolution of Finger: a web frontend¶
Introduction¶
This is the sixth part of the Twisted tutorial Twisted from Scratch, or The Evolution of Finger .
In this part, we demonstrate adding a web frontend using
simple twisted.web.resource.Resource
objects: UserStatusTree , which will
produce a listing of all users at the base URL (/ ) of our
site; UserStatus , which gives the status
of each user at the location /username ;
and UserStatusXR , which exposes an XMLRPC
interface to getUser
and getUsers functions at the
URL /RPC2 .
In this example we construct HTML segments manually. If the web interface was less trivial, we would want to use more sophisticated web templating and design our system so that HTML rendering and logic were clearly separated.
# Do everything properly, and componentize
from twisted.application import internet, service
from twisted.internet import protocol, reactor, defer
from twisted.words.protocols import irc
from twisted.protocols import basic
from twisted.python import components
from twisted.web import resource, server, static, xmlrpc
from zope.interface import Interface, implements
import cgi
class IFingerService(Interface):
def getUser(user):
"""
Return a deferred returning a string.
"""
def getUsers():
"""
Return a deferred returning a list of strings.
"""
class IFingerSetterService(Interface):
def setUser(user, status):
"""
Set the user's status to something.
"""
def catchError(err):
return "Internal error in server"
class FingerProtocol(basic.LineReceiver):
def lineReceived(self, user):
d = self.factory.getUser(user)
d.addErrback(catchError)
def writeValue(value):
self.transport.write(value+'\r\n')
self.transport.loseConnection()
d.addCallback(writeValue)
class IFingerFactory(Interface):
def getUser(user):
"""
Return a deferred returning a string.
"""
def buildProtocol(addr):
"""
Return a protocol returning a string.
"""
class FingerFactoryFromService(protocol.ServerFactory):
implements(IFingerFactory)
protocol = FingerProtocol
def __init__(self, service):
self.service = service
def getUser(self, user):
return self.service.getUser(user)
components.registerAdapter(FingerFactoryFromService,
IFingerService,
IFingerFactory)
class FingerSetterProtocol(basic.LineReceiver):
def connectionMade(self):
self.lines = []
def lineReceived(self, line):
self.lines.append(line)
def connectionLost(self, reason):
if len(self.lines) == 2:
self.factory.setUser(*self.lines)
class IFingerSetterFactory(Interface):
def setUser(user, status):
"""
Return a deferred returning a string.
"""
def buildProtocol(addr):
"""
Return a protocol returning a string.
"""
class FingerSetterFactoryFromService(protocol.ServerFactory):
implements(IFingerSetterFactory)
protocol = FingerSetterProtocol
def __init__(self, service):
self.service = service
def setUser(self, user, status):
self.service.setUser(user, status)
components.registerAdapter(FingerSetterFactoryFromService,
IFingerSetterService,
IFingerSetterFactory)
class IRCReplyBot(irc.IRCClient):
def connectionMade(self):
self.nickname = self.factory.nickname
irc.IRCClient.connectionMade(self)
def privmsg(self, user, channel, msg):
user = user.split('!')[0]
if self.nickname.lower() == channel.lower():
d = self.factory.getUser(msg)
d.addErrback(catchError)
d.addCallback(lambda m: "Status of %s: %s" % (msg, m))
d.addCallback(lambda m: self.msg(user, m))
class IIRCClientFactory(Interface):
"""
@ivar nickname
"""
def getUser(user):
"""
Return a deferred returning a string.
"""
def buildProtocol(addr):
"""
Return a protocol.
"""
class IRCClientFactoryFromService(protocol.ClientFactory):
implements(IIRCClientFactory)
protocol = IRCReplyBot
nickname = None
def __init__(self, service):
self.service = service
def getUser(self, user):
return self.service.getUser(user)
components.registerAdapter(IRCClientFactoryFromService,
IFingerService,
IIRCClientFactory)
class UserStatusTree(resource.Resource):
def __init__(self, service):
resource.Resource.__init__(self)
self.service=service
# add a specific child for the path "RPC2"
self.putChild("RPC2", UserStatusXR(self.service))
# need to do this for resources at the root of the site
self.putChild("", self)
def _cb_render_GET(self, users, request):
userOutput = ''.join(["<li><a href=\"%s\">%s</a></li>" % (user, user)
for user in users])
request.write("""
<html><head><title>Users</title></head><body>
<h1>Users</h1>
<ul>
%s
</ul></body></html>""" % userOutput)
request.finish()
def render_GET(self, request):
d = self.service.getUsers()
d.addCallback(self._cb_render_GET, request)
# signal that the rendering is not complete
return server.NOT_DONE_YET
def getChild(self, path, request):
return UserStatus(user=path, service=self.service)
components.registerAdapter(UserStatusTree, IFingerService, resource.IResource)
class UserStatus(resource.Resource):
def __init__(self, user, service):
resource.Resource.__init__(self)
self.user = user
self.service = service
def _cb_render_GET(self, status, request):
request.write("""<html><head><title>%s</title></head>
<body><h1>%s</h1>
<p>%s</p>
</body></html>""" % (self.user, self.user, status))
request.finish()
def render_GET(self, request):
d = self.service.getUser(self.user)
d.addCallback(self._cb_render_GET, request)
# signal that the rendering is not complete
return server.NOT_DONE_YET
class UserStatusXR(xmlrpc.XMLRPC):
def __init__(self, service):
xmlrpc.XMLRPC.__init__(self)
self.service = service
def xmlrpc_getUser(self, user):
return self.service.getUser(user)
def xmlrpc_getUsers(self):
return self.service.getUsers()
class FingerService(service.Service):
implements(IFingerService)
def __init__(self, filename):
self.filename = filename
self.users = {}
def _read(self):
self.users.clear()
for line in file(self.filename):
user, status = line.split(':', 1)
user = user.strip()
status = status.strip()
self.users[user] = status
self.call = reactor.callLater(30, self._read)
def getUser(self, user):
return defer.succeed(self.users.get(user, "No such user"))
def getUsers(self):
return defer.succeed(self.users.keys())
def startService(self):
self._read()
service.Service.startService(self)
def stopService(self):
service.Service.stopService(self)
self.call.cancel()
application = service.Application('finger', uid=1, gid=1)
f = FingerService('/etc/users')
serviceCollection = service.IServiceCollection(application)
f.setServiceParent(serviceCollection)
internet.TCPServer(79, IFingerFactory(f)
).setServiceParent(serviceCollection)
internet.TCPServer(8000, server.Site(resource.IResource(f))
).setServiceParent(serviceCollection)
i = IIRCClientFactory(f)
i.nickname = 'fingerbot'
internet.TCPClient('irc.freenode.org', 6667, i
).setServiceParent(serviceCollection)
The Evolution of Finger: Twisted client support using Perspective Broker¶
Introduction¶
This is the seventh part of the Twisted tutorial Twisted from Scratch, or The Evolution of Finger .
In this part, we add a Perspective Broker service to the finger application so that Twisted clients can access the finger server. Perspective Broker is introduced in depth in its own section of the core howto index.
Use Perspective Broker¶
We add support for perspective broker, Twisted’s native remote object protocol. Now, Twisted clients will not have to go through XML-RPCish contortions to get information about users.
# Do everything properly, and componentize
from twisted.application import internet, service
from twisted.internet import protocol, reactor, defer
from twisted.words.protocols import irc
from twisted.protocols import basic
from twisted.python import components
from twisted.web import resource, server, static, xmlrpc
from twisted.spread import pb
from zope.interface import Interface, implements
import cgi
class IFingerService(Interface):
def getUser(user):
"""
Return a deferred returning a string.
"""
def getUsers():
"""
Return a deferred returning a list of strings.
"""
class IFingerSetterService(Interface):
def setUser(user, status):
"""
Set the user's status to something.
"""
def catchError(err):
return "Internal error in server"
class FingerProtocol(basic.LineReceiver):
def lineReceived(self, user):
d = self.factory.getUser(user)
d.addErrback(catchError)
def writeValue(value):
self.transport.write(value+'\r\n')
self.transport.loseConnection()
d.addCallback(writeValue)
class IFingerFactory(Interface):
def getUser(user):
"""
Return a deferred returning a string.
"""
def buildProtocol(addr):
"""
Return a protocol returning a string.
"""
class FingerFactoryFromService(protocol.ServerFactory):
implements(IFingerFactory)
protocol = FingerProtocol
def __init__(self, service):
self.service = service
def getUser(self, user):
return self.service.getUser(user)
components.registerAdapter(FingerFactoryFromService,
IFingerService,
IFingerFactory)
class FingerSetterProtocol(basic.LineReceiver):
def connectionMade(self):
self.lines = []
def lineReceived(self, line):
self.lines.append(line)
def connectionLost(self, reason):
if len(self.lines) == 2:
self.factory.setUser(*self.lines)
class IFingerSetterFactory(Interface):
def setUser(user, status):
"""
Return a deferred returning a string.
"""
def buildProtocol(addr):
"""
Return a protocol returning a string.
"""
class FingerSetterFactoryFromService(protocol.ServerFactory):
implements(IFingerSetterFactory)
protocol = FingerSetterProtocol
def __init__(self, service):
self.service = service
def setUser(self, user, status):
self.service.setUser(user, status)
components.registerAdapter(FingerSetterFactoryFromService,
IFingerSetterService,
IFingerSetterFactory)
class IRCReplyBot(irc.IRCClient):
def connectionMade(self):
self.nickname = self.factory.nickname
irc.IRCClient.connectionMade(self)
def privmsg(self, user, channel, msg):
user = user.split('!')[0]
if self.nickname.lower() == channel.lower():
d = self.factory.getUser(msg)
d.addErrback(catchError)
d.addCallback(lambda m: "Status of %s: %s" % (msg, m))
d.addCallback(lambda m: self.msg(user, m))
class IIRCClientFactory(Interface):
"""
@ivar nickname
"""
def getUser(user):
"""
Return a deferred returning a string.
"""
def buildProtocol(addr):
"""
Return a protocol.
"""
class IRCClientFactoryFromService(protocol.ClientFactory):
implements(IIRCClientFactory)
protocol = IRCReplyBot
nickname = None
def __init__(self, service):
self.service = service
def getUser(self, user):
return self.service.getUser(user)
components.registerAdapter(IRCClientFactoryFromService,
IFingerService,
IIRCClientFactory)
class UserStatusTree(resource.Resource):
def __init__(self, service):
resource.Resource.__init__(self)
self.service=service
# add a specific child for the path "RPC2"
self.putChild("RPC2", UserStatusXR(self.service))
# need to do this for resources at the root of the site
self.putChild("", self)
def _cb_render_GET(self, users, request):
userOutput = ''.join(["<li><a href=\"%s\">%s</a></li>" % (user, user)
for user in users])
request.write("""
<html><head><title>Users</title></head><body>
<h1>Users</h1>
<ul>
%s
</ul></body></html>""" % userOutput)
request.finish()
def render_GET(self, request):
d = self.service.getUsers()
d.addCallback(self._cb_render_GET, request)
# signal that the rendering is not complete
return server.NOT_DONE_YET
def getChild(self, path, request):
return UserStatus(user=path, service=self.service)
components.registerAdapter(UserStatusTree, IFingerService, resource.IResource)
class UserStatus(resource.Resource):
def __init__(self, user, service):
resource.Resource.__init__(self)
self.user = user
self.service = service
def _cb_render_GET(self, status, request):
request.write("""<html><head><title>%s</title></head>
<body><h1>%s</h1>
<p>%s</p>
</body></html>""" % (self.user, self.user, status))
request.finish()
def render_GET(self, request):
d = self.service.getUser(self.user)
d.addCallback(self._cb_render_GET, request)
# signal that the rendering is not complete
return server.NOT_DONE_YET
class UserStatusXR(xmlrpc.XMLRPC):
def __init__(self, service):
xmlrpc.XMLRPC.__init__(self)
self.service = service
def xmlrpc_getUser(self, user):
return self.service.getUser(user)
def xmlrpc_getUsers(self):
return self.service.getUsers()
class IPerspectiveFinger(Interface):
def remote_getUser(username):
"""
Return a user's status.
"""
def remote_getUsers():
"""
Return a user's status.
"""
class PerspectiveFingerFromService(pb.Root):
implements(IPerspectiveFinger)
def __init__(self, service):
self.service = service
def remote_getUser(self, username):
return self.service.getUser(username)
def remote_getUsers(self):
return self.service.getUsers()
components.registerAdapter(PerspectiveFingerFromService,
IFingerService,
IPerspectiveFinger)
class FingerService(service.Service):
implements(IFingerService)
def __init__(self, filename):
self.filename = filename
self.users = {}
def _read(self):
self.users.clear()
for line in file(self.filename):
user, status = line.split(':', 1)
user = user.strip()
status = status.strip()
self.users[user] = status
self.call = reactor.callLater(30, self._read)
def getUser(self, user):
return defer.succeed(self.users.get(user, "No such user"))
def getUsers(self):
return defer.succeed(self.users.keys())
def startService(self):
self._read()
service.Service.startService(self)
def stopService(self):
service.Service.stopService(self)
self.call.cancel()
application = service.Application('finger', uid=1, gid=1)
f = FingerService('/etc/users')
serviceCollection = service.IServiceCollection(application)
f.setServiceParent(serviceCollection)
internet.TCPServer(79, IFingerFactory(f)
).setServiceParent(serviceCollection)
internet.TCPServer(8000, server.Site(resource.IResource(f))
).setServiceParent(serviceCollection)
i = IIRCClientFactory(f)
i.nickname = 'fingerbot'
internet.TCPClient('irc.freenode.org', 6667, i
).setServiceParent(serviceCollection)
internet.TCPServer(8889, pb.PBServerFactory(IPerspectiveFinger(f))
).setServiceParent(serviceCollection)
A simple client to test the perspective broker finger:
# test the PB finger on port 8889
# this code is essentially the same as
# the first example in howto/pb-usage
from twisted.spread import pb
from twisted.internet import reactor
def gotObject(object):
print "got object:", object
object.callRemote("getUser","moshez").addCallback(gotData)
# or
# object.callRemote("getUsers").addCallback(gotData)
def gotData(data):
print 'server sent:', data
reactor.stop()
def gotNoObject(reason):
print "no object:",reason
reactor.stop()
factory = pb.PBClientFactory()
reactor.connectTCP("127.0.0.1",8889, factory)
factory.getRootObject().addCallbacks(gotObject,gotNoObject)
reactor.run()
The Evolution of Finger: using a single factory for multiple protocols¶
Introduction¶
This is the eighth part of the Twisted tutorial Twisted from Scratch, or The Evolution of Finger .
In this part, we add HTTPS support to our web frontend, showing how to have a single factory listen on multiple ports. More information on using SSL in Twisted can be found in the SSL howto .
Support HTTPS¶
All we need to do to code an HTTPS site is just write a context factory (in this case, which loads the certificate from a certain file) and then use the twisted.application.internet.SSLServer method. Note that one factory (in this case, a site) can listen on multiple ports with multiple protocols.
# Do everything properly, and componentize
from twisted.application import internet, service
from twisted.internet import protocol, reactor, defer
from twisted.words.protocols import irc
from twisted.protocols import basic
from twisted.python import components
from twisted.web import resource, server, static, xmlrpc
from twisted.spread import pb
from zope.interface import Interface, implements
from OpenSSL import SSL
import cgi
class IFingerService(Interface):
def getUser(user):
"""
Return a deferred returning a string.
"""
def getUsers():
"""
Return a deferred returning a list of strings.
"""
class IFingerSetterService(Interface):
def setUser(user, status):
"""
Set the user's status to something.
"""
def catchError(err):
return "Internal error in server"
class FingerProtocol(basic.LineReceiver):
def lineReceived(self, user):
d = self.factory.getUser(user)
d.addErrback(catchError)
def writeValue(value):
self.transport.write(value+'\r\n')
self.transport.loseConnection()
d.addCallback(writeValue)
class IFingerFactory(Interface):
def getUser(user):
"""
Return a deferred returning a string.
"""
def buildProtocol(addr):
"""
Return a protocol returning a string.
"""
class FingerFactoryFromService(protocol.ServerFactory):
implements(IFingerFactory)
protocol = FingerProtocol
def __init__(self, service):
self.service = service
def getUser(self, user):
return self.service.getUser(user)
components.registerAdapter(FingerFactoryFromService,
IFingerService,
IFingerFactory)
class FingerSetterProtocol(basic.LineReceiver):
def connectionMade(self):
self.lines = []
def lineReceived(self, line):
self.lines.append(line)
def connectionLost(self, reason):
if len(self.lines) == 2:
self.factory.setUser(*self.lines)
class IFingerSetterFactory(Interface):
def setUser(user, status):
"""
Return a deferred returning a string.
"""
def buildProtocol(addr):
"""
Return a protocol returning a string.
"""
class FingerSetterFactoryFromService(protocol.ServerFactory):
implements(IFingerSetterFactory)
protocol = FingerSetterProtocol
def __init__(self, service):
self.service = service
def setUser(self, user, status):
self.service.setUser(user, status)
components.registerAdapter(FingerSetterFactoryFromService,
IFingerSetterService,
IFingerSetterFactory)
class IRCReplyBot(irc.IRCClient):
def connectionMade(self):
self.nickname = self.factory.nickname
irc.IRCClient.connectionMade(self)
def privmsg(self, user, channel, msg):
user = user.split('!')[0]
if self.nickname.lower() == channel.lower():
d = self.factory.getUser(msg)
d.addErrback(catchError)
d.addCallback(lambda m: "Status of %s: %s" % (msg, m))
d.addCallback(lambda m: self.msg(user, m))
class IIRCClientFactory(Interface):
"""
@ivar nickname
"""
def getUser(user):
"""
Return a deferred returning a string.
"""
def buildProtocol(addr):
"""
Return a protocol.
"""
class IRCClientFactoryFromService(protocol.ClientFactory):
implements(IIRCClientFactory)
protocol = IRCReplyBot
nickname = None
def __init__(self, service):
self.service = service
def getUser(self, user):
return self.service.getUser(user)
components.registerAdapter(IRCClientFactoryFromService,
IFingerService,
IIRCClientFactory)
class UserStatusTree(resource.Resource):
def __init__(self, service):
resource.Resource.__init__(self)
self.service=service
# add a specific child for the path "RPC2"
self.putChild("RPC2", UserStatusXR(self.service))
# need to do this for resources at the root of the site
self.putChild("", self)
def _cb_render_GET(self, users, request):
userOutput = ''.join(["<li><a href=\"%s\">%s</a></li>" % (user, user)
for user in users])
request.write("""
<html><head><title>Users</title></head><body>
<h1>Users</h1>
<ul>
%s
</ul></body></html>""" % userOutput)
request.finish()
def render_GET(self, request):
d = self.service.getUsers()
d.addCallback(self._cb_render_GET, request)
# signal that the rendering is not complete
return server.NOT_DONE_YET
def getChild(self, path, request):
return UserStatus(user=path, service=self.service)
components.registerAdapter(UserStatusTree, IFingerService, resource.IResource)
class UserStatus(resource.Resource):
def __init__(self, user, service):
resource.Resource.__init__(self)
self.user = user
self.service = service
def _cb_render_GET(self, status, request):
request.write("""<html><head><title>%s</title></head>
<body><h1>%s</h1>
<p>%s</p>
</body></html>""" % (self.user, self.user, status))
request.finish()
def render_GET(self, request):
d = self.service.getUser(self.user)
d.addCallback(self._cb_render_GET, request)
# signal that the rendering is not complete
return server.NOT_DONE_YET
class UserStatusXR(xmlrpc.XMLRPC):
def __init__(self, service):
xmlrpc.XMLRPC.__init__(self)
self.service = service
def xmlrpc_getUser(self, user):
return self.service.getUser(user)
def xmlrpc_getUsers(self):
return self.service.getUsers()
class IPerspectiveFinger(Interface):
def remote_getUser(username):
"""
Return a user's status.
"""
def remote_getUsers():
"""
Return a user's status.
"""
class PerspectiveFingerFromService(pb.Root):
implements(IPerspectiveFinger)
def __init__(self, service):
self.service = service
def remote_getUser(self, username):
return self.service.getUser(username)
def remote_getUsers(self):
return self.service.getUsers()
components.registerAdapter(PerspectiveFingerFromService,
IFingerService,
IPerspectiveFinger)
class FingerService(service.Service):
implements(IFingerService)
def __init__(self, filename):
self.filename = filename
self.users = {}
def _read(self):
self.users.clear()
for line in file(self.filename):
user, status = line.split(':', 1)
user = user.strip()
status = status.strip()
self.users[user] = status
self.call = reactor.callLater(30, self._read)
def getUser(self, user):
return defer.succeed(self.users.get(user, "No such user"))
def getUsers(self):
return defer.succeed(self.users.keys())
def startService(self):
self._read()
service.Service.startService(self)
def stopService(self):
service.Service.stopService(self)
self.call.cancel()
class ServerContextFactory:
def getContext(self):
"""
Create an SSL context.
This is a sample implementation that loads a certificate from a file
called 'server.pem'.
"""
ctx = SSL.Context(SSL.SSLv23_METHOD)
ctx.use_certificate_file('server.pem')
ctx.use_privatekey_file('server.pem')
return ctx
application = service.Application('finger', uid=1, gid=1)
f = FingerService('/etc/users')
serviceCollection = service.IServiceCollection(application)
f.setServiceParent(serviceCollection)
internet.TCPServer(79, IFingerFactory(f)
).setServiceParent(serviceCollection)
site = server.Site(resource.IResource(f))
internet.TCPServer(8000, site
).setServiceParent(serviceCollection)
internet.SSLServer(443, site, ServerContextFactory()
).setServiceParent(serviceCollection)
i = IIRCClientFactory(f)
i.nickname = 'fingerbot'
internet.TCPClient('irc.freenode.org', 6667, i
).setServiceParent(serviceCollection)
internet.TCPServer(8889, pb.PBServerFactory(IPerspectiveFinger(f))
).setServiceParent(serviceCollection)
The Evolution of Finger: a Twisted finger client¶
Introduction¶
This is the ninth part of the Twisted tutorial Twisted from Scratch, or The Evolution of Finger .
In this part, we develop a client for the finger server: a proxy finger server which forwards requests to another finger server.
Finger Proxy¶
Writing new clients with Twisted is much like writing new servers. We implement the protocol, which just gathers up all the data, and give it to the factory. The factory keeps a deferred which is triggered if the connection either fails or succeeds. When we use the client, we first make sure the deferred will never fail, by producing a message in that case. Implementing a wrapper around client which just returns the deferred is a common pattern. While less flexible than using the factory directly, it’s also more convenient.
# finger proxy
from twisted.application import internet, service
from twisted.internet import defer, protocol, reactor
from twisted.protocols import basic
from twisted.python import components
from zope.interface import Interface, implements
def catchError(err):
return "Internal error in server"
class IFingerService(Interface):
def getUser(user):
"""Return a deferred returning a string"""
def getUsers():
"""Return a deferred returning a list of strings"""
class IFingerFactory(Interface):
def getUser(user):
"""Return a deferred returning a string"""
def buildProtocol(addr):
"""Return a protocol returning a string"""
class FingerProtocol(basic.LineReceiver):
def lineReceived(self, user):
d = self.factory.getUser(user)
d.addErrback(catchError)
def writeValue(value):
self.transport.write(value)
self.transport.loseConnection()
d.addCallback(writeValue)
class FingerFactoryFromService(protocol.ClientFactory):
implements(IFingerFactory)
protocol = FingerProtocol
def __init__(self, service):
self.service = service
def getUser(self, user):
return self.service.getUser(user)
components.registerAdapter(FingerFactoryFromService,
IFingerService,
IFingerFactory)
class FingerClient(protocol.Protocol):
def connectionMade(self):
self.transport.write(self.factory.user+"\r\n")
self.buf = []
def dataReceived(self, data):
self.buf.append(data)
def connectionLost(self, reason):
self.factory.gotData(''.join(self.buf))
class FingerClientFactory(protocol.ClientFactory):
protocol = FingerClient
def __init__(self, user):
self.user = user
self.d = defer.Deferred()
def clientConnectionFailed(self, _, reason):
self.d.errback(reason)
def gotData(self, data):
self.d.callback(data)
def finger(user, host, port=79):
f = FingerClientFactory(user)
reactor.connectTCP(host, port, f)
return f.d
class ProxyFingerService(service.Service):
implements(IFingerService)
def getUser(self, user):
try:
user, host = user.split('@', 1)
except:
user = user.strip()
host = '127.0.0.1'
ret = finger(user, host)
ret.addErrback(lambda _: "Could not connect to remote host")
return ret
def getUsers(self):
return defer.succeed([])
application = service.Application('finger', uid=1, gid=1)
f = ProxyFingerService()
internet.TCPServer(7779, IFingerFactory(f)).setServiceParent(
service.IServiceCollection(application))
The Evolution of Finger: making a finger library¶
Introduction¶
This is the tenth part of the Twisted tutorial Twisted from Scratch, or The Evolution of Finger .
In this part, we separate the application code that launches a finger service from the library code which defines a finger service, placing the application in a Twisted Application Configuration (.tac) file. We also move configuration (such as HTML templates) into separate files. Configuration and deployment with .tac and twistd are introduced in Using the Twisted Application Framework .
Organization¶
Now this code, while quite modular and well-designed, isn’t
properly organized. Everything above the application= belongs in a
module, and the HTML templates all belong in separate files.
We can use the templateFile and templateDirectory
attributes to indicate what HTML template file to use for each Page, and where
to look for it.
# organized-finger.tac
# eg: twistd -ny organized-finger.tac
import finger
from twisted.internet import protocol, reactor, defer
from twisted.spread import pb
from twisted.web import resource, server
from twisted.application import internet, service, strports
from twisted.python import log
application = service.Application('finger', uid=1, gid=1)
f = finger.FingerService('/etc/users')
serviceCollection = service.IServiceCollection(application)
internet.TCPServer(79, finger.IFingerFactory(f)
).setServiceParent(serviceCollection)
site = server.Site(resource.IResource(f))
internet.TCPServer(8000, site
).setServiceParent(serviceCollection)
internet.SSLServer(443, site, finger.ServerContextFactory()
).setServiceParent(serviceCollection)
i = finger.IIRCClientFactory(f)
i.nickname = 'fingerbot'
internet.TCPClient('irc.freenode.org', 6667, i
).setServiceParent(serviceCollection)
internet.TCPServer(8889, pb.PBServerFactory(finger.IPerspectiveFinger(f))
).setServiceParent(serviceCollection)
Note that our program is now quite separated. We have:
- Code (in the module)
- Configuration (file above)
- Presentation (templates)
- Content (
/etc/users) - Deployment (twistd)
Prototypes don’t need this level of separation, so our earlier examples all bunched together. However, real applications do. Thankfully, if we write our code correctly, it is easy to achieve a good separation of parts.
Easy Configuration¶
We can also supply easy configuration for common cases with a makeService method that will also help build .tap files later:
# Easy configuration
# makeService from finger module
def makeService(config):
# finger on port 79
s = service.MultiService()
f = FingerService(config['file'])
h = internet.TCPServer(79, IFingerFactory(f))
h.setServiceParent(s)
# website on port 8000
r = resource.IResource(f)
r.templateDirectory = config['templates']
site = server.Site(r)
j = internet.TCPServer(8000, site)
j.setServiceParent(s)
# ssl on port 443
if config.get('ssl'):
k = internet.SSLServer(443, site, ServerContextFactory())
k.setServiceParent(s)
# irc fingerbot
if 'ircnick' in config:
i = IIRCClientFactory(f)
i.nickname = config['ircnick']
ircserver = config['ircserver']
b = internet.TCPClient(ircserver, 6667, i)
b.setServiceParent(s)
# Pespective Broker on port 8889
if 'pbport' in config:
m = internet.TCPServer(
int(config['pbport']),
pb.PBServerFactory(IPerspectiveFinger(f)))
m.setServiceParent(s)
return s
And we can write simpler files now:
# simple-finger.tac
# eg: twistd -ny simple-finger.tac
from twisted.application import service
import finger
options = { 'file': '/etc/users',
'templates': '/usr/share/finger/templates',
'ircnick': 'fingerbot',
'ircserver': 'irc.freenode.net',
'pbport': 8889,
'ssl': 'ssl=0' }
ser = finger.makeService(options)
application = service.Application('finger', uid=1, gid=1)
ser.setServiceParent(service.IServiceCollection(application))
% twistd -ny simple-finger.tac
Note: the finger user still has ultimate power: they can use makeService, or they can use the lower-level interface if they have specific needs (maybe an IRC server on some other port? Maybe we want the non-SSL webserver to listen only locally? etc. etc.).
This is an important design principle: never force a layer of abstraction; allow usage of layers of abstractions instead.
The pasta theory of design:
- Spaghetti
- Each piece of code interacts with every other piece of code (can be implemented with GOTO, functions, objects).
- Lasagna
- Code has carefully designed layers. Each layer is, in theory independent. However low-level layers usually cannot be used easily, and high-level layers depend on low-level layers.
- Ravioli
- Each part of the code is useful by itself. There is a thin layer of interfaces between various parts (the sauce). Each part can be usefully be used elsewhere.
...but sometimes, the user just wants to order “Ravioli”, so one coarse-grain easily definable layer of abstraction on top of it all can be useful.
The Evolution of Finger: configuration and packaging of the finger service¶
Introduction¶
This is the eleventh part of the Twisted tutorial Twisted from Scratch, or The Evolution of Finger .
In this part, we make it easier for non-programmers to configure a finger server and show how to package it in the .deb and RPM package formats. Plugins are discussed further in the Twisted Plugin System howto. Writing twistd plugins is covered in Writing a twistd Plugin , and .tac applications are covered in Using the Twisted Application Framework .
Plugins¶
So far, the user had to be somewhat of a programmer to be able to configure
stuff. Maybe we can eliminate even that? Move old code
to finger/__init__.py and...
Full source code for finger module here:
# finger.py module
from zope.interface import Interface, implements
from twisted.application import internet, service
from twisted.internet import protocol, reactor, defer
from twisted.words.protocols import irc
from twisted.protocols import basic
from twisted.python import components, log
from twisted.web import resource, server, xmlrpc
from twisted.spread import pb
from OpenSSL import SSL
class IFingerService(Interface):
def getUser(user):
"""
Return a deferred returning a string.
"""
def getUsers():
"""
Return a deferred returning a list of strings.
"""
class IFingerSetterService(Interface):
def setUser(user, status):
"""
Set the user's status to something.
"""
def catchError(err):
return "Internal error in server"
class FingerProtocol(basic.LineReceiver):
def lineReceived(self, user):
d = self.factory.getUser(user)
d.addErrback(catchError)
def writeValue(value):
self.transport.write(value+'\n')
self.transport.loseConnection()
d.addCallback(writeValue)
class IFingerFactory(Interface):
def getUser(user):
"""
Return a deferred returning a string.
"""
def buildProtocol(addr):
"""
Return a protocol returning a string.
"""
class FingerFactoryFromService(protocol.ServerFactory):
implements(IFingerFactory)
protocol = FingerProtocol
def __init__(self, service):
self.service = service
def getUser(self, user):
return self.service.getUser(user)
components.registerAdapter(FingerFactoryFromService,
IFingerService,
IFingerFactory)
class FingerSetterProtocol(basic.LineReceiver):
def connectionMade(self):
self.lines = []
def lineReceived(self, line):
self.lines.append(line)
def connectionLost(self, reason):
if len(self.lines) == 2:
self.factory.setUser(*self.lines)
class IFingerSetterFactory(Interface):
def setUser(user, status):
"""
Return a deferred returning a string.
"""
def buildProtocol(addr):
"""
Return a protocol returning a string.
"""
class FingerSetterFactoryFromService(protocol.ServerFactory):
implements(IFingerSetterFactory)
protocol = FingerSetterProtocol
def __init__(self, service):
self.service = service
def setUser(self, user, status):
self.service.setUser(user, status)
components.registerAdapter(FingerSetterFactoryFromService,
IFingerSetterService,
IFingerSetterFactory)
class IRCReplyBot(irc.IRCClient):
def connectionMade(self):
self.nickname = self.factory.nickname
irc.IRCClient.connectionMade(self)
def privmsg(self, user, channel, msg):
user = user.split('!')[0]
if self.nickname.lower() == channel.lower():
d = self.factory.getUser(msg)
d.addErrback(catchError)
d.addCallback(lambda m: "Status of %s: %s" % (msg, m))
d.addCallback(lambda m: self.msg(user, m))
class IIRCClientFactory(Interface):
"""
@ivar nickname
"""
def getUser(user):
"""
Return a deferred returning a string.
"""
def buildProtocol(addr):
"""
Return a protocol.
"""
class IRCClientFactoryFromService(protocol.ClientFactory):
implements(IIRCClientFactory)
protocol = IRCReplyBot
nickname = None
def __init__(self, service):
self.service = service
def getUser(self, user):
return self.service.getUser(user)
components.registerAdapter(IRCClientFactoryFromService,
IFingerService,
IIRCClientFactory)
class UserStatusTree(resource.Resource):
template = """<html><head><title>Users</title></head><body>
<h1>Users</h1>
<ul>
%(users)s
</ul>
</body>
</html>"""
def __init__(self, service):
resource.Resource.__init__(self)
self.service = service
def getChild(self, path, request):
if path == '':
return self
elif path == 'RPC2':
return UserStatusXR(self.service)
else:
return UserStatus(path, self.service)
def render_GET(self, request):
users = self.service.getUsers()
def cbUsers(users):
request.write(self.template % {'users': ''.join([
# Name should be quoted properly these uses.
'<li><a href="%s">%s</a></li>' % (name, name)
for name in users])})
request.finish()
users.addCallback(cbUsers)
def ebUsers(err):
log.err(err, "UserStatusTree failed")
request.finish()
users.addErrback(ebUsers)
return server.NOT_DONE_YET
components.registerAdapter(UserStatusTree, IFingerService, resource.IResource)
class UserStatus(resource.Resource):
template='''<html><head><title>%(title)s</title></head>
<body><h1>%(name)s</h1><p>%(status)s</p></body></html>'''
def __init__(self, user, service):
resource.Resource.__init__(self)
self.user = user
self.service = service
def render_GET(self, request):
status = self.service.getUser(self.user)
def cbStatus(status):
request.write(self.template % {
'title': self.user,
'name': self.user,
'status': status})
request.finish()
status.addCallback(cbStatus)
def ebStatus(err):
log.err(err, "UserStatus failed")
request.finish()
status.addErrback(ebStatus)
return server.NOT_DONE_YET
class UserStatusXR(xmlrpc.XMLRPC):
def __init__(self, service):
xmlrpc.XMLRPC.__init__(self)
self.service = service
def xmlrpc_getUser(self, user):
return self.service.getUser(user)
def xmlrpc_getUsers(self):
return self.service.getUsers()
class IPerspectiveFinger(Interface):
def remote_getUser(username):
"""
Return a user's status.
"""
def remote_getUsers():
"""
Return a user's status.
"""
class PerspectiveFingerFromService(pb.Root):
implements(IPerspectiveFinger)
def __init__(self, service):
self.service = service
def remote_getUser(self, username):
return self.service.getUser(username)
def remote_getUsers(self):
return self.service.getUsers()
components.registerAdapter(PerspectiveFingerFromService,
IFingerService,
IPerspectiveFinger)
class FingerService(service.Service):
implements(IFingerService)
def __init__(self, filename):
self.filename = filename
def _read(self):
self.users = {}
for line in file(self.filename):
user, status = line.split(':', 1)
user = user.strip()
status = status.strip()
self.users[user] = status
self.call = reactor.callLater(30, self._read)
def getUser(self, user):
return defer.succeed(self.users.get(user, "No such user"))
def getUsers(self):
return defer.succeed(self.users.keys())
def startService(self):
self._read()
service.Service.startService(self)
def stopService(self):
service.Service.stopService(self)
self.call.cancel()
class ServerContextFactory:
def getContext(self):
"""
Create an SSL context.
This is a sample implementation that loads a certificate from a file
called 'server.pem'.
"""
ctx = SSL.Context(SSL.SSLv23_METHOD)
ctx.use_certificate_file('server.pem')
ctx.use_privatekey_file('server.pem')
return ctx
# Easy configuration
def makeService(config):
# finger on port 79
s = service.MultiService()
f = FingerService(config['file'])
h = internet.TCPServer(1079, IFingerFactory(f))
h.setServiceParent(s)
# website on port 8000
r = resource.IResource(f)
r.templateDirectory = config['templates']
site = server.Site(r)
j = internet.TCPServer(8000, site)
j.setServiceParent(s)
# ssl on port 443
# if config.get('ssl'):
# k = internet.SSLServer(443, site, ServerContextFactory())
# k.setServiceParent(s)
# irc fingerbot
if 'ircnick' in config:
i = IIRCClientFactory(f)
i.nickname = config['ircnick']
ircserver = config['ircserver']
b = internet.TCPClient(ircserver, 6667, i)
b.setServiceParent(s)
# Pespective Broker on port 8889
if 'pbport' in config:
m = internet.TCPServer(
int(config['pbport']),
pb.PBServerFactory(IPerspectiveFinger(f)))
m.setServiceParent(s)
return s
# finger/tap.py
from twisted.application import internet, service
from twisted.internet import interfaces
from twisted.python import usage
import finger
class Options(usage.Options):
optParameters = [
['file', 'f', '/etc/users'],
['templates', 't', '/usr/share/finger/templates'],
['ircnick', 'n', 'fingerbot'],
['ircserver', None, 'irc.freenode.net'],
['pbport', 'p', 8889],
]
optFlags = [['ssl', 's']]
def makeService(config):
return finger.makeService(config)
And register it all:
from twisted.application.service import ServiceMaker
finger = ServiceMaker(
'finger', 'finger.tap', 'Run a finger service', 'finger')
Note that the second argument to ServiceMaker ,``finger.tap`` , is a reference to a module
(finger/tap.py ), not to a filename.
And now, the following works
% sudo twistd -n finger --file=/etc/users --ircnick=fingerbot
For more details about this, see the twistd plugin documentation .
OS Integration¶
If we already have the “finger” package installed in
PYTHONPATH (e.g. we added it to site-packages ), we
can achieve easy integration:
% tap2deb --unsigned -m "Foo <foo@example.com>" --type=python finger.tac
% sudo dpkg -i .build/*.deb
% tap2rpm --type=python finger.tac
% sudo rpm -i *.rpm
These packages will properly install and register init.d
scripts, etc. for the given file.
If it doesn’t work on your favorite OS: patches accepted!
Introduction¶
Twisted is a big system. People are often daunted when they approach it. It’s hard to know where to start looking.
This guide builds a full-fledged Twisted application from the ground up, using most of the important bits of the framework. There is a lot of code, but don’t be afraid.
The application we are looking at is a “finger” service, along the lines of the familiar service traditionally provided by UNIX™ servers. We will extend this service slightly beyond the standard, in order to demonstrate some of Twisted’s higher-level features.
Each section of the tutorial dives straight into applications for various Twisted topics. These topics have their own introductory howtos listed in the core howto index and in the documentation for other Twisted projects like Twisted Web and Twisted Words. There are at least three ways to use this tutorial: you may find it useful to read through the rest of the topics listed in the core howto index before working through the finger tutorial, work through the finger tutorial and then go back and hit the introductory material that is relevant to the Twisted project you’re working on, or read the introductory material one piece at a time as it comes up in the finger tutorial.
Contents¶
This tutorial is split into eleven parts:
- The Evolution of Finger: building a simple finger service
- The Evolution of Finger: adding features to the finger service
- The Evolution of Finger: cleaning up the finger code
- The Evolution of Finger: moving to a component based architecture
- The Evolution of Finger: pluggable backends
- The Evolution of Finger: a web frontend
- The Evolution of Finger: Twisted client support using Perspective Broker
- The Evolution of Finger: using a single factory for multiple protocols
- The Evolution of Finger: a Twisted finger client
- The Evolution of Finger: making a finger library
- The Evolution of Finger: configuration and packaging of the finger service
Setting up the TwistedQuotes application¶
Goal¶
This document describes how to set up the TwistedQuotes application used in a number of other documents, such as designing Twisted applications .
Setting up the TwistedQuotes project directory¶
In order to run the Twisted Quotes example, you will need to do the following:
Make a
TwistedQuotesdirectory on your systemPlace the following files in the
TwistedQuotesdirectory:-
""" Twisted Quotes """
(this file marks it as a package, see this section of the Python tutorial for more on packages)
-
from random import choice from zope.interface import implements from TwistedQuotes import quoteproto class StaticQuoter: """ Return a static quote. """ implements(quoteproto.IQuoter) def __init__(self, quote): self.quote = quote def getQuote(self): return self.quote class FortuneQuoter: """ Load quotes from a fortune-format file. """ implements(quoteproto.IQuoter) def __init__(self, filenames): self.filenames = filenames def getQuote(self): quoteFile = file(choice(self.filenames)) quotes = quoteFile.read().split('\n%\n') quoteFile.close() return choice(quotes)
-
from zope.interface import Interface from twisted.internet.protocol import Factory, Protocol class IQuoter(Interface): """ An object that returns quotes. """ def getQuote(): """ Return a quote. """ class QOTD(Protocol): def connectionMade(self): self.transport.write(self.factory.quoter.getQuote()+'\r\n') self.transport.loseConnection() class QOTDFactory(Factory): """ A factory for the Quote of the Day protocol. @type quoter: L{IQuoter} provider @ivar quoter: An object which provides L{IQuoter} which will be used by the L{QOTD} protocol to get quotes to emit. """ protocol = QOTD def __init__(self, quoter): self.quoter = quoter
-
Add the
TwistedQuotesdirectory’s parent to your Python path. For example, if the TwistedQuotes directory’s path is/mystuff/TwistedQuotesorc:\mystuff\TwistedQuotesadd/mystuffto your Python path. On UNIX this would beexport PYTHONPATH=/mystuff:$PYTHONPATH, on Microsoft Windows change thePYTHONPATHvariable through the Systems Properties dialog by adding;c:\mystuffat the end.Test your package by trying to import it in the Python interpreter:
Python 2.1.3 (#1, Apr 20 2002, 22:45:31) [GCC 2.95.4 20011002 (Debian prerelease)] on linux2 Type "copyright", "credits" or "license" for more information. >>> import TwistedQuotes >>> # No traceback means you're fine.
Designing Twisted Applications¶
Goals¶
This document describes how a good Twisted application is structured. It should be useful for beginning Twisted developers who want to structure their code in a clean, maintainable way that reflects current best practices.
Readers will want to be familiar with writing servers and clients using Twisted.
Example of a modular design: TwistedQuotes¶
TwistedQuotes is a very simple plugin which is a great
demonstration of
Twisted’s power. It will export a small kernel of functionality – Quote of
the Day – which can be accessed through every interface that Twisted supports:
web pages, e-mail, instant messaging, a specific Quote of the Day protocol, and
more.
Set up the project directory¶
See the description of setting up the TwistedQuotes example .
A Look at the Heart of the Application¶
from random import choice
from zope.interface import implements
from TwistedQuotes import quoteproto
class StaticQuoter:
"""
Return a static quote.
"""
implements(quoteproto.IQuoter)
def __init__(self, quote):
self.quote = quote
def getQuote(self):
return self.quote
class FortuneQuoter:
"""
Load quotes from a fortune-format file.
"""
implements(quoteproto.IQuoter)
def __init__(self, filenames):
self.filenames = filenames
def getQuote(self):
quoteFile = file(choice(self.filenames))
quotes = quoteFile.read().split('\n%\n')
quoteFile.close()
return choice(quotes)
This code listing shows us what the Twisted Quotes system is all about. The code doesn’t have any way of talking to the outside world, but it provides a library which is a clear and uncluttered abstraction: “give me the quote of the day” .
Note that this module does not import any Twisted functionality at all! The reason for doing things this way is integration. If your “business objects” are not stuck to your user interface, you can make a module that can integrate those objects with different protocols, GUIs, and file formats. Having such classes provides a way to decouple your components from each other, by allowing each to be used independently.
In this manner, Twisted itself has minimal impact on the logic of your program. Although the Twisted “dot products” are highly interoperable, they also follow this approach. You can use them independently because they are not stuck to each other. They communicate in well-defined ways, and only when that communication provides some additional feature. Thus, you can use twisted.web with twisted.enterprise , but neither requires the other, because they are integrated around the concept of Deferreds .
Your Twisted applications should follow this style as much as possible. Have (at least) one module which implements your specific functionality, independent of any user-interface code.
Next, we’re going to need to associate this abstract logic with some way of displaying it to the user. We’ll do this by writing a Twisted server protocol, which will respond to the clients that connect to it by sending a quote to the client and then closing the connection. Note: don’t get too focused on the details of this – different ways to interface with the user are 90% of what Twisted does, and there are lots of documents describing the different ways to do it.
from zope.interface import Interface
from twisted.internet.protocol import Factory, Protocol
class IQuoter(Interface):
"""
An object that returns quotes.
"""
def getQuote():
"""
Return a quote.
"""
class QOTD(Protocol):
def connectionMade(self):
self.transport.write(self.factory.quoter.getQuote()+'\r\n')
self.transport.loseConnection()
class QOTDFactory(Factory):
"""
A factory for the Quote of the Day protocol.
@type quoter: L{IQuoter} provider
@ivar quoter: An object which provides L{IQuoter} which will be used by
the L{QOTD} protocol to get quotes to emit.
"""
protocol = QOTD
def __init__(self, quoter):
self.quoter = quoter
This is a very straightforward Protocol implementation, and the
pattern described above is repeated here. The Protocol contains essentially no
logic of its own, just enough to tie together an object which can generate
quotes (a Quoter ) and an object which can relay
bytes to a TCP connection (a Transport ). When a
client connects to this server, a QOTD instance is
created, and its connectionMade method is called.
The QOTDFactory ‘s role is to specify to the
Twisted framework how to create a Protocol instance
that will handle the connection. Twisted will not instantiate a QOTDFactory ; you will do that yourself later, in a twistd plug-in.
Note: you can read more specifics of Protocol and Factory in the Writing Servers HOWTO.
Once we have an abstraction – a Quoter – and we have a
mechanism to connect it to the network – the QOTD protocol – the
next thing to do is to put the last link in the chain of functionality between
abstraction and user. This last link will allow a user to choose a Quoter and configure the protocol. Writing this configuration is
covered in the Application HOWTO .
Overview of Twisted Internet¶
Twisted Internet is a collection of compatible event-loops for Python.
It contains the code to dispatch events to interested observers and a portable
API so that observers need not care about which event loop is running. Thus,
it is possible to use the same code for different loops, from Twisted’s basic,
yet portable, select -based loop to the loops of various GUI
toolkits like GTK+ or Tk.
Twisted Internet contains the various interfaces to the reactor API, whose usage is documented in the low-level chapter. Those APIs are IReactorCore , IReactorTCP , IReactorSSL , IReactorUNIX , IReactorUDP , IReactorTime , IReactorProcess , IReactorMulticast and IReactorThreads . The reactor APIs allow non-persistent calls to be made.
Twisted Internet also covers the interfaces for the various transports, in ITransport and friends. These interfaces allow Twisted network code to be written without regard to the underlying implementation of the transport.
The IProtocolFactory dictates how factories, which are usually a large part of third party code, are written.
Reactor Overview¶
This HOWTO introduces the Twisted reactor, describes the basics of the reactor and links to the various reactor interfaces.
Reactor Basics¶
The reactor is the core of the event loop within Twisted – the loop which drives applications using Twisted. The event loop is a programming construct that waits for and dispatches events or messages in a program. It works by calling some internal or external “event provider”, which generally blocks until an event has arrived, and then calls the relevant event handler (“dispatches the event”). The reactor provides basic interfaces to a number of services, including network communications, threading, and event dispatching.
For information about using the reactor and the Twisted event loop, see:
- the event dispatching howtos: Scheduling and Using Deferreds ;
- the communication howtos: TCP servers , TCP clients , UDP networking and Using processes ; and
- Using threads .
There are multiple implementations of the reactor, each modified to provide better support for specialized features over the default implementation. More information about these and how to use a particular implementation is available via Choosing a Reactor .
Twisted applications can use the interfaces in twisted.application.service to configure and run the application instead of using boilerplate reactor code. See Using Application for an introduction to Application.
Using the reactor object¶
You can get to the reactor object using the following code:
from twisted.internet import reactor
The reactor usually implements a set of interfaces, but depending on the chosen reactor and the platform, some of the interfaces may not be implemented:
- IReactorCore : Core (required) functionality.
- IReactorFDSet : Use FileDescriptor objects.
- IReactorProcess : Process management. Read the Using Processes document for more information.
- IReactorSSL : SSL networking support.
- IReactorTCP : TCP networking support. More information can be found in the Writing Servers and Writing Clients documents.
- IReactorThreads : Threading use and management. More information can be found within Threading In Twisted .
- IReactorTime : Scheduling interface. More information can be found within Scheduling Tasks .
- IReactorUDP : UDP networking support. More information can be found within UDP Networking .
- IReactorUNIX : UNIX socket support.
- IReactorSocket : Third-party socket support.
Using TLS in Twisted¶
Overview¶
This document describes how to secure your communications using TLS (Transport Layer Security) — also known as SSL (Secure Sockets Layer) — in Twisted servers and clients. It assumes that you know what TLS is, what some of the major reasons to use it are, and how to generate your own certificates. It also assumes that you are comfortable with creating TCP servers and clients as described in the server howto and client howto . After reading this document you should be able to create servers and clients that can use TLS to encrypt their connections, switch from using an unencrypted channel to an encrypted one mid-connection, and require client authentication.
Using TLS in Twisted requires that you have pyOpenSSL installed. A quick test to verify that you do is to run from OpenSSL import SSL at a python prompt and not get an error.
Twisted provides TLS support as a transport — that is, as an alternative to TCP.
When using TLS, use of the TCP APIs you’re already familiar with, TCP4ClientEndpoint and TCP4ServerEndpoint — or reactor.listenTCP and reactor.connectTCP — is replaced by use of parallel TLS APIs (many of which still use the legacy name “SSL” due to age and/or compatibility with older APIs).
To create a TLS server, use SSL4ServerEndpoint or listenSSL .
To create a TLS client, use SSL4ClientEndpoint or connectSSL .
TLS provides transport layer security, but it’s important to understand what “security” means. With respect to TLS it means three things:
- Identity: TLS servers (and sometimes clients) present a certificate, offering proof of who they are, so that you know who you are talking to.
- Confidentiality: once you know who you are talking to, encryption of the connection ensures that the communications can’t be understood by any third parties who might be listening in.
- Integrity: TLS checks the encrypted messages to ensure that they actually came from the party you originally authenticated to. If the messages fail these checks, then they are discarded and your application does not see them.
Without identity, neither confidentiality nor integrity is possible.
If you don’t know who you’re talking to, then you might as easily be talking to your bank or to a thief who wants to steal your bank password.
Each of the APIs listed above with “SSL” in the name requires a configuration object called (for historical reasons) a contextFactory.
(Please pardon the somewhat awkward name.)
The contextFactory serves three purposes:
- It provides the materials to prove your own identity to the other side of the connection: in other words, who you are.
- It expresses your requirements of the other side’s identity: in other words, who you would like to talk to (and who you trust to tell you that you’re talking to the right party).
- It allows you to specify certain specialized options about the way the TLS protocol itself operates.
The requirements of clients and servers are slightly different. Both can provide a certificate to prove their identity, but commonly, TLS servers provide a certificate, whereas TLS clients check the server’s certificate (to make sure they’re talking to the right server) and then later identify themselves to the server some other way, often by offering a shared secret such as a password or API key via an application protocol secured with TLS and not as part of TLS itself.
Since these requirements are slightly different, there are different APIs to construct an appropriate contextFactory value for a client or a server.
For servers, we can use twisted.internet.ssl.CertificateOptions.
In order to prove the server’s identity, you pass the privateKey and certificate arguments to this object.
twisted.internet.ssl.PrivateCertificate.options is a convenient way to create a CertificateOptions instance configured to use a particular key and certificate.
For clients, we can use twisted.internet.ssl.optionsForClientTLS.
This takes two arguments, hostname (which indicates what hostname must be advertised in the server’s certificate) and optionally trustRoot.
By default, optionsForClientTLS tries to obtain the trust roots from your platform, but you can specify your own.
You may obtain an object suitable to pass as the trustRoot= parameter with an explicit list of twisted.internet.ssl.Certificate or twisted.internet.ssl.PrivateCertificate instances by calling twisted.internet.ssl.trustRootFromCertificates. This will cause optionsForClientTLS to accept any connection so long as the server’s certificate is signed by at least one of the certificates passed.
Note
Currently, Twisted only supports loading of OpenSSL’s default trust roots. If you’ve built OpenSSL yourself, you must take care to include these in the appropriate location. If you’re using the OpenSSL shipped as part of Mac OS X 10.5-10.9, this behavior will also be correct. If you’re using Debian, or one of its derivatives like Ubuntu, install the ca-certificates package to ensure you have trust roots available, and this behavior should also be correct. Work is ongoing to make platformTrust — the API that optionsForClientTLS uses by default — more robust. For example, platformTrust should fall back to the “certifi” package if no platform trust roots are available but it doesn’t do that yet. When this happens, you shouldn’t need to change your code.
TLS echo server and client¶
Now that we’ve got the theory out of the way, let’s try some working examples of how to get started with a TLS server.
The following examples rely on the files server.pem (private key and self-signed certificate together) and public.pem (the server’s public certificate by itself).
TLS echo server¶
#!/usr/bin/env python
# Copyright (c) Twisted Matrix Laboratories.
# See LICENSE for details.
import sys
from twisted.internet import ssl, protocol, task, defer
from twisted.python import log
from twisted.python.modules import getModule
import echoserv
def main(reactor):
log.startLogging(sys.stdout)
certData = getModule(__name__).filePath.sibling('server.pem').getContent()
certificate = ssl.PrivateCertificate.loadPEM(certData)
factory = protocol.Factory.forProtocol(echoserv.Echo)
reactor.listenSSL(8000, factory, certificate.options())
return defer.Deferred()
if __name__ == '__main__':
import echoserv_ssl
task.react(echoserv_ssl.main)
This server uses listenSSL to listen for TLS traffic on port 8000, using the certificate and private key contained in the file server.pem.
It uses the same echo example server as the TCP echo server — even going so far as to import its protocol class.
Assuming that you can buy your own TLS certificate from a certificate authority, this is a fairly realistic TLS server.
TLS echo client¶
#!/usr/bin/env python
# Copyright (c) Twisted Matrix Laboratories.
# See LICENSE for details.
from twisted.internet import ssl, task, protocol, endpoints, defer
from twisted.python.modules import getModule
import echoclient
@defer.inlineCallbacks
def main(reactor):
factory = protocol.Factory.forProtocol(echoclient.EchoClient)
certData = getModule(__name__).filePath.sibling('public.pem').getContent()
authority = ssl.Certificate.loadPEM(certData)
options = ssl.optionsForClientTLS(u'example.com', authority)
endpoint = endpoints.SSL4ClientEndpoint(reactor, 'localhost', 8000,
options)
echoClient = yield endpoint.connect(factory)
done = defer.Deferred()
echoClient.connectionLost = lambda reason: done.callback(None)
yield done
if __name__ == '__main__':
import echoclient_ssl
task.react(echoclient_ssl.main)
This client uses SSL4ClientEndpoint to connect to echoserv_ssl.py.
It also uses the same echo example client as the TCP echo client.
Whenever you have a protocol that listens on plain-text TCP it is easy to run it over TLS instead.
It specifies that it only wants to talk to a host named "example.com", and that it trusts the certificate authority in "public.pem" to say who "example.com" is.
Note that the host you are connecting to — localhost — and the host whose identity you are verifying — example.com — can differ.
In this case, our example server.pem certificate identifies a host named “example.com”, but your server is proably running on localhost.
In a realistic client, it’s very important that you pass the same “hostname” your connection API (in this case, SSL4ClientEndpoint) and optionsForClientTLS.
In this case we’re using “localhost” as the host to connect to because you’re probably running this example on your own computer and “example.com” because that’s the value hard-coded in the dummy certificate distributed along with Twisted’s example code.
Connecting To Public Servers¶
Here is a short example, now using the default trust roots for optionsForClientTLS from platformTrust.
from __future__ import print_function
import sys
from twisted.internet import defer, endpoints, protocol, ssl, task, error
def main(reactor, host, port=443):
options = ssl.optionsForClientTLS(hostname=host.decode('utf-8'))
port = int(port)
class ShowCertificate(protocol.Protocol):
def connectionMade(self):
self.transport.write(b"GET / HTTP/1.0\r\n\r\n")
self.done = defer.Deferred()
def dataReceived(self, data):
certificate = ssl.Certificate(self.transport.getPeerCertificate())
print("OK:", certificate)
self.transport.abortConnection()
def connectionLost(self, reason):
print("Lost.")
if not reason.check(error.ConnectionClosed):
print("BAD:", reason.value)
self.done.callback(None)
return endpoints.connectProtocol(
endpoints.SSL4ClientEndpoint(reactor, host, port, options),
ShowCertificate()
).addCallback(lambda protocol: protocol.done)
task.react(main, sys.argv[1:])
You can use this tool fairly simply to retrieve certificates from an HTTPS server with a valid TLS certificate, by running it with a host name. For example:
$ python check_server_certificate.py www.twistedmatrix.com
OK: <Certificate Subject=www.twistedmatrix.com ...>
$ python check_server_certificate.py www.cacert.org
BAD: [(... 'certificate verify failed')]
$ python check_server_certificate.py dornkirk.twistedmatrix.com
BAD: No service reference ID could be validated against certificate.
Note
To properly validate your hostname parameter according to RFC6125, please also install the “service_identity” and “idna” packages from PyPI.
Without this package, Twisted will currently make a conservative guess as to the correctness of the server’s certificate, but this will reject a large number of potentially valid certificates.
service_identity implements the standard correctly and it will be a required dependency for TLS in a future release of Twisted.
Using startTLS¶
If you want to switch from unencrypted to encrypted traffic mid-connection, you’ll need to turn on TLS with startTLS on both ends of the connection at the same time via some agreed-upon signal like the reception of a particular message. You can readily verify the switch to an encrypted channel by examining the packet payloads with a tool like Wireshark .
startTLS server¶
from twisted.internet import ssl, protocol, defer, task, endpoints
from twisted.protocols.basic import LineReceiver
from twisted.python.modules import getModule
class TLSServer(LineReceiver):
def lineReceived(self, line):
print("received: " + line)
if line == "STARTTLS":
print("-- Switching to TLS")
self.sendLine('READY')
self.transport.startTLS(self.factory.options)
def main(reactor):
certData = getModule(__name__).filePath.sibling('server.pem').getContent()
cert = ssl.PrivateCertificate.loadPEM(certData)
factory = protocol.Factory.forProtocol(TLSServer)
factory.options = cert.options()
endpoint = endpoints.TCP4ServerEndpoint(reactor, 8000)
endpoint.listen(factory)
return defer.Deferred()
if __name__ == '__main__':
import starttls_server
task.react(starttls_server.main)
startTLS client¶
from twisted.internet import ssl, endpoints, task, protocol, defer
from twisted.protocols.basic import LineReceiver
from twisted.python.modules import getModule
class StartTLSClient(LineReceiver):
def connectionMade(self):
self.sendLine("plain text")
self.sendLine("STARTTLS")
def lineReceived(self, line):
print("received: " + line)
if line == "READY":
self.transport.startTLS(self.factory.options)
self.sendLine("secure text")
self.transport.loseConnection()
@defer.inlineCallbacks
def main(reactor):
factory = protocol.Factory.forProtocol(StartTLSClient)
certData = getModule(__name__).filePath.sibling('server.pem').getContent()
factory.options = ssl.optionsForClientTLS(
u"example.com", ssl.PrivateCertificate.loadPEM(certData)
)
endpoint = endpoints.HostnameEndpoint(reactor, 'localhost', 8000)
startTLSClient = yield endpoint.connect(factory)
done = defer.Deferred()
startTLSClient.connectionLost = lambda reason: done.callback(None)
yield done
if __name__ == "__main__":
import starttls_client
task.react(starttls_client.main)
startTLS is a transport method that gets passed a contextFactory.
It is invoked at an agreed-upon time in the data reception method of the client and server protocols.
The server uses PrivateCertificate.options to create a contextFactory which will use a particular certificate and private key (a common requirement for TLS servers).
The client creates an uncustomized CertificateOptions which is all that’s necessary for a TLS client to interact with a TLS server.
Client authentication¶
Server and client-side changes to require client authentication fall largely under the dominion of pyOpenSSL, but few examples seem to exist on the web so for completeness a sample server and client are provided here.
TLS server with client authentication via client certificate verification¶
When one or more certificates are passed to PrivateCertificate.options, the resulting contextFactory will use those certificates as trusted authorities and require that the peer present a certificate with a valid chain anchored by one of those authorities.
A server can use this to verify that a client provides a valid certificate signed by one of those certificate authorities; here is an example of such a certificate.
#!/usr/bin/env python
# Copyright (c) Twisted Matrix Laboratories.
# See LICENSE for details.
import sys
from twisted.internet import ssl, protocol, task, defer
from twisted.python import log
from twisted.python.modules import getModule
import echoserv
def main(reactor):
log.startLogging(sys.stdout)
certData = getModule(__name__).filePath.sibling('public.pem').getContent()
authData = getModule(__name__).filePath.sibling('server.pem').getContent()
authority = ssl.Certificate.loadPEM(certData)
certificate = ssl.PrivateCertificate.loadPEM(authData)
factory = protocol.Factory.forProtocol(echoserv.Echo)
reactor.listenSSL(8000, factory, certificate.options(authority))
return defer.Deferred()
if __name__ == '__main__':
import ssl_clientauth_server
task.react(ssl_clientauth_server.main)
Client with certificates¶
The following client then supplies such a certificate as the clientCertificate argument to optionsForClientTLS, while still validating the server’s identity.
#!/usr/bin/env python
# Copyright (c) Twisted Matrix Laboratories.
# See LICENSE for details.
from twisted.internet import ssl, task, protocol, endpoints, defer
from twisted.python.modules import getModule
import echoclient
@defer.inlineCallbacks
def main(reactor):
factory = protocol.Factory.forProtocol(echoclient.EchoClient)
certData = getModule(__name__).filePath.sibling('public.pem').getContent()
authData = getModule(__name__).filePath.sibling('server.pem').getContent()
clientCertificate = ssl.PrivateCertificate.loadPEM(authData)
authority = ssl.Certificate.loadPEM(certData)
options = ssl.optionsForClientTLS(u'example.com', authority,
clientCertificate)
endpoint = endpoints.SSL4ClientEndpoint(reactor, 'localhost', 8000,
options)
echoClient = yield endpoint.connect(factory)
done = defer.Deferred()
echoClient.connectionLost = lambda reason: done.callback(None)
yield done
if __name__ == '__main__':
import ssl_clientauth_client
task.react(ssl_clientauth_client.main)
Notice that these two examples are very, very similar to the TLS echo examples above.
In fact, you can demonstrate a failed authentication by simply running echoclient_ssl.py against ssl_clientauth_server.py; you’ll see no output because the server closed the connection rather than echoing the client’s authenticated input.
TLS Protocol Options¶
For servers, it is desirable to offer Diffie-Hellman based key exchange that provides perfect forward secrecy.
The ciphers are activated by default, however it is necessary to pass an instance of DiffieHellmanParameters to CertificateOptions via the dhParameters option to be able to use them.
For example,
from twisted.internet.ssl import CertificateOptions, DiffieHellmanParameters
from twisted.python.filepath import FilePath
dhFilePath = FilePath('dh_param_1024.pem')
dhParams = DiffieHellmanParameters.fromFile(dhFilePath)
options = CertificateOptions(..., dhParameters=dhParams)
Another part of the TLS protocol which CertificateOptions can control is the version of the TLS or SSL protocol used.
This is often called the context’s “method”.
By default, CertificateOptions creates contexts that require at least the TLSv1 protocol.
CertificateOptions also supports the older SSLv3 protocol (which may be required interoperate with an existing service or piece of software).
To allow SSLv3, just pass OpenSSL.SSL.SSLv3_METHOD to CertificateOptions‘s initializer:
from twisted.internet.ssl import CertificateOptions
from OpenSSL.SSL import SSLv3_METHOD
options = CertificateOptions(..., method=SSLv3_METHOD)
The somewhat confusingly-named OpenSSL.SSL.SSLv23_METHOD is also supported (to enable SSLv3 or better, based on negotiation).
SSLv2 is insecure; it is explicitly not supported and will be disabled in all configurations.
Additionally, it is possible to limit the acceptable ciphers for your connection by passing an IAcceptableCiphers object to CertificateOptions.
Since Twisted uses a secure cipher configuration by default, it is discouraged to do so unless absolutely necessary.
Application Layer Protocol Negotiation (ALPN) and Next Protocol Negotiation (NPN)¶
ALPN and NPN are TLS extensions that can be used by clients and servers to negotiate what application-layer protocol will be spoken once the encrypted connection is established. This avoids the need for extra custom round trips once the encrypted connection is established. It is implemented as a standard part of the TLS handshake.
NPN is supported from OpenSSL version 1.0.1. ALPN is the newer of the two protocols, supported in OpenSSL versions 1.0.2 onward. These functions require pyOpenSSL version 0.15 or higher. To query the methods supported by your system, use twisted.internet.ssl.protocolNegotiationMechanisms. It will return a collection of flags indicating support for NPN and/or ALPN.
twisted.internet.ssl.CertificateOptions and twisted.internet.ssl.optionsForClientTLS allow for selecting the protocols your program is willing to speak after the connection is established.
On the server=side you will have:
from twisted.internet.ssl import CertificateOptions
options = CertificateOptions(..., acceptableProtocols=[b'h2', b'http/1.1'])
and for clients:
from twisted.internet.ssl import optionsForClientTLS
options = optionsForClientTLS(hostname=hostname, acceptableProtocols=[b'h2', b'http/1.1'])
Twisted will attempt to use both ALPN and NPN, if they’re available, to maximise compatibility with peers. If both ALPN and NPN are supported by the peer, the result from ALPN is preferred.
For NPN, the client selects the protocol to use; For ALPN, the server does. If Twisted is acting as the peer who is supposed to select the protocol, it will prefer the earliest protocol in the list that is supported by both peers.
To determine what protocol was negotiated, after the connection is done, use TLSMemoryBIOProtocol.negotiatedProtocol.
It will return one of the protocol names passed to the acceptableProtocols parameter.
It will return None if the peer did not offer ALPN or NPN.
It can also return None if no overlap could be found and the connection was established regardless (some peers will do this: Twisted will not).
In this case, the protocol that should be used is whatever protocol would have been used if negotiation had not been attempted at all.
Warning
If ALPN or NPN are used and no overlap can be found, then the remote peer may choose to terminate the connection. This may cause the TLS handshake to fail, or may result in the connection being torn down immediately after being made. If Twisted is the selecting peer (that is, Twisted is the server and ALPN is being used, or Twisted is the client and NPN is being used), and no overlap can be found, Twisted will always choose to fail the handshake rather than allow an ambiguous connection to set up.
An example of using this functionality can be found in this example script for clients and this example script for servers.
Conclusion¶
After reading through this tutorial, you should be able to:
- Use
listenSSLandconnectSSLto create servers and clients that use TLS - Use
startTLSto switch a channel from being unencrypted to using TLS mid-connection - Add server and client support for client authentication
UDP Networking¶
Overview¶
Unlike TCP, UDP has no notion of connections. A UDP socket can receive datagrams from any server on the network and send datagrams to any host on the network. In addition, datagrams may arrive in any order, never arrive at all, or be duplicated in transit.
Since there are no connections, we only use a single object, a protocol, for each UDP socket. We then use the reactor to connect this protocol to a UDP transport, using the twisted.internet.interfaces.IReactorUDP reactor API.
DatagramProtocol¶
The class where you actually implement the protocol parsing and handling will usually be descended from twisted.internet.protocol.DatagramProtocol or from one of its convenience children.
The DatagramProtocol class receives datagrams and can send them out over the network.
Received datagrams include the address they were sent from.
When sending datagrams the destination address must be specified.
Here is a simple example:
from twisted.internet.protocol import DatagramProtocol
from twisted.internet import reactor
class Echo(DatagramProtocol):
def datagramReceived(self, data, (host, port)):
print "received %r from %s:%d" % (data, host, port)
self.transport.write(data, (host, port))
reactor.listenUDP(9999, Echo())
reactor.run()
As you can see, the protocol is registered with the reactor. This means it may be persisted if it’s added to an application, and thus it has startProtocol and stopProtocol methods that will get called when the protocol is connected and disconnected from a UDP socket.
The protocol’s transport attribute will implement the twisted.internet.interfaces.IUDPTransport interface.
Notice that the host argument should be an IP address, not a hostname.
If you only have the hostname use reactor.resolve() to resolve the address (see twisted.internet.interfaces.IReactorCore.resolve).
Adopted Datagram Ports¶
It is also possible to add an existing SOCK_DGRAM file descriptor to the reactor using the adoptDatagramPort API.
Here is a simple example:
import socket
from twisted.internet.protocol import DatagramProtocol
from twisted.internet import reactor
class Echo(DatagramProtocol):
def datagramReceived(self, data, (host, port)):
print "received %r from %s:%d" % (data, host, port)
self.transport.write(data, (host, port))
portSocket = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
# Make the port non-blocking and start it listening.
portSocket.setblocking(False)
portSocket.bind(('127.0.0.1', 9999))
# Now pass the port file descriptor to the reactor
port = reactor.adoptDatagramPort(
portSocket.fileno(), socket.AF_INET, Echo())
# The portSocket should be cleaned up by the process that creates it.
portSocket.close()
reactor.run()
Note
- You must ensure that the socket is non-blocking before passing its file descriptor to adoptDatagramPort.
- adoptDatagramPort cannot (currently) detect the family of the adopted socket so you must ensure that you pass the correct socket family argument.
- The reactor will not shutdown the socket. It is the responsibility of the process that created the socket to shutdown and clean up the socket when it is no longer needed.
Connected UDP¶
A connected UDP socket is slightly different from a standard one as it can only send and receive datagrams to/from a single address. However this does not in any way imply a connection as datagrams may still arrive in any order and the port on the other side may have no one listening. The benefit of the connected UDP socket is that it may provide notification of undelivered packages. This depends on many factors (almost all of which are out of the control of the application) but still presents certain benefits which occasionally make it useful.
Unlike a regular UDP protocol, we do not need to specify where to send datagrams and are not told where they came from since they can only come from the address to which the socket is ‘connected’.
from twisted.internet.protocol import DatagramProtocol
from twisted.internet import reactor
class Helloer(DatagramProtocol):
def startProtocol(self):
host = "192.168.1.1"
port = 1234
self.transport.connect(host, port)
print "now we can only send to host %s port %d" % (host, port)
self.transport.write("hello") # no need for address
def datagramReceived(self, data, (host, port)):
print "received %r from %s:%d" % (data, host, port)
# Possibly invoked if there is no server listening on the
# address to which we are sending.
def connectionRefused(self):
print "No one listening"
# 0 means any port, we don't care in this case
reactor.listenUDP(0, Helloer())
reactor.run()
Note that connect(), like write() will only accept IP addresses, not unresolved hostnames.
To obtain the IP of a hostname use reactor.resolve() , e.g.:
from twisted.internet import reactor
def gotIP(ip):
print "IP of 'example.com' is", ip
reactor.callLater(3, reactor.stop)
reactor.resolve('example.com').addCallback(gotIP)
reactor.run()
Connecting to a new address after a previous connection or making a connected port unconnected are not currently supported, but likely will be in the future.
Multicast UDP¶
Multicast allows a process to contact multiple hosts with a single packet, without knowing the specific IP address of any of the hosts. This is in contrast to normal, or unicast, UDP, where each datagram has a single IP as its destination. Multicast datagrams are sent to special multicast group addresses (in the IPv4 range 224.0.0.0 to 239.255.255.255), along with a corresponding port. In order to receive multicast datagrams, you must join that specific group address. However, any UDP socket can send to multicast addresses.
from twisted.internet.protocol import DatagramProtocol
from twisted.internet import reactor
class MulticastPingPong(DatagramProtocol):
def startProtocol(self):
"""
Called after protocol has started listening.
"""
# Set the TTL>1 so multicast will cross router hops:
self.transport.setTTL(5)
# Join a specific multicast group:
self.transport.joinGroup("228.0.0.5")
def datagramReceived(self, datagram, address):
print "Datagram %s received from %s" % (repr(datagram), repr(address))
if datagram == "Client: Ping":
# Rather than replying to the group multicast address, we send the
# reply directly (unicast) to the originating port:
self.transport.write("Server: Pong", address)
# We use listenMultiple=True so that we can run MulticastServer.py and
# MulticastClient.py on same machine:
reactor.listenMulticast(8005, MulticastPingPong(),
listenMultiple=True)
reactor.run()
As with UDP, with multicast there is no server/client differentiation at the protocol level.
Our server example is very simple and closely resembles a normal listenUDP protocol implementation.
The main difference is that instead of listenUDP, listenMulticast is called with the port number.
The server calls joinGroup to join a multicast group.
A DatagramProtocol that is listening with multicast and has joined a group can receive multicast datagrams, but also unicast datagrams sent directly to its address.
The server in the example above sends such a unicast message in reply to the multicast message it receives from the client.
from twisted.internet.protocol import DatagramProtocol
from twisted.internet import reactor
class MulticastPingClient(DatagramProtocol):
def startProtocol(self):
# Join the multicast address, so we can receive replies:
self.transport.joinGroup("228.0.0.5")
# Send to 228.0.0.5:8005 - all listeners on the multicast address
# (including us) will receive this message.
self.transport.write('Client: Ping', ("228.0.0.5", 8005))
def datagramReceived(self, datagram, address):
print "Datagram %s received from %s" % (repr(datagram), repr(address))
reactor.listenMulticast(8005, MulticastPingClient(), listenMultiple=True)
reactor.run()
Note that a multicast socket will have a default TTL (time to live) of 1. That is, datagrams won’t traverse more than one router hop, unless a higher TTL is set with setTTL. Other functionality provided by the multicast transport includes setOutgoingInterface and setLoopbackMode – see IMulticastTransport for more information.
Broadcast UDP¶
Broadcast allows a different way of contacting several unknown hosts.
Broadcasting via UDP sends a packet out to all hosts on the local network by sending to a magic broadcast address ("<broadcast>").
This broadcast is filtered by routers by default, and there are no “groups” like multicast, only different ports.
Broadcast is enabled by passing True to setBroadcastAllowed on the port.
Checking the broadcast status can be done with getBroadcastAllowed on the port.
For a complete example of this feature, see udpbroadcast.py.
IPv6¶
UDP sockets can also bind to IPv6 addresses to support sending and receiving datagrams over IPv6.
By passing an IPv6 address to listenUDP‘s interface argument, the reactor will start an IPv6 socket that can be used to send and receive UDP datagrams.
from twisted.internet.protocol import DatagramProtocol
from twisted.internet import reactor
class Echo(DatagramProtocol):
def datagramReceived(self, data, addr):
print "received %r from %s" % (data, addr)
self.transport.write(data, addr)
reactor.listenUDP(8006, Echo(), interface='::')
reactor.run()
Using Processes¶
Overview¶
Along with connection to servers across the internet, Twisted also connects to local processes with much the same API. The API is described in more detail in the documentation of:
Running Another Process¶
Processes are run through the reactor,
using reactor.spawnProcess . Pipes are created to the child process,
and added to the reactor core so that the application will not block while
sending data into or pulling data out of the new
process. reactor.spawnProcess requires two arguments,
processProtocol and executable , and optionally takes
several more: args , environment ,
path , userID , groupID ,
usePTY , and childFDs . Not all of these are
available on Windows.
from twisted.internet import reactor
processProtocol = MyProcessProtocol()
reactor.spawnProcess(processProtocol, executable, args=[program, arg1, arg2],
env={'HOME': os.environ['HOME']}, path,
uid, gid, usePTY, childFDs)
processProtocolshould be an instance of a subclass of twisted.internet.protocol.ProcessProtocol . The interface is described below.executableis the full path of the program to run. It will be connected to processProtocol.argsis a list of command line arguments to be passed to the process.args[0]should be the name of the process.envis a dictionary containing the environment to pass through to the process.pathis the directory to run the process in. The child will switch to the given directory just before starting the new program. The default is to stay in the current directory.uidandgidare the user ID and group ID to run the subprocess as. Of course, changing identities will be more likely to succeed if you start as root.usePTYspecifies whether the child process should be run with a pty, or if it should just get a pair of pipes. Whether a program needs to be run with a PTY or not depends on the particulars of that program. Often, programs which primarily interact with users via a terminal do need a PTY.childFDslets you specify how the child’s file descriptors should be set up. Each key is a file descriptor number (an integer) as seen by the child. 0, 1, and 2 are usually stdin, stdout, and stderr, but some programs may be instructed to use additional fds through command-line arguments or environment variables. Each value is either an integer specifying one of the parent’s current file descriptors, the string “r” which creates a pipe that the parent can read from, or the string “w” which creates a pipe that the parent can write to. IfchildFDsis not provided, a default is used which creates the usual stdin-writer, stdout-reader, and stderr-reader pipes.
args and env have empty default values, but
many programs depend upon them to be set correctly. At the very least,
args[0] should probably be the same as executable .
If you just provide os.environ for env , the child
program will inherit the environment from the current process, which is
usually the civilized thing to do (unless you want to explicitly clean the
environment as a security precaution). The default is to give an empty env to the child.
reactor.spawnProcess returns an instance that implements IProcessTransport.
Writing a ProcessProtocol¶
The ProcessProtocol you pass to spawnProcess is your
interaction with the process. It has a very similar signature to a regular
Protocol, but it has several extra methods to deal with events specific to
a process. In our example, we will interface with ‘wc’ to create a word count
of user-given text. First, we’ll start by importing the required modules, and
writing the initialization for our ProcessProtocol.
from twisted.internet import protocol
class WCProcessProtocol(protocol.ProcessProtocol):
def __init__(self, text):
self.text = text
When the ProcessProtocol is connected to the protocol, it has the connectionMade method called. In our protocol, we will write our text to the standard input of our process and then close standard input, to let the process know we are done writing to it.
...
def connectionMade(self):
self.transport.write(self.text)
self.transport.closeStdin()
At this point, the process has received the data, and it’s time for us
to read the results. Instead of being received in dataReceived ,
data from standard output is received in outReceived . This is
to distinguish it from data on standard error.
...
def outReceived(self, data):
fieldLength = len(data) / 3
lines = int(data[:fieldLength])
words = int(data[fieldLength:fieldLength*2])
chars = int(data[fieldLength*2:])
self.transport.loseConnection()
self.receiveCounts(lines, words, chars)
Now, the process has parsed the output, and ended the connection to the process. Then it sends the results on to the final method, receiveCounts. This is for users of the class to override, so as to do other things with the data. For our demonstration, we will just print the results.
...
def receiveCounts(self, lines, words, chars):
print 'Received counts from wc.'
print 'Lines:', lines
print 'Words:', words
print 'Characters:', chars
We’re done! To use our WCProcessProtocol, we create an instance, and pass it to spawnProcess.
from twisted.internet import reactor
wcProcess = WCProcessProtocol("accessing protocols through Twisted is fun!\n")
reactor.spawnProcess(wcProcess, 'wc', ['wc'])
reactor.run()
Things that can happen to your ProcessProtocol¶
These are the methods that you can usefully override in your subclass of
ProcessProtocol :
.connectionMade(): This is called when the program is started, and makes a good place to write data into the stdin pipe (usingself.transport.write)..outReceived(data): This is called with data that was received from the process’ stdout pipe. Pipes tend to provide data in larger chunks than sockets (one kilobyte is a common buffer size), so you may not experience the “random dribs and drabs” behavior typical of network sockets, but regardless you should be prepared to deal if you don’t get all your data in a single call. To do it properly,outReceivedought to simply accumulate the data and put off doing anything with it until the process has finished..errReceived(data): This is called with data from the process’ stderr pipe. It behaves just likeoutReceived..inConnectionLost: This is called when the reactor notices that the process’ stdin pipe has closed. Programs don’t typically close their own stdin, so this will probably get called when your ProcessProtocol has shut down the write side withself.transport.loseConnection..outConnectionLost: This is called when the program closes its stdout pipe. This usually happens when the program terminates..errConnectionLost: Same asoutConnectionLost, but for stderr instead of stdout..processExited(status): This is called when the child process has been reaped, and receives information about the process’ exit status. The status is passed in the form of a Failure instance, created with a.valuethat either holds a ProcessDone object if the process terminated normally (it died of natural causes instead of receiving a signal, and if the exit code was 0), or a ProcessTerminated object (with an.exitCodeattribute) if something went wrong..processEnded(status): This is called when all the file descriptors associated with the child process have been closed and the process has been reaped. This means it is the last callback which will be made onto aProcessProtocol. Thestatusparameter has the same meaning as it does forprocessExited.
The base-class definitions of most of these functions are no-ops. This will result in all stdout and stderr being thrown away. Note that it is important for data you don’t care about to be thrown away: if the pipe were not read, the child process would eventually block as it tried to write to a full pipe.
Things you can do from your ProcessProtocol¶
The following are the basic ways to control the child process:
self.transport.write(data): Stuff some data in the stdin pipe. Note that thiswritemethod will queue any data that can’t be written immediately. Writing will resume in the future when the pipe becomes writable again.self.transport.closeStdin: Close the stdin pipe. Programs which act as filters (reading from stdin, modifying the data, writing to stdout) usually take this as a sign that they should finish their job and terminate. For these programs, it is important to close stdin when you’re done with it, otherwise the child process will never quit.self.transport.closeStdout: Not usually called, since you’re putting the process into a state where any attempt to write to stdout will cause a SIGPIPE error. This isn’t a nice thing to do to the poor process.self.transport.closeStderr: Not usually called, same reason ascloseStdout.self.transport.loseConnection: Close all three pipes.self.transport.signalProcess('KILL'): Kill the child process. This will eventually result inprocessEndedbeing called.
Verbose Example¶
Here is an example that is rather verbose about exactly when all the
methods are called. It writes a number of lines into the wc
program and then parses the output.
#!/usr/bin/env python
# Copyright (c) Twisted Matrix Laboratories.
# See LICENSE for details.
from twisted.internet import protocol
from twisted.internet import reactor
import re
class MyPP(protocol.ProcessProtocol):
def __init__(self, verses):
self.verses = verses
self.data = ""
def connectionMade(self):
print "connectionMade!"
for i in range(self.verses):
self.transport.write("Aleph-null bottles of beer on the wall,\n" +
"Aleph-null bottles of beer,\n" +
"Take one down and pass it around,\n" +
"Aleph-null bottles of beer on the wall.\n")
self.transport.closeStdin() # tell them we're done
def outReceived(self, data):
print "outReceived! with %d bytes!" % len(data)
self.data = self.data + data
def errReceived(self, data):
print "errReceived! with %d bytes!" % len(data)
def inConnectionLost(self):
print "inConnectionLost! stdin is closed! (we probably did it)"
def outConnectionLost(self):
print "outConnectionLost! The child closed their stdout!"
# now is the time to examine what they wrote
#print "I saw them write:", self.data
(dummy, lines, words, chars, file) = re.split(r'\s+', self.data)
print "I saw %s lines" % lines
def errConnectionLost(self):
print "errConnectionLost! The child closed their stderr."
def processExited(self, reason):
print "processExited, status %d" % (reason.value.exitCode,)
def processEnded(self, reason):
print "processEnded, status %d" % (reason.value.exitCode,)
print "quitting"
reactor.stop()
pp = MyPP(10)
reactor.spawnProcess(pp, "wc", ["wc"], {})
reactor.run()
The exact output of this program depends upon the relative timing of some un-synchronized events. In particular, the program may observe the child process close its stderr pipe before or after it reads data from the stdout pipe. One possible transcript would look like this:
% ./process.py
connectionMade!
inConnectionLost! stdin is closed! (we probably did it)
errConnectionLost! The child closed their stderr.
outReceived! with 24 bytes!
outConnectionLost! The child closed their stdout!
I saw 40 lines
processEnded, status 0
quitting
Main loop terminated.
%
Doing it the Easy Way¶
Frequently, one just needs a simple way to get all the output from a
program. In the blocking world, you might use commands.getoutput from the standard library, but
using that in an event-driven program will cause everything else to stall
until the command finishes. (in addition, the SIGCHLD handler used by that
function does not play well with Twisted’s own signal handling). For these
cases, the twisted.internet.utils.getProcessOutput
function can be used. Here is a simple example:
from twisted.internet import protocol, utils, reactor
from twisted.python import failure
from cStringIO import StringIO
class FortuneQuoter(protocol.Protocol):
fortune = '/usr/games/fortune'
def connectionMade(self):
output = utils.getProcessOutput(self.fortune)
output.addCallbacks(self.writeResponse, self.noResponse)
def writeResponse(self, resp):
self.transport.write(resp)
self.transport.loseConnection()
def noResponse(self, err):
self.transport.loseConnection()
if __name__ == '__main__':
f = protocol.Factory()
f.protocol = FortuneQuoter
reactor.listenTCP(10999, f)
reactor.run()
If you only need the final exit code (like commands.getstatusoutput(cmd)[0] ), the twisted.internet.utils.getProcessValue function is
useful. Here is an example:
from twisted.internet import utils, reactor
def printTrueValue(val):
print "/bin/true exits with rc=%d" % val
output = utils.getProcessValue('/bin/false')
output.addCallback(printFalseValue)
def printFalseValue(val):
print "/bin/false exits with rc=%d" % val
reactor.stop()
output = utils.getProcessValue('/bin/true')
output.addCallback(printTrueValue)
reactor.run()
Mapping File Descriptors¶
“stdin” , “stdout” , and “stderr” are just conventions. Programs which operate as filters generally accept input on fd0, write their output on fd1, and emit error messages on fd2. This is common enough that the standard C library provides macros like “stdin” to mean fd0, and shells interpret the pipe character “|” to mean “redirect fd1 from one command into fd0 of the next command” .
But these are just conventions, and programs are free to use additional file descriptors or even ignore the standard three entirely. The”childFDs” argument allows you to specify exactly what kind of files descriptors the child process should be given.
Each child FD can be put into one of three states:
- Mapped to a parent FD: this causes the child’s reads and writes to come from or go to the same source/destination as the parent.
- Feeding into a pipe which can be read by the parent.
- Feeding from a pipe which the parent writes into.
Mapping the child FDs to the parent’s is very commonly used to send the child’s stderr output to the same place as the parent’s. When you run a program from the shell, it will typically leave fds 0, 1, and 2 mapped to the shell’s 0, 1, and 2, allowing you to see the child program’s output on the same terminal you used to launch the child. Likewise, inetd will typically map both stdin and stdout to the network socket, and may map stderr to the same socket or to some kind of logging mechanism. This allows the child program to be implemented with no knowledge of the network: it merely speaks its protocol by doing reads on fd0 and writes on fd1.
Feeding into a parent’s read pipe is used to gather output from the child, and is by far the most common way of interacting with child processes.
Feeding from a parent’s write pipe allows the parent to control the child. Programs like “bc” or “ftp” can be controlled this way, by writing commands into their stdin stream.
The “childFDs” dictionary maps file descriptor numbers (as will be seen by the child process) to one of these three states. To map the fd to one of the parent’s fds, simply provide the fd number as the value. To map it to a read pipe, use the string “r” as the value. To map it to a write pipe, use the string “w” .
For example, the default mapping sets up the standard stdin/stdout/stderr pipes. It is implemented with the following dictionary:
childFDs = { 0: "w", 1: "r", 2: "r" }
To launch a process which reads and writes to the same places that the parent python program does, use this:
childFDs = { 0: 0, 1: 1, 2: 2}
To write into an additional fd (say it is fd number 4), use this:
childFDs = { 0: "w", 1: "r", 2: "r" , 4: "w"}
ProcessProtocols with extra file descriptors¶
When you provide a “childFDs” dictionary with more than the normal
three fds, you need additional methods to access those pipes. These methods
are more generalized than the .outReceived ones described above.
In fact, those methods (outReceived and
errReceived ) are actually just wrappers left in for
compatibility with older code, written before this generalized fd mapping was
implemented. The new list of things that can happen to your ProcessProtocol
is as follows:
.connectionMade: This is called when the program is started..childDataReceived(childFD, data): This is called with data that was received from one of the process’ output pipes (i.e. where the childFDs value was “r” . The actual file number (from the point of view of the child process) is in “childFD” . For compatibility, the default implementation of.childDataReceiveddispatches to.outReceivedor.errReceivedwhen “childFD” is 1 or 2..childConnectionLost(childFD): This is called when the reactor notices that one of the process’ pipes has been closed. This either means you have just closed down the parent’s end of the pipe (with.transport.closeChildFD), the child closed the pipe explicitly (sometimes to indicate EOF), or the child process has terminated and the kernel has closed all of its pipes. The “childFD” argument tells you which pipe was closed. Note that you can only find out about file descriptors which were mapped to pipes: when they are mapped to existing fds the parent has no way to notice when they’ve been closed. For compatibility, the default implementation dispatches to.inConnectionLost,.outConnectionLost, or.errConnectionLost..processEnded(status): This is called when the child process has been reaped, and all pipes have been closed. This insures that all data written by the child prior to its death will be received before.processEndedis invoked.
In addition to those methods, there are other methods available to influence the child process:
self.transport.writeToChild(childFD, data): Stuff some data into an input pipe..writesimply writes to childFD=0.self.transport.closeChildFD(childFD): Close one of the child’s pipes. Closing an input pipe is a common way to indicate EOF to the child process. Closing an output pipe is neither very friendly nor very useful.
Examples¶
GnuPG, the encryption program, can use additional file descriptors to accept a passphrase and emit status output. These are distinct from stdin (used to accept the crypttext), stdout (used to emit the plaintext), and stderr (used to emit human-readable status/warning messages). The passphrase FD reads until the pipe is closed and uses the resulting string to unlock the secret key that performs the actual decryption. The status FD emits machine-parseable status messages to indicate the validity of the signature, which key the message was encrypted to, etc.
gpg accepts command-line arguments to specify what these fds are, and then assumes that they have been opened by the parent before the gpg process is started. It simply performs reads and writes to these fd numbers.
To invoke gpg in decryption/verification mode, you would do something like the following:
class GPGProtocol(ProcessProtocol):
def __init__(self, crypttext):
self.crypttext = crypttext
self.plaintext = ""
self.status = ""
def connectionMade(self):
self.transport.writeToChild(3, self.passphrase)
self.transport.closeChildFD(3)
self.transport.writeToChild(0, self.crypttext)
self.transport.closeChildFD(0)
def childDataReceived(self, childFD, data):
if childFD == 1: self.plaintext += data
if childFD == 4: self.status += data
def processEnded(self, status):
rc = status.value.exitCode
if rc == 0:
self.deferred.callback(self)
else:
self.deferred.errback(rc)
def decrypt(crypttext):
gp = GPGProtocol(crypttext)
gp.deferred = Deferred()
cmd = ["gpg", "--decrypt", "--passphrase-fd", "3", "--status-fd", "4",
"--batch"]
p = reactor.spawnProcess(gp, cmd[0], cmd, env=None,
childFDs={0:"w", 1:"r", 2:2, 3:"w", 4:"r"})
return gp.deferred
In this example, the status output could be parsed after the fact. It could, of course, be parsed on the fly, as it is a simple line-oriented protocol. Methods from LineReceiver could be mixed in to make this parsing more convenient.
The stderr mapping (“2:2” ) used will cause any GPG errors to be emitted by the parent program, just as if those errors had caused in the parent itself. This is sometimes desirable (it roughly corresponds to letting exceptions propagate upwards), especially if you do not expect to encounter errors in the child process and want them to be more visible to the end user. The alternative is to map stderr to a read-pipe and handle any such output from within the ProcessProtocol (roughly corresponding to catching the exception locally).
Introduction to Deferreds¶
This document introduces Deferreds, Twisted’s preferred mechanism for controlling the flow of asynchronous code. Don’t worry if you don’t know what that means yet – that’s why you are here!
It is intended for newcomers to Twisted, and was written particularly to help people read and understand code that already uses Deferreds.
This document assumes you have a good working knowledge of Python. It assumes no knowledge of Twisted.
By the end of the document, you should understand what Deferreds are and how they can be used to coordinate asynchronous code. In particular, you should be able to:
- Read and understand code that uses Deferreds
- Translate from synchronous code to asynchronous code and back again
- Implement any sort of error-handling for asynchronous code that you wish
The joy of order¶
When you write Python code, one prevailing, deep, unassailled assumption is that a line of code within a block is only ever executed after the preceding line is finished.
pod_bay_doors.open()
pod.launch()
The pod bay doors open, and only then does the pod launch. That’s wonderful. One-line-after-another is a built-in mechanism in the language for encoding the order of execution. It’s clear, terse, and unambiguous.
Exceptions make things more complicated.
If pod_bay_doors.open() raises an exception, then we cannot know with certainty that it completed, and so it would be wrong to proceed blithely to the next line.
Thus, Python gives us try, except, finally, and else, which together model almost every conceivable way of handling a raised exception, and tend to work really well.
Function application is the other way we encode order of execution:
pprint(sorted(x.get_names()))
First x.get_names() gets called, then sorted is called with its return value, and then pprint with whatever sorted returns.
It can also be written as:
names = x.get_names()
sorted_names = sorted(names)
pprint(sorted_names)
Sometimes it leads us to encode the order when we don’t need to, as in this example:
total = 0
for account in accounts:
total += account.get_balance()
print "Total balance $%s" % (total,)
But that’s normally not such a big deal.
All in all, things are pretty good, and all of the explanation above is laboring familiar and obvious points. One line comes after another and one thing happens after another, and both facts are inextricably tied.
But what if we had to do it differently?
A hypothetical problem¶
What if we could no longer rely on the previous line of code being finished (whatever that means) before we started to interpret & execute the next line of code?
What if pod_bay_doors.open() returned immediately, triggering something somewhere else that would eventually open the pod bay doors, recklessly sending the Python interpreter plunging into pod.launch() ?
That is, what would we do if the order of execution did not match the order of lines of Python? If “returning” no longer meant “finishing”?
Asynchronous operations?
How would we prevent our pod from hurtling into the still-closed doors? How could we respond to a potential failure to open the doors at all? What if opening the doors gave us some crucial information that we needed in order to launch the pod? How would we get access to that information?
And, crucially, since we are writing code, how can we write our code so that we can build other code on top of it?
The components of a solution¶
We would still need a way of saying “do this only when that has finished”.
We would need a way of distinguishing between successful completion and interrupted processing, normally modeled with try, expect, else, and finally.
We need a mechanism for getting return failures and exception information from the thing that just executed to the thing that needs to happen next.
We need somehow to be able to operate on results that we don’t have yet. Instead of acting, we need to make and encode plans for how we would act if we could.
Unless we hack the interpreter somehow, we would need to build this with the Python language constructs we are given: methods, functions, objects, and the like.
Perhaps we want something that looks a little like this:
placeholder = pod_bay_doors.open()
placeholder.when_done(pod.launch)
One solution: Deferred¶
Twisted tackles this problem with Deferreds, a type of object designed to do one thing, and one thing only: encode an order of execution separately from the order of lines in Python source code.
It doesn’t deal with threads, parallelism, signals, or subprocesses. It doesn’t know anything about an event loop, greenlets, or scheduling. All it knows about is what order to do things in. How does it know that? Because we explicitly tell it the order that we want.
Thus, instead of writing:
pod_bay_doors.open()
pod.launch()
We write:
d = pod_bay_doors.open()
d.addCallback(lambda ignored: pod.launch())
That introduced a dozen new concepts in a couple of lines of code, so let’s break it down. If you think you’ve got it, you might want to skip to the next section.
Here, pod_bay_doors.open() is returning a Deferred, which we assign to d.
We can think of d as a placeholder, representing the value that open() will eventually return when it finally gets around to finishing.
To “do this next”, we add a callback to d.
A callback is a function that will be called with whatever open() eventually returns.
In this case, we don’t care, so we make a function with a single, ignored parameter that just calls pod.launch().
So, we’ve replaced the “order of lines is order of execution” with a deliberate, in-Python encoding of the order of execution, where d represents the particular flow and d.addCallback replaces “new line”.
Of course, programs generally consist of more than two lines, and we still don’t know how to deal with failure.
Getting it right: The failure cases¶
In what follows, we are going to take each way of expressing order of operations in normal Python (using lines of code and try/except) and translate them into an equivalent code built with Deferred objects.
This is going to be a bit painstaking, but if you want to really understand how to use Deferreds and maintain code that uses them, it is worth understanding each example below.
One thing, then another, then another¶
Recall our example from earlier:
pprint(sorted(x.get_names()))
Also written as:
names = x.get_names()
sorted_names = sorted(names)
pprint(sorted_names)
What if neither get_names nor sorted can be relied on to finish before they return?
That is, if both are asynchronous operations?
Well, in Twisted-speak they would return Deferreds and so we would write:
d = x.get_names()
d.addCallback(sorted)
d.addCallback(pprint)
Eventually, sorted will get called with whatever get_names finally delivers.
When sorted finishes, pprint will be called with whatever it delivers.
We could also write this as:
x.get_names().addCallback(sorted).addCallback(pprint)
Since d.addCallback returns d.
Simple failure handling¶
We often want to write code equivalent to this:
try:
x.get_names()
except Exception, e:
report_error(e)
How would we write this with Deferreds?
d = x.get_names()
d.addErrback(report_error)
errback is the Twisted name for a callback that is called when an error is received.
This glosses over an important detail.
Instead of getting the exception object e, report_error would get a Failure object, which has all of the useful information that e does, but is optimized for use with Deferreds.
We’ll dig into that a bit later, after we’ve dealt with all of the other combinations of exceptions.
Handle an error, but do something else on success¶
What if we want to do something after our try block if it actually worked?
Abandoning our contrived examples and reaching for generic variable names, we get:
try:
y = f()
except Exception, e:
g(e)
else:
h(y)
Well, we’d write it like this with Deferreds:
d = f()
d.addCallbacks(h, g)
Where addCallbacks means “add a callback and an errback at the same time”.
h is the callback, g is the errback.
Now that we have addCallbacks along with addErrback and addCallback, we can match any possible combination of try, except, else, and finally by varying the order in which we call them.
Explaining exactly how it works is tricky (although the Deferred reference does rather a good job), but once we’re through all of the examples it ought to be clearer.
Handle an error, then proceed anyway¶
What if we want to do something after our try/except block, regardless of whether or not there was an exception?
That is, what if we wanted to do the equivalent of this generic code:
try:
y = f()
except Exception, e:
y = g(e)
h(y)
And with Deferreds:
d = f()
d.addErrback(g)
d.addCallback(h)
Because addErrback returns d, we can chain the calls like so:
f().addErrback(g).addCallback(h)
The order of addErrback and addCallback matters.
In the next section, we can see what would happen when we swap them around.
Handle an error for the entire operation¶
What if we want to wrap up a multi-step operation in one exception handler?
try:
y = f()
z = h(y)
except Exception, e:
g(e)
With Deferreds, it would look like this:
d = f()
d.addCallback(h)
d.addErrback(g)
Or, more succinctly:
d = f().addCallback(h).addErrback(g)
Do something regardless¶
What about finally?
How do we do something regardless of whether or not there was an exception?
How do we translate this:
try:
y = f()
finally:
g()
Well, roughly we do this:
d = f()
d.addBoth(g)
This adds g as both the callback and the errback.
It is equivalent to:
d.addCallbacks(g, g)
Why “roughly”?
Because if f raises, g will be passed a Failure object representing the exception.
Otherwise, g will be passed the asynchronous equivalent of the return value of f() (i.e. y).
Inline callbacks - using ‘yield’¶
Twisted features a decorator named inlineCallbacks which allows you to work with Deferreds without writing callback functions.
This is done by writing your code as generators, which yield Deferreds instead of attaching callbacks.
Consider the following function written in the traditional Deferred style:
def getUsers():
d = makeRequest("GET", "/users")
d.addCallback(json.loads)
return d
using inlineCallbacks, we can write this as:
from twisted.internet.defer import inlineCallbacks, returnValue
@inlineCallbacks
def getUsers(self):
responseBody = yield makeRequest("GET", "/users")
returnValue(json.loads(responseBody))
a couple of things are happening here:
- instead of calling
addCallbackon theDeferredreturned bymakeRequest, we yield it. This causes Twisted to return theDeferred‘s result to us. - we use
returnValueto propagate the final result of our function. Because this function is a generator, we cannot use the return statement; that would be a syntax error.
Note
New in version 15.0.
On Python 3.3 and above, instead of writing returnValue(json.loads(responseBody)) you can instead write return json.loads(responseBody).
This can be a significant readability advantage, but unfortunately if you need compatibility with Python 2, this isn’t an option.
Both versions of getUsers present exactly the same API to their callers: both return a Deferred that fires with the parsed JSON body of the request.
Though the inlineCallbacks version looks like synchronous code, which blocks while waiting for the request to finish, each yield statement allows other code to run while waiting for the Deferred being yielded to fire.
inlineCallbacks become even more powerful when dealing with complex control flow and error handling.
For example, what if makeRequest fails due to a connection error?
For the sake of this example, let’s say we want to log the exception and return an empty list.
def getUsers():
d = makeRequest("GET", "/users")
def connectionError(failure):
failure.trap(ConnectionError)
log.failure("makeRequest failed due to connection error",
failure)
return []
d.addCallbacks(json.loads, connectionError)
return d
With inlineCallbacks, we can rewrite this as:
@inlineCallbacks
def getUsers(self):
try:
responseBody = yield makeRequest("GET", "/users")
except ConnectionError:
log.failure("makeRequest failed due to connection error")
returnValue([])
returnValue(json.loads(responseBody))
Our exception handling is simplified because we can use Python’s familiar try / except syntax for handling ConnectionErrors.
Conclusion¶
You have been introduced to asynchronous code and have seen how to use Deferreds to:
- Do something after an asynchronous operation completes successfully
- Use the result of a successful asynchronous operation
- Catch errors in asynchronous operations
- Do one thing if an operation succeeds, and a different thing if it fails
- Do something after an error has been handled successfully
- Wrap multiple asynchronous operations with one error handler
- Do something after an asynchronous operation, regardless of whether it succeeded or failed
- Write code without callbacks using
inlineCallbacks
These are very basic uses of Deferred. For detailed information about how they work, how to combine multiple Deferreds, and how to write code that mixes synchronous and asynchronous APIs, see the Deferred reference. Alternatively, read about how to write functions that generate Deferreds.
Deferred Reference¶
This document is a guide to the behaviour of the twisted.internet.defer.Deferred object, and to various ways you can use them when they are returned by functions.
This document assumes that you are familiar with the basic principle that the Twisted framework is structured around: asynchronous, callback-based programming, where instead of having blocking code in your program or using threads to run blocking code, you have functions that return immediately and then begin a callback chain when data is available.
After reading this document, the reader should expect to be able to deal with most simple APIs in Twisted and Twisted-using code that return Deferreds.
- what sorts of things you can do when you get a Deferred from a function call; and
- how you can write your code to robustly handle errors in Deferred code.
Deferreds¶
Twisted uses the Deferred object to manage the callback
sequence. The client application attaches a series of functions to the
deferred to be called in order when the results of the asynchronous request are
available (this series of functions is known as a series of callbacks , or a callback chain ), together
with a series of functions to be called if there is an error in the
asynchronous request (known as a series of errbacks or an**errback chain** ). The asynchronous library code calls the first
callback when the result is available, or the first errback when an error
occurs, and the Deferred object then hands the results of each
callback or errback function to the next function in the chain.
Callbacks¶
A twisted.internet.defer.Deferred is a promise that a function will at some point have a result. We can attach callback functions to a Deferred, and once it gets a result these callbacks will be called. In addition Deferreds allow the developer to register a callback for an error, with the default behavior of logging the error. The deferred mechanism standardizes the application programmer’s interface with all sorts of blocking or delayed operations.
from twisted.internet import reactor, defer
def getDummyData(inputData):
"""
This function is a dummy which simulates a delayed result and
returns a Deferred which will fire with that result. Don't try too
hard to understand this.
"""
print('getDummyData called')
deferred = defer.Deferred()
# simulate a delayed result by asking the reactor to fire the
# Deferred in 2 seconds time with the result inputData * 3
reactor.callLater(2, deferred.callback, inputData * 3)
return deferred
def cbPrintData(result):
"""
Data handling function to be added as a callback: handles the
data by printing the result
"""
print('Result received: {}'.format(result))
deferred = getDummyData(3)
deferred.addCallback(cbPrintData)
# manually set up the end of the process by asking the reactor to
# stop itself in 4 seconds time
reactor.callLater(4, reactor.stop)
# start up the Twisted reactor (event loop handler) manually
print('Starting the reactor')
reactor.run()
Multiple callbacks¶
Multiple callbacks can be added to a Deferred. The first callback in the Deferred’s callback chain will be called with the result, the second with the result of the first callback, and so on. Why do we need this? Well, consider a Deferred returned by twisted.enterprise.adbapi - the result of a SQL query. A web widget might add a callback that converts this result into HTML, and pass the Deferred onwards, where the callback will be used by twisted to return the result to the HTTP client. The callback chain will be bypassed in case of errors or exceptions.
from twisted.internet import reactor, defer
class Getter:
def gotResults(self, x):
"""
The Deferred mechanism provides a mechanism to signal error
conditions. In this case, odd numbers are bad.
This function demonstrates a more complex way of starting
the callback chain by checking for expected results and
choosing whether to fire the callback or errback chain
"""
if self.d is None:
print("Nowhere to put results")
return
d = self.d
self.d = None
if x % 2 == 0:
d.callback(x*3)
else:
d.errback(ValueError("You used an odd number!"))
def _toHTML(self, r):
"""
This function converts r to HTML.
It is added to the callback chain by getDummyData in
order to demonstrate how a callback passes its own result
to the next callback
"""
return "Result: %s" % r
def getDummyData(self, x):
"""
The Deferred mechanism allows for chained callbacks.
In this example, the output of gotResults is first
passed through _toHTML on its way to printData.
Again this function is a dummy, simulating a delayed result
using callLater, rather than using a real asynchronous
setup.
"""
self.d = defer.Deferred()
# simulate a delayed result by asking the reactor to schedule
# gotResults in 2 seconds time
reactor.callLater(2, self.gotResults, x)
self.d.addCallback(self._toHTML)
return self.d
def cbPrintData(result):
print(result)
def ebPrintError(failure):
import sys
sys.stderr.write(str(failure))
# this series of callbacks and errbacks will print an error message
g = Getter()
d = g.getDummyData(3)
d.addCallback(cbPrintData)
d.addErrback(ebPrintError)
# this series of callbacks and errbacks will print "Result: 12"
g = Getter()
d = g.getDummyData(4)
d.addCallback(cbPrintData)
d.addErrback(ebPrintError)
reactor.callLater(4, reactor.stop)
reactor.run()
Note
- Pay particular attention to the handling of
self.din thegotResultsmethod. Before theDeferredis fired with a result or an error, the attribute is set toNoneso that theGetterinstance no longer has a reference to theDeferredabout to be fired. This has several benefits. First, it avoids any chanceGetter.gotResultswill accidentally fire the sameDeferredmore than once (which would result in anAlreadyCalledErrorexception). Second, it allows a callback on thatDeferredto callGetter.getDummyData(which sets a new value for thedattribute) without causing problems. Third, it makes the Python garbage collector’s job easier by eliminating a reference cycle.
Visual Explanation¶
- Requesting method (data sink) requests data, gets Deferred object.
- Requesting method attaches callbacks to Deferred object.
- When the result is ready, give it to the Deferred
object.
.callback(result)if the operation succeeded,.errback(failure)if it failed. Note thatfailureis typically an instance of a twisted.python.failure.Failure instance. - Deferred object triggers previously-added (call/err)back
with the
resultorfailure. Execution then follows the following rules, going down the chain of callbacks to be processed.- Result of the callback is always passed as the first argument to the next callback, creating a chain of processors.
- If a callback raises an exception, switch to errback.
- An unhandled failure gets passed down the line of
errbacks, this creating an asynchronous analog to a
series to a series of
except:statements. - If an errback doesn’t raise an exception or return a twisted.python.failure.Failure instance, switch to callback.
Errbacks¶
Deferred’s error handling is modeled after Python’s exception handling. In the case that no errors occur, all the callbacks run, one after the other, as described above.
If the errback is called instead of the callback (e.g. because a DB query
raised an error), then a twisted.python.failure.Failure is passed into the first
errback (you can add multiple errbacks, just like with callbacks). You can
think of your errbacks as being like except blocks
of ordinary Python code.
Unless you explicitly raise an error in an except
block, the Exception is caught and stops
propagating, and normal execution continues. The same thing happens with
errbacks: unless you explicitly return a Failure or (re-)raise an exception, the error stops
propagating, and normal callbacks continue executing from that point (using the
value returned from the errback). If the errback does return a Failure or raise an exception, then that is passed to the
next errback, and so on.
Note: If an errback doesn’t return anything, then it effectively
returns None , meaning that callbacks will continue
to be executed after this errback. This may not be what you expect to happen,
so be careful. Make sure your errbacks return a Failure (probably the one that was passed to it), or a
meaningful return value for the next callback.
Also, twisted.python.failure.Failure instances have a useful method called trap, allowing you to effectively do the equivalent of:
try:
# code that may throw an exception
cookSpamAndEggs()
except (SpamException, EggException):
# Handle SpamExceptions and EggExceptions
...
You do this by:
def errorHandler(failure):
failure.trap(SpamException, EggException)
# Handle SpamExceptions and EggExceptions
d.addCallback(cookSpamAndEggs)
d.addErrback(errorHandler)
If none of arguments passed to failure.trap
match the error encapsulated in that Failure , then
it re-raises the error.
There’s another potential “gotcha” here. There’s a
method twisted.internet.defer.Deferred.addCallbacks
which is similar to, but not exactly the same as, addCallback followed by addErrback . In particular, consider these two cases:
# Case 1
d = getDeferredFromSomewhere()
d.addCallback(callback1) # A
d.addErrback(errback1) # B
d.addCallback(callback2)
d.addErrback(errback2)
# Case 2
d = getDeferredFromSomewhere()
d.addCallbacks(callback1, errback1) # C
d.addCallbacks(callback2, errback2)
If an error occurs in callback1 , then for Case 1 errback1 will be called with the failure. For Case
2, errback2 will be called. Be careful with your
callbacks and errbacks.
What this means in a practical sense is in Case 1, the callback in line
A will handle a success condition from getDeferredFromSomewhere ,
and the errback in line B will handle any errors that occur from either the upstream source, or that occur in A . In Case 2, the errback in line C will only handle an error condition raised by getDeferredFromSomewhere ,
it will not do any handling of errors
raised in callback1 .
Unhandled Errors¶
If a Deferred is garbage-collected with an unhandled error (i.e. it would call the next errback if there was one), then Twisted will write the error’s traceback to the log file. This means that you can typically get away with not adding errbacks and still get errors logged. Be careful though; if you keep a reference to the Deferred around, preventing it from being garbage-collected, then you may never see the error (and your callbacks will mysteriously seem to have never been called). If unsure, you should explicitly add an errback after your callbacks, even if all you do is:
# Make sure errors get logged
from twisted.python import log
d.addErrback(log.err)
Handling either synchronous or asynchronous results¶
In some applications, there are functions that might be either asynchronous or synchronous. For example, a user authentication function might be able to check in memory whether a user is authenticated, allowing the authentication function to return an immediate result, or it may need to wait on network data, in which case it should return a Deferred to be fired when that data arrives. However, a function that wants to check if a user is authenticated will then need to accept both immediate results and Deferreds.
In this example, the library function authenticateUser uses the application function isValidUser to authenticate a user:
def authenticateUser(isValidUser, user):
if isValidUser(user):
print("User is authenticated")
else:
print("User is not authenticated")
However, it assumes that isValidUser returns immediately,
whereas isValidUser may actually authenticate the user
asynchronously and return a Deferred. It is possible to adapt this
trivial user authentication code to accept either a
synchronous isValidUser or an
asynchronous isValidUser , allowing the library to handle
either type of function. It is, however, also possible to adapt
synchronous functions to return Deferreds. This section describes both
alternatives: handling functions that might be synchronous or
asynchronous in the library function (authenticateUser )
or in the application code.
Handling possible Deferreds in the library code¶
Here is an example of a synchronous user authentication function that might be
passed to authenticateUser :
def synchronousIsValidUser(user):
'''
Return true if user is a valid user, false otherwise
'''
return user in ["Alice", "Angus", "Agnes"]
However, here’s an asynchronousIsValidUser function that returns
a Deferred:
from twisted.internet import reactor, defer
def asynchronousIsValidUser(user):
d = defer.Deferred()
reactor.callLater(2, d.callback, user in ["Alice", "Angus", "Agnes"])
return d
Our original implementation of authenticateUser expected isValidUser to be synchronous, but now we need to change it to handle both
synchronous and asynchronous implementations of isValidUser . For this, we
use maybeDeferred to
call isValidUser , ensuring that the result of isValidUser is a Deferred,
even if isValidUser is a synchronous function:
from twisted.internet import defer
def printResult(result):
if result:
print("User is authenticated")
else:
print("User is not authenticated")
def authenticateUser(isValidUser, user):
d = defer.maybeDeferred(isValidUser, user)
d.addCallback(printResult)
Now isValidUser could be either synchronousIsValidUser or asynchronousIsValidUser .
It is also possible to modify synchronousIsValidUser to return a Deferred, see Generating Deferreds for more information.
Cancellation¶
Motivation¶
A Deferred may take any amount of time to be called back; in fact, it may never be called back. Your users may not be that patient. Since all actions taken when the Deferred completes are in your application or library’s callback code, you always have the option of simply disregarding the result when you receive it, if it’s been too long. However, while you’re ignoring it, the underlying operation represented by that Deferred is still chugging along in the background, possibly consuming resources such as CPU time, memory, network bandwidth and maybe even disk space. So, when the user has closed the window, hit the cancel button, disconnected from your server or sent a “stop” network message, you will want to announce your indifference to the result of that operation so that the originator of the Deferred can clean everything up and free those resources to be put to better use.
Cancellation for Applications which Consume Deferreds¶
Here’s a simple example. You’re connecting to an external host with an endpoint , but that host is really slow. You want to put a “cancel” button into your application to terminate the connection attempt, so the user can try connecting to a different host instead. Here’s a simple sketch of such an application, with the actual user interface left as an exercise for the reader:
def startConnecting(someEndpoint):
def connected(it):
"Do something useful when connected."
return someEndpoint.connect(myFactory).addCallback(connected)
# ...
connectionAttempt = startConnecting(endpoint)
def cancelClicked():
connectionAttempt.cancel()
Obviously (I hope), startConnecting is meant to be called by some UI element
that lets the user choose what host to connect to and then constructs an
appropriate endpoint (perhaps using twisted.internet.endpoints.clientFromString ). Then, a cancel
button, or similar, is hooked up to the cancelClicked .
When connectionAttempt.cancel is invoked, that will:
- cause the underlying connection operation to be terminated, if it is still ongoing
- cause the connectionAttempt Deferred to be completed, one way or another, in a timely manner
- likely cause the connectionAttempt Deferred to be errbacked with CancelledError
You may notice that that set of consequences is very heavily qualified. Although cancellation indicates the calling API’s desire for the underlying operation to be stopped, the underlying operation cannot necessarily react immediately. Even in this very simple example, there is already one thing that might not be interruptible: platform-native name resolution blocks, and therefore needs to be executed in a thread; the connection operation can’t be cancelled if it’s stuck waiting for a name to be resolved in this manner. So, the Deferred that you are cancelling may not callback or errback right away.
A Deferred may wait upon another Deferred at any point in its callback chain
(see “Handling...asynchronous results”, above). There’s no way for a particular
point in the callback chain to know if everything is finished. Since multiple
layers of the callback chain may wish to cancel the same Deferred, any layer may
call .cancel() at any time. The .cancel() method never
raises any exception or returns any value; you may call it repeatedly, even on a
Deferred which has already fired, or which has no remaining callbacks.
The main reason for all these qualifications, aside from specific examples, is that anyone who instantiates a Deferred may supply it with a cancellation function; that function can do absolutely anything that it wants to. Ideally, anything it does will be in the service of stopping the operation your requested, but there’s no way to guarantee any exact behavior across all Deferreds that might be cancelled. Cancellation of Deferreds is best effort. This may be the case for a number of reasons:
- The
Deferreddoesn’t know how to cancel the underlying operation. - The underlying operation may have reached an uncancellable state, because some irreversible operation has been done.
- The
Deferredmay already have a result, and so there’s nothing to cancel.
Calling cancel() will always succeed without an error
regardless of whether or not cancellation was possible. In cases 1 and 2 the Deferred may well errback with a twisted.internet.defer.CancelledError while the underlying
operation continues. Deferred s that support cancellation should
document what they do when cancelled, if they are uncancellable in certain edge
cases, etc..
If the cancelled Deferred is waiting on another Deferred , the cancellation will be forwarded to the other Deferred .
Default Cancellation Behavior¶
All Deferreds support cancellation. However, by default, they support a very rudimentary form of cancellation which doesn’t free any resources.
Consider this example of a Deferred which is ignorant of cancellation:
operation = Deferred()
def x(result):
print("Hooray, a result:" + repr(x))
operation.addCallback(x)
# ...
def operationDone():
operation.callback("completed")
A caller of an API that receives operation may call cancel on it. Since operation does not have a
cancellation function, one of two things will happen.
If
operationDonehas been called, and the operation has completed, nothing much will change.operationwill still have a result, and there are no more callbacks, so there’s no observable change in behavior.If
operationDonehas not yet been invoked, thenoperationwill be immediately errbacked with aCancelledError.However, once it’s cancelled, there’s no way to tell
operationDonenot to run; it will eventually calloperation.callbacklater. In normal operation, issuingcallbackon aDeferredthat has already called back results in anAlreadyCalledError, and this would cause an ugly traceback that could not be caught. Therefore,.callbackcan be invoked exactly once, causing a no-op, on aDeferredwhich has been cancelled but has no canceller. If you call it multiple times, you will still get anAlreadyCalledErrorexception.
Creating Cancellable Deferreds: Custom Cancellation Functions¶
Let’s imagine you are implementing an HTTP client, which returns a Deferred firing with the response from the server. Cancellation is best achieved by closing the connection. In order to make cancellation do that, all you have to do is pass a function to the constructor of the Deferred (it will get called with the Deferred that is being cancelled):
class HTTPClient(Protocol):
def request(self, method, path):
self.resultDeferred = Deferred(
lambda ignore: self.transport.abortConnection())
request = b"%s %s HTTP/1.0\r\n\r\n" % (method, path)
self.transport.write(request)
return self.resultDeferred
def dataReceived(self, data):
# ... parse HTTP response ...
# ... eventually call self.resultDeferred.callback() ...
Now if someone calls cancel() on the Deferred
returned from HTTPClient.request() , the HTTP request will be
cancelled (assuming it’s not too late to do so). Care should be taken not to callback() a Deferred that has already been cancelled.
DeferredList¶
Sometimes you want to be notified after several different events have all happened, rather than waiting for each one individually. For example, you may want to wait for all the connections in a list to close. twisted.internet.defer.DeferredList is the way to do this.
To create a DeferredList from multiple Deferreds, you simply pass a list of the Deferreds you want it to wait for:
# Creates a DeferredList
dl = defer.DeferredList([deferred1, deferred2, deferred3])
You can now treat the DeferredList like an ordinary Deferred; you can call addCallbacks and so on. The DeferredList will call its callback
when all the deferreds have completed. The callback will be called with a list
of the results of the Deferreds it contains, like so:
# A callback that unpacks and prints the results of a DeferredList
def printResult(result):
for (success, value) in result:
if success:
print('Success:', value)
else:
print('Failure:', value.getErrorMessage())
# Create three deferreds.
deferred1 = defer.Deferred()
deferred2 = defer.Deferred()
deferred3 = defer.Deferred()
# Pack them into a DeferredList
dl = defer.DeferredList([deferred1, deferred2, deferred3], consumeErrors=True)
# Add our callback
dl.addCallback(printResult)
# Fire our three deferreds with various values.
deferred1.callback('one')
deferred2.errback(Exception('bang!'))
deferred3.callback('three')
# At this point, dl will fire its callback, printing:
# Success: one
# Failure: bang!
# Success: three
# (note that defer.SUCCESS == True, and defer.FAILURE == False)
A standard DeferredList will never call errback, but failures in Deferreds
passed to a DeferredList will still errback unless consumeErrors
is passed True . See below for more details about this and other
flags which modify the behavior of DeferredList.
Note
If you want to apply callbacks to the individual Deferreds that go into the DeferredList, you should be careful about when those callbacks are added. The act of adding a Deferred to a DeferredList inserts a callback into that Deferred (when that callback is run, it checks to see if the DeferredList has been completed yet). The important thing to remember is that it is this callback which records the value that goes into the result list handed to the DeferredList’s callback.
Therefore, if you add a callback to the Deferred after adding the Deferred to the DeferredList, the value returned by that callback will not be given to the DeferredList’s callback. To avoid confusion, we recommend not adding callbacks to a Deferred once it has been used in a DeferredList.
def printResult(result):
print(result)
def addTen(result):
return result + " ten"
# Deferred gets callback before DeferredList is created
deferred1 = defer.Deferred()
deferred2 = defer.Deferred()
deferred1.addCallback(addTen)
dl = defer.DeferredList([deferred1, deferred2])
dl.addCallback(printResult)
deferred1.callback("one") # fires addTen, checks DeferredList, stores "one ten"
deferred2.callback("two")
# At this point, dl will fire its callback, printing:
# [(1, 'one ten'), (1, 'two')]
# Deferred gets callback after DeferredList is created
deferred1 = defer.Deferred()
deferred2 = defer.Deferred()
dl = defer.DeferredList([deferred1, deferred2])
deferred1.addCallback(addTen) # will run *after* DeferredList gets its value
dl.addCallback(printResult)
deferred1.callback("one") # checks DeferredList, stores "one", fires addTen
deferred2.callback("two")
# At this point, dl will fire its callback, printing:
# [(1, 'one), (1, 'two')]
Other behaviours¶
DeferredList accepts three keyword arguments that modify its behaviour:fireOnOneCallback , fireOnOneErrback and consumeErrors . If fireOnOneCallback is set, the
DeferredList will immediately call its callback as soon as any of its Deferreds
call their callback. Similarly, fireOnOneErrback will call errback
as soon as any of the Deferreds call their errback. Note that DeferredList is
still one-shot, like ordinary Deferreds, so after a callback or errback has been
called the DeferredList will do nothing further (it will just silently ignore
any other results from its Deferreds).
The fireOnOneErrback option is particularly useful when you
want to wait for all the results if everything succeeds, but also want to know
immediately if something fails.
The consumeErrors argument will stop the DeferredList from
propagating any errors along the callback chains of any Deferreds it contains
(usually creating a DeferredList has no effect on the results passed along the
callbacks and errbacks of their Deferreds). Stopping errors at the DeferredList
with this option will prevent “Unhandled error in Deferred” warnings from
the Deferreds it contains without needing to add extra errbacks [1] . Passing a true value
for the consumeErrors parameter will not change the behavior of fireOnOneCallback or fireOnOneErrback .
gatherResults¶
A common use for DeferredList is to “join” a number of parallel asynchronous operations, finishing successfully if all of the operations were successful, or failing if any one of the operations fails. In this case, twisted.internet.defer.gatherResults is a useful shortcut:
from twisted.internet import defer
d1 = defer.Deferred()
d2 = defer.Deferred()
d = defer.gatherResults([d1, d2], consumeErrors=True)
def cbPrintResult(result):
print(result)
d.addCallback(cbPrintResult)
d1.callback("one")
# nothing is printed yet; d is still awaiting completion of d2
d2.callback("two")
# printResult prints ["one", "two"]
The consumeErrors argument has the same meaning as it does
for NEEDS A TITLE : if true, it causes gatherResults to consume any errors in the passed-in Deferreds.
Always use this argument unless you are adding further callbacks or errbacks to
the passed-in Deferreds, or unless you know that they will not fail.
Otherwise, a failure will result in an unhandled error being logged by Twisted.
This argument is available since Twisted 11.1.0.
Class Overview¶
This is an overview API reference for Deferred from the point of using a Deferred returned by a function. It is not meant to be a substitute for the docstrings in the Deferred class, but can provide guidelines for its use.
There is a parallel overview of functions used by the Deferred’s creator in Generating Deferreds .
Basic Callback Functions¶
addCallbacks(self, callback[, errback, callbackArgs, callbackKeywords, errbackArgs, errbackKeywords])This is the method you will use to interact with Deferred. It adds a pair of callbacks “parallel” to each other (see diagram above) in the list of callbacks made when the Deferred is called back to. The signature of a method added using addCallbacks should be
myMethod(result, *methodArgs, **methodKeywords). If your method is passed in the callback slot, for example, all arguments in the tuplecallbackArgswill be passed as*methodArgsto your method.There are various convenience methods that are derivative of addCallbacks. I will not cover them in detail here, but it is important to know about them in order to create concise code.
addCallback(callback, *callbackArgs, **callbackKeywords)Adds your callback at the next point in the processing chain, while adding an errback that will re-raise its first argument, not affecting further processing in the error case.
Note that, while addCallbacks (plural) requires the arguments to be passed in a tuple, addCallback (singular) takes all its remaining arguments as things to be passed to the callback function. The reason is obvious: addCallbacks (plural) cannot tell whether the arguments are meant for the callback or the errback, so they must be specifically marked by putting them into a tuple. addCallback (singular) knows that everything is destined to go to the callback, so it can use Python’s “*” and “**” syntax to collect the remaining arguments.
addErrback(errback, *errbackArgs, **errbackKeywords)Adds your errback at the next point in the processing chain, while adding a callback that will return its first argument, not affecting further processing in the success case.
addBoth(callbackOrErrback, *callbackOrErrbackArgs, **callbackOrErrbackKeywords)This method adds the same callback into both sides of the processing chain at both points. Keep in mind that the type of the first argument is indeterminate if you use this method! Use it for
finally:style blocks.
Chaining Deferreds¶
If you need one Deferred to wait on another, all you need to do is return a Deferred from a method added to addCallbacks. Specifically, if you return Deferred B from a method added to Deferred A using A.addCallbacks, Deferred A’s processing chain will stop until Deferred B’s .callback() method is called; at that point, the next callback in A will be passed the result of the last callback in Deferred B’s processing chain at the time.
Note
If a Deferred is somehow returned from its own callbacks (directly or indirectly), the behavior is undefined. The Deferred code will make an attempt to detect this situation and produce a warning. In the future, this will become an exception.
If this seems confusing, don’t worry about it right now – when you run into a situation where you need this behavior, you will probably recognize it immediately and realize why this happens. If you want to chain deferreds manually, there is also a convenience method to help you.
chainDeferred(otherDeferred)Add
otherDeferredto the end of this Deferred’s processing chain. When self.callback is called, the result of my processing chain up to this point will be passed tootherDeferred.callback. Further additions to my callback chain do not affectotherDeferredThis is the same as
self.addCallbacks(otherDeferred.callback, otherDeferred.errback)
See also¶
- Generating Deferreds , an introduction to writing asynchronous functions that return Deferreds.
Footnotes
| [1] | Unless of course a later callback starts a fresh error — but as we’ve already noted, adding callbacks to a Deferred after its used in a DeferredList is confusing and usually avoided. |
Generating Deferreds¶
Deferred objects are signals that a function you have called does not yet have the data you want available. When a function returns a Deferred object, your calling function attaches callbacks to it to handle the data when available.
This document addresses the other half of the question: writing functions that return Deferreds, that is, constructing Deferred objects, arranging for them to be returned immediately without blocking until data is available, and firing their callbacks when the data is available.
This document assumes that you are familiar with the asynchronous model used by Twisted, and with using deferreds returned by functions .
Class overview¶
This is an overview API reference for Deferred from the point of creating a Deferred and firing its callbacks and errbacks. It is not meant to be a substitute for the docstrings in the Deferred class, but can provide guidelines for its use.
There is a parallel overview of functions used by calling function which the Deferred is returned to at Using Deferreds .
Basic Callback Functions¶
callback(result)Run success callbacks with the given result. This can only be run once. Later calls to this or
errbackwill raise twisted.internet.defer.AlreadyCalledError . If further callbacks or errbacks are added after this point, addCallbacks will run the callbacks immediately.errback(failure)Run error callbacks with the given failure. This can only be run once. Later calls to this or
callbackwill raise twisted.internet.defer.AlreadyCalledError . If further callbacks or errbacks are added after this point, addCallbacks will run the callbacks immediately.
What Deferreds don’t do: make your code asynchronous¶
Deferreds do not make the code magically not block.
Let’s take this function as an example:
from twisted.internet import defer
TARGET = 10000
def largeFibonnaciNumber():
# create a Deferred object to return:
d = defer.Deferred()
# calculate the ten thousandth Fibonnaci number
first = 0
second = 1
for i in xrange(TARGET - 1):
new = first + second
first = second
second = new
if i % 100 == 0:
print "Progress: calculating the %dth Fibonnaci number" % i
# give the Deferred the answer to pass to the callbacks:
d.callback(second)
# return the Deferred with the answer:
return d
import time
timeBefore = time.time()
# call the function and get our Deferred
d = largeFibonnaciNumber()
timeAfter = time.time()
print "Total time taken for largeFibonnaciNumber call: %0.3f seconds" % \
(timeAfter - timeBefore)
# add a callback to it to print the number
def printNumber(number):
print "The %dth Fibonacci number is %d" % (TARGET, number)
print "Adding the callback now."
d.addCallback(printNumber)
You will notice that despite creating a Deferred in the largeFibonnaciNumber function, these things happened:
- the “Total time taken for largeFibonnaciNumber call” output shows that the function did not return immediately as asynchronous functions are expected to do; and
- rather than the callback being added before the result was available and called after the result is available, it isn’t even added until after the calculation has been completed.
The function completed its calculation before returning, blocking the process until it had finished, which is exactly what asynchronous functions are not meant to do. Deferreds are not a non-blocking talisman: they are a signal for asynchronous functions to use to pass results onto callbacks, but using them does not guarantee that you have an asynchronous function.
Advanced Processing Chain Control¶
pause()Cease calling any methods as they are added, and do not respond to
callback, untilself.unpause()is called.unpause()If
callbackhas been called on this Deferred already, call all the callbacks that have been added to this Deferred sincepausewas called.Whether it was called or not, this will put this Deferred in a state where further calls to
addCallbacksorcallbackwill work as normal.
Returning Deferreds from synchronous functions¶
Sometimes you might wish to return a Deferred from a synchronous function. There are several reasons why, the major two are maintaining API compatibility with another version of your function which returns a Deferred, or allowing for the possibility that in the future your function might need to be asynchronous.
In the Using Deferreds reference, we gave the following example of a synchronous function:
def synchronousIsValidUser(user):
'''
Return true if user is a valid user, false otherwise
'''
return user in ["Alice", "Angus", "Agnes"]
While we can require that callers of our function wrap our synchronous result in a Deferred using maybeDeferred , for the sake of API compatibility it is better to return a Deferred ourselves using defer.succeed :
from twisted.internet import defer
def immediateIsValidUser(user):
'''
Returns a Deferred resulting in true if user is a valid user, false
otherwise
'''
result = user in ["Alice", "Angus", "Agnes"]
# return a Deferred object already called back with the value of result
return defer.succeed(result)
There is an equivalent defer.fail method to return a Deferred with the errback chain already fired.
Integrating blocking code with Twisted¶
At some point, you are likely to need to call a blocking function: many functions in third party libraries will have long running blocking functions. There is no way to ‘force’ a function to be asynchronous: it must be written that way specifically. When using Twisted, your own code should be asynchronous, but there is no way to make third party functions asynchronous other than rewriting them.
In this case, Twisted provides the ability to run the blocking code in a separate thread rather than letting it block your application. The twisted.internet.threads.deferToThread function will set up a thread to run your blocking function, return a Deferred and later fire that Deferred when the thread completes.
Let’s assume our largeFibonnaciNumber function
from above is in a third party library (returning the result of the
calculation, not a Deferred) and is not easily modifiable to be finished in
discrete blocks. This example shows it being called in a thread, unlike in the
earlier section we’ll see that the operation does not block our entire
program:
def largeFibonnaciNumber():
"""
Represent a long running blocking function by calculating
the TARGETth Fibonnaci number
"""
TARGET = 10000
first = 0
second = 1
for i in xrange(TARGET - 1):
new = first + second
first = second
second = new
return second
from twisted.internet import threads, reactor
def fibonacciCallback(result):
"""
Callback which manages the largeFibonnaciNumber result by
printing it out
"""
print "largeFibonnaciNumber result =", result
# make sure the reactor stops after the callback chain finishes,
# just so that this example terminates
reactor.stop()
def run():
"""
Run a series of operations, deferring the largeFibonnaciNumber
operation to a thread and performing some other operations after
adding the callback
"""
# get our Deferred which will be called with the largeFibonnaciNumber result
d = threads.deferToThread(largeFibonnaciNumber)
# add our callback to print it out
d.addCallback(fibonacciCallback)
print "1st line after the addition of the callback"
print "2nd line after the addition of the callback"
if __name__ == '__main__':
run()
reactor.run()
Possible sources of error¶
Deferreds greatly simplify the process of writing asynchronous code by providing a standard for registering callbacks, but there are some subtle and sometimes confusing rules that you need to follow if you are going to use them. This mostly applies to people who are writing new systems that use Deferreds internally, and not writers of applications that just add callbacks to Deferreds produced and processed by other systems. Nevertheless, it is good to know.
Firing Deferreds more than once is impossible¶
Deferreds are one-shot. You can only call Deferred.callback or Deferred.errback once. The processing chain continues each time
you add new callbacks to an already-called-back-to Deferred.
Synchronous callback execution¶
If a Deferred already has a result available, addCallback may call the callback synchronously: that is, immediately
after it’s been added. In situations where callbacks modify state, it is
might be desirable for the chain of processing to halt until all callbacks are
added. For this, it is possible to pause and unpause
a Deferred’s processing chain while you are adding lots of callbacks.
Be careful when you use these methods! If you pause a
Deferred, it is your responsibility to make sure that you unpause it.
The function adding the callbacks must unpause a paused Deferred, it should never be the responsibility of the code that actually fires the
callback chain by calling callback or errback as
this would negate its usefulness!
Scheduling tasks for the future¶
Let’s say we want to run a task X seconds in the future. The way to do that is defined in the reactor interface twisted.internet.interfaces.IReactorTime :
from twisted.internet import reactor
def f(s):
print "this will run 3.5 seconds after it was scheduled: %s" % s
reactor.callLater(3.5, f, "hello, world")
# f() will only be called if the event loop is started.
reactor.run()
If the result of the function is important or if it may be necessary to handle exceptions it raises, then the twisted.internet.task.deferLater utility conveniently takes care of creating a Deferred and setting up a delayed call:
from twisted.internet import task
from twisted.internet import reactor
def f(s):
return "This will run 3.5 seconds after it was scheduled: %s" % s
d = task.deferLater(reactor, 3.5, f, "hello, world")
def called(result):
print result
d.addCallback(called)
# f() will only be called if the event loop is started.
reactor.run()
If we want a task to run every X seconds repeatedly, we can use twisted.internet.task.LoopingCall:
from twisted.internet import task
from twisted.internet import reactor
loopTimes = 3
failInTheEnd = False
_loopCounter = 0
def runEverySecond():
"""
Called at ever loop interval.
"""
global _loopCounter
if _loopCounter < loopTimes:
_loopCounter += 1
print 'A new second has passed.'
return
if failInTheEnd:
raise Exception('Failure during loop execution.')
# We looped enough times.
loop.stop()
return
def cbLoopDone(result):
"""
Called when loop was stopped with success.
"""
print "Loop done."
reactor.stop()
def ebLoopFailed(failure):
"""
Called when loop execution failed.
"""
print failure.getBriefTraceback()
reactor.stop()
loop = task.LoopingCall(runEverySecond)
# Start looping every 1 second.
loopDeferred = loop.start(1.0)
# Add callbacks for stop and failure.
loopDeferred.addCallback(cbLoopDone)
loopDeferred.addErrback(ebLoopFailed)
reactor.run()
If we want to cancel a task that we’ve scheduled:
from twisted.internet import reactor
def f():
print "I'll never run."
callID = reactor.callLater(5, f)
callID.cancel()
reactor.run()
As with all reactor-based code, in order for scheduling to work the reactor must be started using reactor.run() .
Using Threads in Twisted¶
How Twisted Uses Threads Itself¶
All callbacks registered with the reactor - for example, dataReceived, connectionLost, or any higher-level method that comes from these, such as render_GET in twisted.web, or a callback added to a Deferred - are called from reactor.run.
The terminology around this is that we say these callbacks are run in the “main thread”, or “reactor thread” or “I/O thread”.
Therefore, internally, Twisted makes very little use of threads.
This is not to say that it makes no use of threads; there are plenty of APIs which have no non-blocking equivalent, so when Twisted needs to call those, it calls them in a thread.
One prominent example of this is system hostname resolution: unless you have configured Twisted to use its own DNS client in twisted.names, it will have to use your operating system’s blocking APIs to map host names to IP addresses, in the reactor’s thread pool.
However, this is something you only need to know about for resource-tuning purposes, like setting the number of threads to use; otherwise, it is an implementation detail you can ignore.
It is a common mistake is to think that because Twisted can manage multiple connections once, things are happening in multiple threads, and so you need to carefully manage locks. Lucky for you, Twisted does most things in one thread! This document will explain how to interact with existing APIs which need to be run within their own threads because they block. If you’re just using Twisted’s own APIs, the rule for threads is simply “don’t use them”.
Invoking Twisted From Other Threads¶
Methods within Twisted may only be invoked from the reactor thread unless otherwise noted. Very few things within Twisted are thread-safe. For example, writing data to a transport from a protocol is not thread-safe. This means that if you start a thread and call a Twisted method, you might get correct behavior... or you might get hangs, crashes, or corrupted data. So don’t do it.
The right way to call methods on the reactor from another thread, and therefore any objects which might call methods on the reactor, is to give a function to the reactor to execute within its own thread. This can be done using the function callFromThread:
from twisted.internet import reactor
def notThreadSafe(someProtocol, message):
someProtocol.transport.write(b"a message: " + message)
def callFromWhateverThreadYouWant():
reactor.callFromThread(notThreadSafe, b"hello")
In this example, callFromWhateverThreadYouWant is thread-safe and can be invoked by any thread, but notThreadSafe should only ever be called by code running in the thread where reactor.run is running.
Note
There are many objects within Twisted that represent values - for example, FilePath and URLPath - which you may construct yourself.
These may be safely constructed and used within a non-reactor thread as long as they are not shared with other threads.
However, you should be sure that these objects do not share any state, especially not with the reactor.
One good rule of thumb is that any object whose methods return Deferreds is almost certainly touching the reactor at some point, and should never be accessed from a non-reactor thread.
Running Code In Threads¶
Sometimes we may want to run code in a non-reactor thread, to avoid blocking the reactor. Twisted provides an API for doing so, the callInThread method on the reactor.
For example, to run a method in a non-reactor thread we can do:
from twisted.internet import reactor
def aSillyBlockingMethod(x):
import time
time.sleep(2)
print(x)
reactor.callInThread(aSillyBlockingMethod, "2 seconds have passed")
reactor.run()
callInThread will put your code into a queue, to be run by the next available thread in the reactor’s thread pool.
This means that depending on what other work has been submitted to the pool, your method may not run immediately.
Note
Keep in mind that callInThread can only concurrently run a fixed maximum number of tasks, and all users of the reactor are sharing that limit.
Therefore, you should not submit tasks which depend on other tasks in order to complete to be executed by callInThread.
An example of such a task would be something like this:
q = Queue()
def blocker():
print(q.get() + q.get())
def unblocker(a, b):
q.put(a)
q.put(b)
In this case, blocker will block forever unless unblocker can successfully run to give it inputs; similarly, unblocker might block forever if blocker is not run to consume its outputs.
So if you had a threadpool of maximum size X, and you ran for each in range(X): reactor.callInThread(blocker), the reactor threadpool would be wedged forever, unable to process more work or even shut down.
See “Managing the Reactor Thread Pool” below to tune these limits.
Getting Results¶
callInThread and callFromThread allow you to move the execution of your code out of and into the reactor thread, respectively, but that isn’t always enough.
When we run some code, we often want to know what its result was. For this, Twisted provides two methods: deferToThread and blockingCallFromThread, defined in the twisted.internet.threads module.
To get a result from some blocking code back into the reactor thread, we can use deferToThread to execute it instead of callFromThread.
from twisted.internet import reactor, threads
- def doLongCalculation():
- # .... do long calculation here ... return 3
- def printResult(x):
- print(x)
# run method in thread and get result as defer.Deferred d = threads.deferToThread(doLongCalculation) d.addCallback(printResult) reactor.run()
Similarly, you want some code running in a non-reactor thread wants to invoke some code in the reactor thread and get its result, you can use blockingCallFromThread:
from twisted.internet import threads, reactor, defer
from twisted.web.client import getPage
from twisted.web.error import Error
def inThread():
try:
result = threads.blockingCallFromThread(
reactor, getPage, "http://twistedmatrix.com/")
except Error, exc:
print exc
else:
print result
reactor.callFromThread(reactor.stop)
reactor.callInThread(inThread)
reactor.run()
blockingCallFromThread will return the object or raise the exception returned or raised by the function passed to it.
If the function passed to it returns a Deferred, it will return the value the Deferred is called back with or raise the exception it is errbacked with.
Managing the Reactor Thread Pool¶
We may want to modify the size of the thread pool, increasing or decreasing the number of threads in use. We can do this:
from twisted.internet import reactor
reactor.suggestThreadPoolSize(30)
The default size of the thread pool depends on the reactor being used; the default reactor uses a minimum size of 0 and a maximum size of 10.
The reactor thread pool is implemented by ThreadPool. To access methods on this object for more advanced tuning and monitoring (see the API documentation for details) you can get the thread pool with getThreadPool.
Producers and Consumers: Efficient High-Volume Streaming¶
The purpose of this guide is to describe the Twisted producer and consumer system. The producer system allows applications to stream large amounts of data in a manner which is both memory and CPU efficient, and which does not introduce a source of unacceptable latency into the reactor.
Readers should have at least a passing familiarity with the terminology associated with interfaces.
Push Producers¶
A push producer is one which will continue to generate data without external prompting until told to stop; a pull producer will generate one chunk of data at a time in response to an explicit request for more data.
The push producer API is defined by the IPushProducer interface. It is best to create a push producer when data generation is closedly tied to an event source. For example, a proxy which forwards incoming bytes from one socket to another outgoing socket might be implemented using a push producer: the dataReceived takes the role of an event source from which the producer generates bytes, and requires no external intervention in order to do so.
There are three methods which may be invoked on a push producer at various points in its lifetime: pauseProducing , resumeProducing , and stopProducing .
pauseProducing()¶
In order to avoid the possibility of using an unbounded amount of memory to buffer produced data which cannot be processed quickly enough, it is necessary to be able to tell a push producer to stop producing data for a while. This is done using the pauseProducing method. Implementers of a push producer should temporarily stop producing data when this method is invoked.
resumeProducing()¶
After a push producer has been paused for some time, the excess of data which it produced will have been processed and the producer may again begin producing data. When the time for this comes, the push producer will have resumeProducing invoked on it.
stopProducing()¶
Most producers will generate some finite (albeit, perhaps, unknown in advance) amount of data and then stop, having served their intended purpose. However, it is possible that before this happens an event will occur which renders the remaining, unproduced data irrelevant. In these cases, producing it anyway would be wasteful. The stopProducing method will be invoked on the push producer. The implementation should stop producing data and clean up any resources owned by the producer.
Pull Producers¶
The pull producer API is defined by the IPullProducer interface. Pull producers are useful in cases where there is no clear event source involved with the generation of data. For example, if the data is the result of some algorithmic process that is bound only by CPU time, a pull producer is appropriate.
Pull producers are defined in terms of only two methods: resumeProducing and stopProducing .
resumeProducing()¶
Unlike push producers, a pull producer is expected to only produce data in response to resumeProducing being called. This method will be called whenever more data is required. How much data to produce in response to this method call depends on various factors: too little data and runtime costs will be dominated by the back-and-forth event notification associated with a buffer becoming empty and requesting more data to process; too much data and memory usage will be driven higher than it needs to be and the latency associated with creating so much data will cause overall performance in the application to suffer. A good rule of thumb is to generate between 16 and 64 kilobytes of data at a time, but you should experiment with various values to determine what is best for your application.
stopProducing()¶
This method has the same meaning for pull producers as it does for push producers.
Consumers¶
This far, I’ve discussed the various external APIs of the two kinds of producers supported by Twisted. However, I have not mentioned where the data a producer generates actually goes, nor what entity is responsible for invoking these APIs. Both of these roles are filled by consumers . Consumers are defined by the one interface IConsumer .
IConsumer , defines three methods: registerProducer , unregisterProducer , and write .
registerProducer(producer, streaming)¶
So that a consumer can invoke methods on a producer, the consumer needs to be told about the producer. This is done with the registerProducer method. The first argument is either a IPullProducer or IPushProducer provider; the second argument indicates which of these interfaces is provided: True for push producers, False for pull producers.
unregisterProducer()¶
Eventually a consumer will not longer be interested in a producer. This could be because the producer has finished generating all its data, or because the consumer is moving on to something else, or any number of other reasons. In any case, this method reverses the effects of registerProducer .
write(data)¶
As you might guess, this is the method which a producer calls when it has generated some data. Push producers should call it as frequently as they like as long as they are not paused. Pull producers should call it once for each time resumeProducing is called on them.
Further Reading¶
An example push producer application can be found in doc/examples/streaming.py .
- Components: Interfaces and Adapters
- FileSender : A Simple Pull Producer
Choosing a Reactor and GUI Toolkit Integration¶
Overview¶
Twisted provides a variety of implementations of the twisted.internet.reactor . The specialized implementations are suited for different purposes and are designed to integrate better with particular platforms.
The epoll()-based reactor is Twisted’s default on Linux. Other platforms use poll() , or the most cross-platform reactor, select() .
Platform-specific reactor implementations exist for:
- Poll for Linux
- Epoll for Linux 2.6
- WaitForMultipleObjects (WFMO) for Win32
- Input/Output Completion Port (IOCP) for Win32
- KQueue for FreeBSD and Mac OS X
- CoreFoundation for Mac OS X
The remaining custom reactor implementations provide support for integrating with the native event loops of various graphical toolkits. This lets your Twisted application use all of the usual Twisted APIs while still being a graphical application.
Twisted currently integrates with the following graphical toolkits:
When using applications that are runnable using twistd , e.g.
TACs or plugins, there is no need to choose a reactor explicitly, since
this can be chosen using twistd ‘s -r option.
In all cases, the event loop is started by calling reactor.run() . In all cases, the event loop
should be stopped with reactor.stop() .
IMPORTANT: installing a reactor should be the first thing
done in the app, since any code that does
from twisted.internet import reactor will automatically
install the default reactor if the code hasn’t already installed one.
Reactor Functionality¶
| Status | TCP | SSL | UDP | Threading | Processes | Scheduling | Platforms | |
|---|---|---|---|---|---|---|---|---|
| select() | Stable | Y | Y | Y | Y | Y | Y | Unix, Win32 |
| poll | Stable | Y | Y | Y | Y | Y | Y | Unix |
| WaitForMultipleObjects (WFMO) for Win32 | Experimental | Y | Y | Y | Y | Y | Y | Win32 |
| Input/Output Completion Port (IOCP) for Win32 | Experimental | Y | Y | Y | Y | Y | Y | Win32 |
| CoreFoundation | Unmaintained | Y | Y | Y | Y | Y | Y | Mac OS X |
| epoll | Stable | Y | Y | Y | Y | Y | Y | Linux 2.6 |
| GTK+ | Stable | Y | Y | Y | Y | Y | Y | Unix, Win32 |
| wx | Experimental | Y | Y | Y | Y | Y | Y | Unix, Win32 |
| kqueue | Stable | Y | Y | Y | Y | Y | Y | FreeBSD |
General Purpose Reactors¶
Select()-based Reactor¶
The select reactor is the default on platforms that don’t
provide a better alternative that covers all use cases. If
the select reactor is desired, it may be installed via:
from twisted.internet import selectreactor
selectreactor.install()
from twisted.internet import reactor
Platform-Specific Reactors¶
Poll-based Reactor¶
The PollReactor will work on any platform that provides select.poll . With larger numbers of connected
sockets, it may provide for better performance than the SelectReactor.
from twisted.internet import pollreactor
pollreactor.install()
from twisted.internet import reactor
KQueue¶
The KQueue Reactor allows Twisted to use FreeBSD’s kqueue mechanism for event scheduling. See instructions in the twisted.internet.kqreactor ‘s docstring for installation notes.
from twisted.internet import kqreactor
kqreactor.install()
from twisted.internet import reactor
WaitForMultipleObjects (WFMO) for Win32¶
The Win32 reactor is not yet complete and has various limitations and issues that need to be addressed. The reactor supports GUI integration with the win32gui module, so it can be used for native Win32 GUI applications.
from twisted.internet import win32eventreactor
win32eventreactor.install()
from twisted.internet import reactor
Input/Output Completion Port (IOCP) for Win32¶
Windows provides a fast, scalable event notification system known as IO Completion Ports, or IOCP for short. Twisted includes a reactor based on IOCP which is nearly complete.
from twisted.internet import iocpreactor
iocpreactor.install()
from twisted.internet import reactor
Epoll-based Reactor¶
The EPollReactor will work on any platform that provides
epoll , today only Linux 2.6 and over. The
implementation of the epoll reactor currently uses the Level Triggered
interface, which is basically like poll() but scales much better.
from twisted.internet import epollreactor
epollreactor.install()
from twisted.internet import reactor
GUI Integration Reactors¶
GTK+¶
Twisted integrates with PyGTK version
2.0 using the gtk2reactor . An example Twisted application that
uses GTK+ can be found
in doc/core/examples/pbgtk2.py .
GTK-2.0 split the event loop out of the GUI toolkit and into a separate
module called “glib” . To run an application using the glib event loop,
use the glib2reactor . This will be slightly faster
than gtk2reactor (and does not require a working X display),
but cannot be used to run GUI applications.
from twisted.internet import gtk2reactor # for gtk-2.0
gtk2reactor.install()
from twisted.internet import reactor
from twisted.internet import glib2reactor # for non-GUI apps
glib2reactor.install()
from twisted.internet import reactor
GTK+ 3.0 and GObject Introspection¶
Twisted integrates with GTK+ 3 and GObject
through PyGObject’s
introspection using the gtk3reactor
and gireactor reactors.
from twisted.internet import gtk3reactor
gtk3reactor.install()
from twisted.internet import reactor
from twisted.internet import gireactor # for non-GUI apps
gireactor.install()
from twisted.internet import reactor
GLib 3.0 introduces the concept of GApplication , a class
that handles application uniqueness in a cross-platform way and provides
its own main loop. Its counterpart GtkApplication also
handles application lifetime with respect to open windows. Twisted
supports registering these objects with the event loop, which should be
done before running the reactor:
from twisted.internet import gtk3reactor
gtk3reactor.install()
from gi.repository import Gtk
app = Gtk.Application(...)
from twisted import reactor
reactor.registerGApplication(app)
reactor.run()
wxPython¶
Twisted currently supports two methods of integrating wxPython. Unfortunately, neither method will work on all wxPython platforms (such as GTK2 or Windows). It seems that the only portable way to integrate with wxPython is to run it in a separate thread. One of these methods may be sufficient if your wx app is limited to a single platform.
As with Tkinter , the support for integrating Twisted with a wxPython application uses specialized support code rather than a simple reactor.
from wxPython.wx import *
from twisted.internet import wxsupport, reactor
myWxAppInstance = wxApp(0)
wxsupport.install(myWxAppInstance)
However, this has issues when running on Windows, so Twisted now comes with alternative wxPython support using a reactor. Using this method is probably better. Initialization is done in two stages. In the first, the reactor is installed:
from twisted.internet import wxreactor
wxreactor.install()
from twisted.internet import reactor
Later, once a wxApp instance has
been created, but before reactor.run()
is called:
from twisted.internet import reactor
myWxAppInstance = wxApp(0)
reactor.registerWxApp(myWxAppInstance)
An example Twisted application that uses wxPython can be found
in doc/core/examples/wxdemo.py .
Non-Reactor GUI Integration¶
Tkinter¶
The support for Tkinter doesn’t use a specialized reactor. Instead, there is some specialized support code:
from Tkinter import *
from twisted.internet import tksupport, reactor
root = Tk()
# Install the Reactor support
tksupport.install(root)
# at this point build Tk app as usual using the root object,
# and start the program with "reactor.run()", and stop it
# with "reactor.stop()".
PyUI¶
As with Tkinter , the support for integrating Twisted with a PyUI application uses specialized support code rather than a simple reactor.
from twisted.internet import pyuisupport, reactor
pyuisupport.install(args=(640, 480), kw={'renderer': 'gl'})
An example Twisted application that uses PyUI can be found in doc/core/examples/pyuidemo.py .
Getting Connected with Endpoints¶
Introduction¶
On a network, one can think of any given connection as a long wire, stretched between two points. Lots of stuff can happen along the length of that wire - routers, switches, network address translation, and so on, but that is usually invisible to the application passing data across it. Twisted strives to make the nature of the “wire” as transparent as possible, with highly abstract interfaces for passing and receiving data, such as ITransport and IProtocol.
However, the application can’t be completely ignorant of the wire. In particular, it must do something to start the connection, and to do so, it must identify the end points of the wire. There are different names for the roles of each end point - “initiator” and “responder”, “connector” and “listener”, or “client” and “server” - but the common theme is that one side of the connection waits around for someone to connect to it, and the other side does the connecting.
In Twisted 10.1, several new interfaces were introduced to describe each of these roles for stream-oriented connections: IStreamServerEndpoint and IStreamClientEndpoint. The word “stream”, in this case, refers to endpoints which treat a connection as a continuous stream of bytes, rather than a sequence of discrete datagrams: TCP is a “stream” protocol whereas UDP is a “datagram” protocol.
Constructing and Using Endpoints¶
In both Writing Servers and Writing Clients, we covered basic usage of endpoints;
you construct an appropriate type of server or client endpoint, and then call listen (for servers) or connect (for clients).
In both of those tutorials, we constructed specific types of endpoints directly. However, in most programs, you will want to allow the user to specify where to listen or connect, in a way which will allow the user to request different strategies, without having to adjust your program. In order to allow this, you should use clientFromString or serverFromString.
There’s Not Much To It¶
Each type of endpoint is just an interface with a single method that
takes an argument. serverEndpoint.listen(factory) will start
listening on that endpoint with your protocol factory, and clientEndpoint.connect(factory) will start a single connection
attempt. Each of these APIs returns a value, though, which can be important.
However, if you are not already, you should be very familiar with Deferreds, as they are returned by both connect and listen methods, to indicate when the connection has connected or the listening port is up and running.
Servers and Stopping¶
IStreamServerEndpoint.listen returns a Deferred that fires with an IListeningPort. Note that this deferred may errback. The most common cause of such an error would be that another program is already using the requested port number, but the exact cause may vary depending on what type of endpoint you are listening on. If you receive such an error, it means that your application is not actually listening, and will not receive any incoming connections. It’s important to somehow alert an administrator of your server, in this case, especially if you only have one listening port!
Note also that once this has succeeded, it will continue listening forever.
If you need to stop listening for some reason, in response to anything other than a full server shutdown (reactor.stop and / or twistd will usually handle that case for you), make sure you keep a reference around to that listening port object so you can call IListeningPort.stopListening on it.
Finally, keep in mind that stopListening itself returns a Deferred, and the port may not have fully stopped listening until that Deferred has fired.
Most server applications will not need to worry about these details.
One example of a case where you would need to be concerned with all of these events would be an implementation of a protocol like non-PASV FTP, where new listening ports need to be bound for the lifetime of a particular action, then disposed of.
Clients and Cancelling¶
connectProtocol connects a Protocol instance to a given IStreamClientEndpoint. It returns a Deferred which fires with the Protocol once the connection has been made.
Connection attempts may fail, and so that Deferred may also errback.
If it does so, you will have to try again; no further attempts will be made.
See the client documentation for an example use.
connectProtocol is a wrapper around a lower-level API:
IStreamClientEndpoint.connect will use a protocol factory for a new outgoing connection attempt.
It returns a Deferred which fires with the IProtocol returned from the factory’s buildProtocol method, or errbacks with the connection failure.
Connection attempts may also take a long time, and your users may become bored and wander off.
If this happens, and your code decides, for whatever reason, that you’ve been waiting for the connection too long, you can call Deferred.cancel on the Deferred returned from connect or connectProtocol, and the underlying machinery should give up on the connection.
This should cause the Deferred to errback, usually with CancelledError;
although you should consult the documentation for your particular endpoint type to see if it may do something different.
Although some endpoint types may imply a built-in timeout, the interface does not guarantee one. If you don’t have any way for the application to cancel a wayward connection attempt, the attempt may just keep waiting forever. For example, a very simple 30-second timeout could be implemented like this:
attempt = connectProtocol(myEndpoint, myProtocol)
reactor.callLater(30, attempt.cancel)
Note
If you’ve used ClientFactory before, keep in mind that the connect method takes a Factory, not a ClientFactory.
Even if you pass a ClientFactory to endpoint.connect, its clientConnectionFailed and clientConnectionLost methods will not be called.
Maximizing the Return on your Endpoint Investment¶
Directly constructing an endpoint in your application is rarely the best option, because it ties your application to a particular type of transport. The strength of the endpoints API is in separating the construction of the endpoint (figuring out where to connect or listen) and its activation (actually connecting or listening).
If you are implementing a library that needs to listen for connections or make outgoing connections, when possible, you should write your code to accept client and server endpoints as parameters to functions or to your objects’ constructors. That way, application code that calls your library can provide whatever endpoints are appropriate.
If you are writing an application and you need to construct endpoints yourself, you can allow users to specify arbitrary endpoints described by a string using the clientFromString and serverFromString APIs. Since these APIs just take a string, they provide flexibility: if Twisted adds support for new types of endpoints (for example, IPv6 endpoints, or WebSocket endpoints), your application will automatically be able to take advantage of them with no changes to its code.
Endpoints Aren’t Always the Answer¶
For many use-cases, especially the common case of a twistd plugin which runs a long-running server that just binds a simple port, you might not want to use the endpoints APIs directly.
Instead, you may want to construct an IService, using strports.service, which will fit neatly into the required structure of the twistd plugin API.
This doesn’t give your application much control - the port starts listening at startup and stops listening at shutdown - but it does provide the same flexibility in terms of what type of server endpoint your application will support.
It is, however, almost always preferable to use an endpoint rather than calling a lower-level APIs like connectTCP, listenTCP, etc, directly. By accepting an arbitrary endpoint rather than requiring a specific reactor interface, you leave your application open to lots of interesting transport-layer extensibility for the future.
Endpoint Types Included With Twisted¶
The parser used by clientFromString and serverFromString is extensible via third-party plugins, so the endpoints available on your system depend on what packages you have installed.
However, Twisted itself includes a set of basic endpoints that will always be available.
Clients¶
- TCP
Supported arguments:
host,port,timeout.timeoutis optional.For example,
tcp:host=twistedmatrix.com:port=80:timeout=15.- TLS
Required arguments:
host,port.Optional arguments:
timeout,bindAddress,certificate,privateKey,trustRoots,endpoint.hostis a (UTF-8 encoded) hostname to connect to, as well as the host name to verify against.portis a numeric port number to connect to.timeoutandbindAddresshave the same meaning as thetimeoutandbindAddressfor TCP clients.certificateis the certificate to use for the client; it should be the path name of a PEM file containing a certificate for whichprivateKeyis the private key.privateKeyis the client’s private key, matching the certificate specified bycertificate. It should be the path name of a PEM file containing an X.509 client certificate. Ifcertificateis specified butprivateKeyis unspecified, Twisted will look for the certificate in the same file as specified bycertificate.trustRootsspecifies a path to a directory of PEM-encoded certificate files. If you leave this unspecified, Twisted will do its best to use the platform default set of trust roots, which should be the default WebTrust set.- the optional
endpointparameter changes the meaning of thetls:endpoint slightly. Rather than the default of connecting over TCP with the same hostname used for verification, you can connect over any endpoint type. If you specify the endpoint here,hostandportare used for certificate verification purposes only. Bear in mind you will need to backslash-escape the colons in the endpoint description here.
This client connects to the supplied hostname, validates the server’s hostname against the supplied hostname, and then upgrades to TLS immediately after validation succeeds.
The simplest example of this would be:
tls:example.com:443.You can use the
endpoint:feature with TCP if you want to connect to a host name; for example, if your DNS is not working, but you know that the IP address 7.6.5.4 points toawesome.site.example.com, you could specify:tls:awesome.site.example.com:443:endpoint=tcp\:7.6.5.4\:443.You can use it with any other endpoint type as well, though; for example, if you had a local UNIX socket that established a tunnel to
awesome.site.example.comin/var/run/awesome.sock, you could instead dotls:awesome.site.example.com:443:endpoint=unix\:/var/run/awesome.sock.Or, from python code:
wrapped = HostnameEndpoint('example.com', 443) contextFactory = optionsForClientTLS(hostname=u'example.com') endpoint = wrapClientTLS(contextFactory, wrapped) conn = endpoint.connect(Factory.forProtocol(Protocol))
- UNIX
Supported arguments:
path,timeout,checkPID.pathgives a filesystem path to a listening UNIX domain socket server.checkPID(optional) enables a check of the lock file Twisted-based UNIX domain socket servers use to prove they are still running.For example,
unix:path=/var/run/web.sock.- TCP (Hostname)
Supported arguments:
host,port,timeout.hostis a hostname to connect to.timeoutis optional. It is a name-based TCP endpoint that returns the connection which is established first amongst the resolved addresses.For example,
endpoint = HostnameEndpoint(reactor, "twistedmatrix.com", 80) conn = endpoint.connect(Factory.forProtocol(Protocol))
SSL (Deprecated)
Note
You should generally prefer the “TLS” client endpoint, above, unless you need to work with versions of Twisted older than 16.0. Among other things:
- the
ssl:client endpoint requires that you pass ‘’both’’hostname=(for hostname verification) as well ashost=(for a TCP connection address) in order to get hostname verification, which is required for security, whereastls:does the correct thing by default by using the same hostname for both.- the
ssl:client endpoint doesn’t work with IPv6, and thetls:endpoint does.All TCP arguments are supported, plus:
certKey,privateKey,caCertsDir.certKey(optional) gives a filesystem path to a certificate (PEM format).privateKey(optional) gives a filesystem path to a private key (PEM format).caCertsDir(optional) gives a filesystem path to a directory containing trusted CA certificates to use to verify the server certificate.For example,
ssl:host=twistedmatrix.com:port=443:caCertsDir=/etc/ssl/certs.
Servers¶
- TCP (IPv4)
Supported arguments:
port,interface,backlog.interfaceandbacklogare optional.interfaceis an IP address (belonging to the IPv4 address family) to bind to.For example,
tcp:port=80:interface=192.168.1.1.- TCP (IPv6)
All TCP (IPv4) arguments are supported, with
interfacetaking an IPv6 address literal instead.For example,
tcp6:port=80:interface=2001\:0DB8\:f00e\:eb00\:\:1.- SSL
All TCP arguments are supported, plus:
certKey,privateKey,extraCertChain,sslmethod, anddhParameters.certKey(optional, defaults to the value of privateKey) gives a filesystem path to a certificate (PEM format).privateKeygives a filesystem path to a private key (PEM format).extraCertChaingives a filesystem path to a file with one or more concatenated certificates in PEM format that establish the chain from a root CA to the one that signed your certificate.sslmethodindicates which SSL/TLS version to use (a value likeTLSv1_METHOD).dhParametersgives a filesystem path to a file in PEM format with parameters that are required for Diffie-Hellman key exchange. Since the this is required for theDHE-family of ciphers that offer perfect forward secrecy (PFS), it is recommended to specify one. Such a file can be created usingopenssl dhparam -out dh_param_1024.pem -2 1024. Please refer to OpenSSL’s documentation on dhparam for further details.For example,
ssl:port=443:privateKey=/etc/ssl/server.pem:extraCertChain=/etc/ssl/chain.pem:sslmethod=SSLv3_METHOD:dhParameters=dh_param_1024.pem.- UNIX
Supported arguments:
address,mode,backlog,lockfile.addressgives a filesystem path to listen on with a UNIX domain socket server.mode(optional) gives the filesystem permission/mode (in octal) to apply to that socket.lockfileenables use of a separate lock file to prove the server is still running.For example,
unix:address=/var/run/web.sock:lockfile=1.- systemd
Supported arguments:
domain,index.domainindicates which socket domain the inherited file descriptor belongs to (eg INET, INET6).indexindicates an offset into the array of file descriptors which have been inherited from systemd.For example,
systemd:domain=INET6:index=3.See also Deploying Twisted with systemd.
Components: Interfaces and Adapters¶
Object oriented programming languages allow programmers to reuse portions of existing code by creating new “classes” of objects which subclass another class. When a class subclasses another, it is said to inherit all of its behaviour. The subclass can then “override” and “extend” the behavior provided to it by the superclass. Inheritance is very useful in many situations, but because it is so convenient to use, often becomes abused in large software systems, especially when multiple inheritance is involved. One solution is to use delegation instead of “inheritance” where appropriate. Delegation is simply the act of asking another object to perform a task for an object. To support this design pattern, which is often referred to as the components pattern because it involves many small interacting components, interfaces and adapters were created by the Zope 3 team.
“Interfaces” are simply markers which objects can use to say “I implement this interface”. Other objects may then make requests like “Please give me an object which implements interface X for object type Y”. Objects which implement an interface for another object type are called “adapters”.
The superclass-subclass relationship is said to be an is-a relationship. When designing object hierarchies, object modellers use subclassing when they can say that the subclass is the same class as the superclass. For example:
class Shape:
sideLength = 0
def getSideLength(self):
return self.sideLength
def setSideLength(self, sideLength):
self.sideLength = sideLength
def area(self):
raise NotImplementedError, "Subclasses must implement area"
class Triangle(Shape):
def area(self):
return (self.sideLength * self.sideLength) / 2
class Square(Shape):
def area(self):
return self.sideLength * self.sideLength
In the above example, a Triangle is-a Shape, so it subclasses Shape, and a Square is-a Shape, so it also subclasses Shape.
However, subclassing can get complicated, especially when Multiple Inheritance enters the picture. Multiple Inheritance allows a class to inherit from more than one base class. Software which relies heavily on inheritance often ends up having both very wide and very deep inheritance trees, meaning that one class inherits from many superclasses spread throughout the system. Since subclassing with Multiple Inheritance means implementation inheritance, locating a method’s actual implementation and ensuring the correct method is actually being invoked becomes a challenge. For example:
class Area:
sideLength = 0
def getSideLength(self):
return self.sideLength
def setSideLength(self, sideLength):
self.sideLength = sideLength
def area(self):
raise NotImplementedError, "Subclasses must implement area"
class Color:
color = None
def setColor(self, color):
self.color = color
def getColor(self):
return self.color
class Square(Area, Color):
def area(self):
return self.sideLength * self.sideLength
The reason programmers like using implementation inheritance is because it makes code easier to read since the implementation details of Area are in a separate place than the implementation details of Color. This is nice, because conceivably an object could have a color but not an area, or an area but not a color. The problem, though, is that Square is not really an Area or a Color, but has an area and color. Thus, we should really be using another object oriented technique called composition, which relies on delegation rather than inheritance to break code into small reusable chunks. Let us continue with the Multiple Inheritance example, though, because it is often used in practice.
What if both the Color and the Area base class defined the same method, perhaps calculate?
Where would the implementation come from?
The implementation that is located for Square().calculate() depends on the method resolution order, or MRO, and can change when programmers change seemingly unrelated things by refactoring classes in other parts of the system, causing obscure bugs.
Our first thought might be to change the calculate method name to avoid name clashes, to perhaps calculateArea and calculateColor.
While explicit, this change could potentially require a large number of changes throughout a system, and is error-prone, especially when attempting to integrate two systems which you didn’t write.
Let’s imagine another example. We have an electric appliance, say a hair dryer.
The hair dryer is American voltage.
We have two electric sockets, one of them an American 120 Volt socket, and one of them a United Kingdom 240 Volt socket.
If we plug the hair dryer into the 240 Volt socket, it is going to expect 120 Volt current and errors will result.
Going back and changing the hair dryer to support both plug120Volt and plug240Volt methods would be tedious, and what if we decided we needed to plug the hair dryer into yet another type of socket?
For example:
class HairDryer:
def plug(self, socket):
if socket.voltage() == 120:
print "I was plugged in properly and am operating."
else:
print "I was plugged in improperly and "
print "now you have no hair dryer any more."
class AmericanSocket:
def voltage(self):
return 120
class UKSocket:
def voltage(self):
return 240
Given these classes, the following operations can be performed:
>>> hd = HairDryer()
>>> am = AmericanSocket()
>>> hd.plug(am)
I was plugged in properly and am operating.
>>> uk = UKSocket()
>>> hd.plug(uk)
I was plugged in improperly and
now you have no hair dryer any more.
We are going to attempt to solve this problem by writing an Adapter for the UKSocket which converts the voltage for use with an American hair dryer.
An Adapter is a class which is constructed with one and only one argument, the “adaptee” or “original” object.
In this example, we will show all code involved for clarity:
class AdaptToAmericanSocket:
def __init__(self, original):
self.original = original
def voltage(self):
return self.original.voltage() / 2
Now, we can use it as so:
>>> hd = HairDryer()
>>> uk = UKSocket()
>>> adapted = AdaptToAmericanSocket(uk)
>>> hd.plug(adapted)
I was plugged in properly and am operating.
So, as you can see, an adapter can ‘override’ the original implementation. It can also ‘extend’ the interface of the original object by providing methods the original object did not have. Note that an Adapter must explicitly delegate any method calls it does not wish to modify to the original, otherwise the Adapter cannot be used in places where the original is expected. Usually this is not a problem, as an Adapter is created to conform an object to a particular interface and then discarded.
Interfaces and Components in Twisted code¶
Adapters are a useful way of using multiple classes to factor code into discrete chunks. However, they are not very interesting without some more infrastructure. If each piece of code which wished to use an adapted object had to explicitly construct the adapter itself, the coupling between components would be too tight. We would like to achieve “loose coupling”, and this is where twisted.python.components comes in.
First, we need to discuss Interfaces in more detail.
As we mentioned earlier, an Interface is nothing more than a class which is used as a marker.
Interfaces should be subclasses of zope.interface.Interface, and have a very odd look to python programmers not used to them:
from zope.interface import Interface
class IAmericanSocket(Interface):
def voltage():
"""
Return the voltage produced by this socket object, as an integer.
"""
Notice how it looks just like a regular class definition, other than inheriting from Interface?
However, the method definitions inside the class block do not have any method body!
Since Python does not have any native language-level support for Interfaces like Java does, this is what distinguishes an Interface definition from a Class.
Now that we have a defined Interface, we can talk about objects using terms like this:
“The AmericanSocket class implements the IAmericanSocket interface” and “Please give me an object which adapts UKSocket to the IAmericanSocket interface”.
We can make declarations about what interfaces a certain class implements, and we can request adapters which implement a certain interface for a specific class.
Let’s look at how we declare that a class implements an interface:
from zope.interface import implementer
@implementer(IAmericanSocket)
class AmericanSocket:
def voltage(self):
return 120
So, to declare that a class implements an interface, we simply decorate it with zope.interface.implementer.
Now, let’s say we want to rewrite the AdaptToAmericanSocket class as a real adapter.
In this case we also specify it as implementing IAmericanSocket:
from zope.interface import implementer
@implementer(IAmericanSocket)
class AdaptToAmericanSocket:
def __init__(self, original):
"""
Pass the original UKSocket object as original
"""
self.original = original
def voltage(self):
return self.original.voltage() / 2
Notice how we placed the implements declaration on this adapter class.
So far, we have not achieved anything by using components other than requiring us to type more.
In order for components to be useful, we must use the component registry.
Since AdaptToAmericanSocket implements IAmericanSocket and regulates the voltage of a UKSocket object, we can register AdaptToAmericanSocket as an IAmericanSocket adapter for the UKSocket class.
It is easier to see how this is done in code than to describe it:
from zope.interface import Interface, implementer
from twisted.python import components
class IAmericanSocket(Interface):
def voltage():
"""
Return the voltage produced by this socket object, as an integer.
"""
@implementer(IAmericanSocket)
class AmericanSocket:
def voltage(self):
return 120
class UKSocket:
def voltage(self):
return 240
@implementer(IAmericanSocket)
class AdaptToAmericanSocket:
def __init__(self, original):
self.original = original
def voltage(self):
return self.original.voltage() / 2
components.registerAdapter(
AdaptToAmericanSocket,
UKSocket,
IAmericanSocket)
Now, if we run this script in the interactive interpreter, we can discover a little more about how to use components. The first thing we can do is discover whether an object implements an interface or not:
>>> IAmericanSocket.implementedBy(AmericanSocket)
True
>>> IAmericanSocket.implementedBy(UKSocket)
False
>>> am = AmericanSocket()
>>> uk = UKSocket()
>>> IAmericanSocket.providedBy(am)
True
>>> IAmericanSocket.providedBy(uk)
False
As you can see, the AmericanSocket instance claims to implement IAmericanSocket, but the UKSocket does not.
If we wanted to use the HairDryer with the AmericanSocket, we could know that it would be safe to do so by checking whether it implements IAmericanSocket.
However, if we decide we want to use HairDryer with a UKSocket instance, we must adapt it to IAmericanSocket before doing so.
We use the interface object to do this:
>>> IAmericanSocket(uk)
<__main__.AdaptToAmericanSocket instance at 0x1a5120>
When calling an interface with an object as an argument, the interface looks in the adapter registry for an adapter which implements the interface for the given instance’s class.
If it finds one, it constructs an instance of the Adapter class, passing the constructor the original instance, and returns it.
Now the HairDryer can safely be used with the adapted UKSocket .
But what happens if we attempt to adapt an object which already implements IAmericanSocket?
We simply get back the original instance:
>>> IAmericanSocket(am)
<__main__.AmericanSocket instance at 0x36bff0>
So, we could write a new “smart” HairDryer which automatically looked up an adapter for the socket you tried to plug it into:
class HairDryer:
def plug(self, socket):
adapted = IAmericanSocket(socket)
assert adapted.voltage() == 120, "BOOM"
print "I was plugged in properly and am operating"
Now, if we create an instance of our new “smart” HairDryer and attempt to plug it in to various sockets, the HairDryer will adapt itself automatically depending on the type of socket it is plugged in to:
>>> am = AmericanSocket()
>>> uk = UKSocket()
>>> hd = HairDryer()
>>> hd.plug(am)
I was plugged in properly and am operating
>>> hd.plug(uk)
I was plugged in properly and am operating
Voila; the magic of components.
Components and Inheritance¶
If you inherit from a class which implements some interface, and your new subclass declares that it implements another interface, the implements will be inherited by default.
For example, pb.Root is a class which implements IPBRoot.
This interface indicates that an object has remotely-invokable methods and can be used as the initial object served by a new Broker instance.
It has an implements setting like:
from zope.interface import implementer
@implementer(IPBRoot)
class Root(Referenceable):
pass
Suppose you have your own class which implements your IMyInterface interface:
from zope.interface import implementer, Interface
class IMyInterface(Interface):
pass
@implementer(IMyInterface)
class MyThing:
pass
Now if you want to make this class inherit from pb.Root, the interfaces code will automatically determine that it also implements IPBRoot:
from twisted.spread import pb
from zope.interface import implementer, Interface
class IMyInterface(Interface):
pass
@implementer(IMyInterface)
class MyThing(pb.Root):
pass
>>> from twisted.spread.flavors import IPBRoot
>>> IPBRoot.implementedBy(MyThing)
True
If you want MyThing to inherit from pb.Root but not implement IPBRoot like pb.Root does, use @implementer_only:
from twisted.spread import pb
from zope.interface import implementer_only, Interface
class IMyInterface(Interface):
pass
@implementer_only(IMyInterface)
class MyThing(pb.Root):
pass
>>> from twisted.spread.pb import IPBRoot
>>> IPBRoot.implementedBy(MyThing)
False
Cred: Pluggable Authentication¶
Goals¶
Cred is a pluggable authentication system for servers. It allows any number of network protocols to connect and authenticate to a system, and communicate to those aspects of the system which are meaningful to the specific protocol. For example, Twisted’s POP3 support passes a “username and password” set of credentials to get back a mailbox for the specified email account. IMAP does the same, but retrieves a slightly different view of the same mailbox, enabling those features specific to IMAP which are not available in other mail protocols.
Cred is designed to allow both the backend implementation of the business logic - called the avatar - and the authentication database - called the credential checker - to be decided during deployment. For example, the same POP3 server should be able to authenticate against the local UNIX password database or an LDAP server without having to know anything about how or where mail is stored.
To sketch out how this works - a “Realm” corresponds to an application domain and is in charge of avatars, which are network-accessible business logic objects. To connect this to an authentication database, a top-level object called a Portal stores a realm, and a number of credential checkers. Something that wishes to log in, such as a Protocol , stores a reference to the portal. Login consists of passing credentials and a request interface (e.g. POP3’s IMailbox ) to the portal. The portal passes the credentials to the appropriate credential checker, which returns an avatar ID. The ID is passed to the realm, which returns the appropriate avatar. For a Portal that has a realm that creates mailbox objects and a credential checker that checks /etc/passwd, login consists of passing in a username/password and the IMailbox interface to the portal. The portal passes this to the /etc/passwd credential checker, gets back a avatar ID corresponding to an email account, passes that to the realm and gets back a mailbox object for that email account.
Putting all this together, here’s how a login request will typically be processed:
Cred objects¶
The Portal¶
This is the core of login, the point of integration between all the objects in the cred system. There is one concrete implementation of Portal, and no interface - it does a very simple task. A Portal associates one (1) Realm with a collection of CredentialChecker instances. (More on those later.)
If you are writing a protocol that needs to authenticate against something, you will need a reference to a Portal, and to nothing else. This has only 2 methods -
login
(credentials, mind, *interfaces)The docstring is quite expansive (see twisted.cred.portal ), but in brief, this is what you call when you need to call in order to connect a user to the system. Typically you only pass in one interface, and the mind is
None. The interfaces are the possible interfaces the returned avatar is expected to implement, in order of preference. The result is a deferred which fires a tuple of:- interface the avatar implements (which was one of the interfaces passed in the
*interfacestuple) - an object that implements that interface (an avatar)
- logout, a 0-argument callable which disconnects the connection that was established by this call to login
The logout method has to be called when the avatar is logged out. For POP3 this means when the protocol is disconnected or logged out, etc..
- interface the avatar implements (which was one of the interfaces passed in the
registerChecker
(checker, *credentialInterfaces)which adds a CredentialChecker to the portal. The optional list of interfaces are interfaces of credentials that the checker is able to check.
The CredentialChecker¶
This is an object implementing ICredentialsChecker which resolves some credentials to an avatar ID.
Whether the credentials are stored in an in-memory data structure, an
Apache-style htaccess file, a UNIX password database, an SSH key database,
or any other form, an implementation of ICredentialsChecker is
how this data is connected to cred.
A credential checker stipulates some requirements of the credentials it can check by specifying a credentialInterfaces attribute, which is a list of interfaces. Credentials passed to its requestAvatarId method must implement one of those interfaces.
For the most part, these things will just check usernames and passwords and produce the username as the result, but hopefully we will be seeing some public-key, challenge-response, and certificate based credential checker mechanisms soon.
A credential checker should raise an error if it cannot authenticate
the user, and return twisted.cred.checkers.ANONYMOUS
for anonymous access.
The Credentials¶
Oddly enough, this represents some credentials that the user presents. Usually this will just be a small static blob of data, but in some cases it will actually be an object connected to a network protocol. For example, a username/password pair is static, but a challenge/response server is an active state-machine that will require several method calls in order to determine a result.
Twisted comes with a number of credentials interfaces and implementations in the twisted.cred.credentials module, such as IUsernamePassword and IUsernameHashedPassword .
The Realm¶
A realm is an interface which connects your universe of “business objects” to the authentication system.
IRealm is another one-method interface:
requestAvatar
(avatarId, mind, *interfaces)This method will typically be called from ‘Portal.login’. The avatarId is the one returned by a CredentialChecker.
Note
Note that
avatarIdmust always be a string. In particular, do not use unicode strings. If internationalized support is needed, it is recommended to use UTF-8, and take care of decoding in the realm.The important thing to realize about this method is that if it is being called, the user has already authenticated . Therefore, if possible, the Realm should create a new user if one does not already exist whenever possible. Of course, sometimes this will be impossible without more information, and that is the case that the interfaces argument is for.
Since requestAvatar should be called from a Deferred callback, it may return a Deferred or a synchronous result.
The Avatar¶
An avatar is a business logic object for a specific user. For POP3, it’s a mailbox, for a first-person-shooter it’s the object that interacts with the game, the actor as it were. Avatars are specific to an application, and each avatar represents a single “user” .
The Mind¶
As mentioned before, the mind is usually None , so you can skip this
bit if you want.
Masters of Perspective Broker already know this object as the ill-named “client object” . There is no “mind” class, or even interface, but it is an object which serves an important role - any notifications which are to be relayed to an authenticated client are passed through a ‘mind’. In addition, it allows passing more information to the realm during login in addition to the avatar ID.
The name may seem rather unusual, but considering that a Mind is representative of the entity on the “other end” of a network connection that is both receiving updates and issuing commands, I believe it is appropriate.
Although many protocols will not use this, it serves an important role. It is provided as an argument both to the Portal and to the Realm, although a CredentialChecker should interact with a client program exclusively through a Credentials instance.
Unlike the original Perspective Broker “client object” , a Mind’s implementation is most often dictated by the protocol that is connecting rather than the Realm. A Realm which requires a particular interface to issue notifications will need to wrap the Protocol’s mind implementation with an adapter in order to get one that conforms to its expected interface - however, Perspective Broker will likely continue to use the model where the client object has a pre-specified remote interface.
(If you don’t quite understand this, it’s fine. It’s hard to explain, and it’s not used in simple usages of cred, so feel free to pass None until you find yourself requiring something like this.)
Responsibilities¶
Server protocol implementation¶
The protocol implementor should define the interface the avatar should implement, and design the protocol to have a portal attached. When a user logs in using the protocol, a credential object is created, passed to the portal, and an avatar with the appropriate interface is requested. When the user logs out or the protocol is disconnected, the avatar should be logged out.
The protocol designer should not hardcode how users are authenticated or the realm implemented. For example, a POP3 protocol implementation would require a portal whose realm returns avatars implementing IMailbox and whose credential checker accepts username/password credentials, but that is all. Here’s a sketch of how the code might look - note that USER and PASS are the protocol commands used to login, and the DELE command can only be used after you are logged in:
# Copyright (c) Twisted Matrix Laboratories.
# See LICENSE for details.
from zope.interface import Interface
from twisted.protocols import basic
from twisted.python import log
from twisted.cred import credentials, error
from twisted.internet import defer
class IMailbox(Interface):
"""
Interface specification for mailbox.
"""
def deleteMessage(index):
pass
class POP3(basic.LineReceiver):
# ...
def __init__(self, portal):
self.portal = portal
def do_DELE(self, i):
# uses self.mbox, which is set after login
i = int(i)-1
self.mbox.deleteMessage(i)
self.successResponse()
def do_USER(self, user):
self._userIs = user
self.successResponse('USER accepted, send PASS')
def do_PASS(self, password):
if self._userIs is None:
self.failResponse("USER required before PASS")
return
user = self._userIs
self._userIs = None
d = defer.maybeDeferred(self.authenticateUserPASS, user, password)
d.addCallback(self._cbMailbox, user)
def authenticateUserPASS(self, user, password):
if self.portal is not None:
return self.portal.login(
credentials.UsernamePassword(user, password),
None,
IMailbox
)
raise error.UnauthorizedLogin()
def _cbMailbox(self, ial, user):
interface, avatar, logout = ial
if interface is not IMailbox:
self.failResponse('Authentication failed')
log.err("_cbMailbox() called with an interface other than IMailbox")
return
self.mbox = avatar
self._onLogout = logout
self.successResponse('Authentication succeeded')
log.msg("Authenticated login for " + user)
Application implementation¶
The application developer can implement realms and credential checkers. For example, they might implement a realm that returns IMailbox implementing avatars, using MySQL for storage, or perhaps a credential checker that uses LDAP for authentication. In the following example, the Realm for a simple remote object service (using Twisted’s Perspective Broker protocol) is implemented:
from twisted.spread import pb
from twisted.cred.portal import IRealm
class SimplePerspective(pb.Avatar):
def perspective_echo(self, text):
print 'echoing',text
return text
def logout(self):
print self, "logged out"
class SimpleRealm:
implements(IRealm)
def requestAvatar(self, avatarId, mind, *interfaces):
if pb.IPerspective in interfaces:
avatar = SimplePerspective()
return pb.IPerspective, avatar, avatar.logout
else:
raise NotImplementedError("no interface")
Deployment¶
Deployment involves tying together a protocol, an appropriate realm and a credential checker. For example, a POP3 server can be constructed by attaching to it a portal that wraps the MySQL-based realm and an /etc/passwd credential checker, or perhaps the LDAP credential checker if that is more useful. The following example shows how the SimpleRealm in the previous example is deployed using an in-memory credential checker:
from twisted.spread import pb
from twisted.internet import reactor
from twisted.cred.portal import Portal
from twisted.cred.checkers import InMemoryUsernamePasswordDatabaseDontUse
portal = Portal(SimpleRealm())
checker = InMemoryUsernamePasswordDatabaseDontUse()
checker.addUser("guest", "password")
portal.registerChecker(checker)
reactor.listenTCP(9986, pb.PBServerFactory(portal))
reactor.run()
Cred plugins¶
Authentication with cred plugins¶
Cred offers a plugin architecture for authentication methods. The primary API for this architecture is the command-line; the plugins are meant to be specified by the end-user when deploying a TAP (twistd plugin).
For more information on writing a twistd plugin and using cred plugins for your application, please refer to the Writing a twistd plugin document.
Building a cred plugin¶
To build a plugin for cred, you should first define an authType , a short one-word string that defines
your plugin to the command-line. Once you have this, the convention is
to create a file named myapp_plugins.py in the twisted.plugins module path.
Below is an example file structure for an application that defines such a plugin:
- MyApplication/
- setup.py
- myapp/
- __init__.py
- cred.py
- server.py
- twisted/
- plugins/
- myapp_plugins.py
- plugins/
Once you have created this structure within your application, you can create the code for your cred plugin by building a factory class which implements ICheckerFactory . These factory classes should not consist of a tremendous amount of code. Most of the real application logic should reside in the cred checker itself. (For help on building those, scroll up.)
The core purpose of the CheckerFactory is to translate an argstring , which is passed on the command line,
into a suitable set of initialization parameters for a Checker
class. In most cases this should be little more than constructing a
dictionary or a tuple of arguments, then passing them along to a new
checker instance.
from zope.interface import implements
from twisted import plugin
from twisted.cred.strcred import ICheckerFactory
from myapp.cred import SpecialChecker
class SpecialCheckerFactory(object):
"""
A checker factory for a specialized (fictional) API.
"""
# The class needs to implement both of these interfaces
# for the plugin system to find our factory.
implements(ICheckerFactory, plugin.IPlugin)
# This tells AuthOptionsMixin how to find this factory.
authType = "special"
# This is a one-line explanation of what arguments, if any,
# your particular cred plugin requires at the command-line.
argStringFormat = "A colon-separated key=value list."
# This help text can be multiple lines. It will be displayed
# when someone uses the "--help-auth-type special" command.
authHelp = """Some help text goes here ..."""
# This will be called once per command-line.
def generateChecker(self, argstring=""):
argdict = dict((x.split('=') for x in argstring.split(':')))
return SpecialChecker(**argdict)
# We need to instantiate our class for the plugin to work.
theSpecialCheckerFactory = SpecialCheckerFactory()
For more information on how your plugin can be used in your application (and by other application developers), please see the Writing a twistd plugin document.
Conclusion¶
After reading through this tutorial, you should be able to
- Understand how the cred architecture applies to your application
- Integrate your application with cred’s object model
- Deploy an application that uses cred for authentication
- Allow your users to use command-line authentication plugins
The Twisted Plugin System¶
The purpose of this guide is to describe the preferred way to write extensible Twisted applications (and consequently, also to describe how to extend applications written in such a way). This extensibility is achieved through the definition of one or more APIs and a mechanism for collecting code plugins which implement this API to provide some additional functionality. At the base of this system is the twisted.plugin module.
Making an application extensible using the plugin system has several strong advantages over other techniques:
- It allows third-party developers to easily enhance your software in a way that is loosely coupled: only the plugin API is required to remain stable.
- It allows new plugins to be discovered flexibly. For example, plugins can be loaded and saved when a program is first run, or re-discovered each time the program starts up, or they can be polled for repeatedly at runtime (allowing the discovery of new plugins installed after the program has started).
Writing Extensible Programs¶
Taking advantage of twisted.plugin is a two step process:
Define an interface which plugins will be required to implement. This is done using the zope.interface package in the same way one would define an interface for any other purpose.
A convention for defining interfaces is do so in a file named like ProjectName/projectname/iprojectname.py . The rest of this document will follow that convention: consider the following interface definition be in
Matsim/matsim/imatsim.py, an interface definition module for a hypothetical material simulation package.At one or more places in your program, invoke twisted.plugin.getPlugins and iterate over its result.
As an example of the first step, consider the following interface definition for a physical modelling system.
from zope.interface import Interface, Attribute
class IMaterial(Interface):
"""
An object with specific physical properties
"""
def yieldStress(temperature):
"""
Returns the pressure this material can support without
fracturing at the given temperature.
@type temperature: C{float}
@param temperature: Kelvins
@rtype: C{float}
@return: Pascals
"""
dielectricConstant = Attribute("""
@type dielectricConstant: C{complex}
@ivar dielectricConstant: The relative permittivity, with the
real part giving reflective surface properties and the
imaginary part giving the radio absorption coefficient.
""")
In another module, we might have a function that operates on
objects providing the IMaterial interface:
def displayMaterial(m):
print 'A material with yield stress %s at 500 K' % (m.yieldStress(500),)
print 'Also a dielectric constant of %s.' % (m.dielectricConstant,)
The last piece of required code is that which collects
IMaterial providers and passes them to the
displayMaterial function.
from twisted.plugin import getPlugins
from matsim import imatsim
def displayAllKnownMaterials():
for material in getPlugins(imatsim.IMaterial):
displayMaterial(material)
Third party developers may now contribute different materials
to be used by this modelling system by implementing one or more
plugins for the IMaterial interface.
Extending an Existing Program¶
The above code demonstrates how an extensible program might be written using Twisted’s plugin system. How do we write plugins for it, though? Essentially, we create objects which provide the required interface and then make them available at a particular location. Consider the following example.
from zope.interface import implements
from twisted.plugin import IPlugin
from matsim import imatsim
class SimpleMaterial(object):
implements(IPlugin, imatsim.IMaterial)
def __init__(self, yieldStressFactor, dielectricConstant):
self._yieldStressFactor = yieldStressFactor
self.dielectricConstant = dielectricConstant
def yieldStress(self, temperature):
return self._yieldStressFactor * temperature
steelPlate = SimpleMaterial(2.06842719e11, 2.7 + 0.2j)
brassPlate = SimpleMaterial(1.03421359e11, 1.4 + 0.5j)
steelPlate and brassPlate now provide both
IPlugin and IMaterial .
All that remains is to make this module available at an appropriate
location. For this, there are two options. The first of these is
primarily useful during development: if a directory which
has been added to sys.path (typically by adding it to the
PYTHONPATH environment variable) contains a
directory named twisted/plugins/ ,
each .py file in that directory will be loaded
as a source of plugins. This directory must not be a Python
package: including __init__.py will cause the
directory to be skipped and no plugins loaded from it. Second, each
module in the installed version of Twisted’s twisted.plugins package will also be loaded as a source of
plugins.
Once this plugin is installed in one of these two ways,
displayAllKnownMaterials can be run and we will see
two pairs of output: one for a steel plate and one for a brass
plate.
Alternate Plugin Packages¶
getPlugins takes one
additional argument not mentioned above. If passed in, the 2nd argument
should be a module or package to be used instead of
twisted.plugins as the plugin meta-package. If you
are writing a plugin for a Twisted interface, you should never
need to pass this argument. However, if you have developed an
interface of your own, you may want to mandate that plugins for it
are installed in your own plugins package, rather than in
Twisted’s.
You may want to support yourproject/plugins/
directories for ease of development. To do so, you should make yourproject/plugins/__init__.py contain at least
the following lines.
from twisted.plugin import pluginPackagePaths
__path__.extend(pluginPackagePaths(__name__))
__all__ = []
The key behavior here is that interfaces are essentially paired with a particular plugin package. If plugins are installed in a different package than the one the code which relies on the interface they provide, they will not be found when the application goes to load them.
Plugin Caching¶
In the course of using the Twisted plugin system, you may
notice dropin.cache files appearing at
various locations. These files are used to cache information
about what plugins are present in the directory which contains
them. At times, this cached information may become out of date.
Twisted uses the mtimes of various files involved in the plugin
system to determine when this cache may have become invalid.
Twisted will try to re-write the cache each time it tries to use
it but finds it out of date.
For a site-wide install, it may not (indeed, should not) be possible for applications running as normal users to rewrite the cache file. While these applications will still run and find correct plugin information, they may run more slowly than they would if the cache was up to date, and they may also report exceptions if certain plugins have been removed but which the cache still references. For these reasons, when installing or removing software which provides Twisted plugins, the site administrator should be sure the cache is regenerated. Well-behaved package managers for such software should take this task upon themselves, since it is trivially automatable. The canonical way to regenerate the cache is to run the following Python code:
from twisted.plugin import IPlugin, getPlugins
list(getPlugins(IPlugin))
As mentioned, it is normal for exceptions to be raised once here if plugins have been removed.
Further Reading¶
The Basics¶
Application¶
Twisted programs usually work with twisted.application.service.Application. This class usually holds all persistent configuration of a running server, such as:
- ports to bind to,
- places where connections to must be kept or attempted,
- periodic actions to do,
- and almost everything else to do with your Application.
It is the root object in a tree of services implementing twisted.application.service.IService.
Other howtos describe how to write custom code for Applications, but this one describes how to use already written code (which can be part of Twisted or from a third-party Twisted plugin developer).
The Twisted distribution comes with an important tool to deal with Applications: twistd(1).
Applications are just Python objects, which can be created and manipulated in the same ways as any other object.
twistd¶
The Twisted Daemon is a program that knows how to run Applications.
Strictly speaking, twistd is not necessary.
Fetching the application, getting the IService component, calling startService(), scheduling stopService() when the reactor shuts down, and then calling reactor.run() could be done manually.
However, twistd supplies many options which are highly useful for program set up:
- choosing a reactor (for more on reactors, see Choosing a Reactor),
- logging configuration (see the logger documentation for more),
- daemonizing (forking to the background),
- and more.
twistd supports all Applications mentioned above – and an additional one.
Sometimes it is convenient to write the code for building a class in straight Python.
One big source of such Python files is the examples directory.
When a straight Python file which defines an Application object called application is used, use the -y option.
When twistd runs, it records its process id in a twistd.pid file (this can be configured via a command line switch).
In order to shutdown the twistd process, kill that pid.
The usual way to do this would be:
kill `cat twistd.pid`
To prevent twistd from daemonizing, you can pass it the --no-daemon option (or -n, in conjunction with other short options).
As always, the gory details are in the manual page.
OS Integration¶
If you have an Application that runs with twistd, you can deploy it on RedHat Linux or Debian GNU/Linux based systems using the tap2deb or tap2rpm tools.
These take a Twisted Application file (of any of the supported formats — Python source, XML or pickle), and build a Debian or RPM package (respectively) that installs the Application as a system service.
The package includes the Application file, a default /etc/init.d/ script that starts and stops the process with twistd, and post-installation scripts that configure the Application to be run in the appropriate init levels.
Note
tap2rpm and tap2deb do not package your entire application and dependent code, just the Twisted Application file.
You will need to find some other way to package your Python code, such as distutils‘ bdist_rpm command.
For more savvy users, these tools also generate the source package, allowing you to modify and polish things which automated software cannot detect (such as dependencies or relationships to virtual packages).
Using the Twisted Application Framework¶
Introduction¶
Audience¶
The target audience of this document is a Twisted user who wants to deploy a significant amount of Twisted code in a re-usable, standard and easily configurable fashion. A Twisted user who wishes to use the Application framework needs to be familiar with developing Twisted servers and/or clients.
Goals¶
- To introduce the Twisted Application infrastructure.
- To explain how to deploy your Twisted application using
.tacfiles andtwistd. - To outline the existing Twisted services.
Overview¶
The Twisted Application infrastructure takes care of running and stopping your application. Using this infrastructure frees you from from having to write a large amount of boilerplate code by hooking your application into existing tools that manage daemonization, logging, choosing a reactor and more.
The major tool that manages Twisted applications is a command-line utility called twistd.
twistd is cross platform, and is the recommended tool for running Twisted applications.
The core component of the Twisted Application infrastructure is the twisted.application.service.Application object – an object which represents your application. However, Application doesn’t provide anything that you’d want to manipulate directly. Instead, Application acts as a container of any “Services” (objects implementing IService) that your application provides. Most of your interaction with the Application infrastructure will be done through Services.
By “Service”, we mean anything in your application that can be started and stopped. Typical services include web servers, FTP servers and SSH clients. Your Application object can contain many services, and can even contain structured hierarchies of Services using MultiService or your own custom IServiceCollection implementations. You will most likely want to use these to manage Services which are dependent on other Services. For example, a proxying Twisted application might want its server Service to only start up after the associated Client service.
An IService has two basic methods, startService() which is used to start the service, and stopService() which is used to stop the service.
The latter can return a Deferred, indicating service shutdown is not over until the result fires.
For example:
from twisted.internet import reactor
from twisted.application import service
from somemodule import EchoFactory
class EchoService(service.Service):
def __init__(self, portNum):
self.portNum = portNum
def startService(self):
self._port = reactor.listenTCP(self.portNum, EchoFactory())
def stopService(self):
return self._port.stopListening()
See Writing Servers for an explanation of EchoFactory and listenTCP.
Using Services and Application¶
twistd and tac¶
To handle start-up and configuration of your Twisted application, the Twisted Application infrastructure uses .tac files.
.tac are Python files which configure an Application object and assign this object to the top-level variable “application” .
The following is a simple example of a .tac file:
# You can run this .tac file directly with:
# twistd -ny service.tac
"""
This is an example .tac file which starts a webserver on port 8080 and
serves files from the current working directory.
The important part of this, the part that makes it a .tac file, is
the final root-level section, which sets up the object called 'application'
which twistd will look for
"""
import os
from twisted.application import service, internet
from twisted.web import static, server
def getWebService():
"""
Return a service suitable for creating an application object.
This service is a simple web server that serves files on port 8080 from
underneath the current working directory.
"""
# create a resource to serve static files
fileServer = server.Site(static.File(os.getcwd()))
return internet.TCPServer(8080, fileServer)
# this is the core part of any tac file, the creation of the root-level
# application object
application = service.Application("Demo application")
# attach the service to its parent application
service = getWebService()
service.setServiceParent(application)
twistd is a program that runs Twisted applications using a .tac file. In its most simple form, it takes a single argument -y and a tac file name.
For example, you can run the above server with the command twistd -y service.tac.
By default, twistd daemonizes and logs to a file called twistd.log.
More usually, when debugging, you will want your application to run in the foreground and log to the command line.
To run the above file like this, use the command twistd -noy service.tac.
For more information, see the twistd man page.
Customizing twistd logging¶
twistd logging can be customized using the command line.
This requires that a log observer factory be importable.
Given a file named my.py with the code:
from twisted.python.log import FileLogObserver
def logger():
return FileLogObserver(open("/tmp/my.log", "w")).emit
Invoking twistd --logger my.logger ... will log to a file named /tmp/my.log (this simple example could easily be replaced with use of the --logfile parameter to twistd).
Alternatively, the logging behavior can be customized through an API accessible from .tac files.
The ILogObserver component can be set on an Application in order to customize the default log observer that twistd will use.
Here is an example of how to use DailyLogFile, which rotates the log once per day.
from twisted.application.service import Application
from twisted.python.log import ILogObserver, FileLogObserver
from twisted.python.logfile import DailyLogFile
application = Application("myapp")
logfile = DailyLogFile("my.log", "/tmp")
application.setComponent(ILogObserver, FileLogObserver(logfile).emit)
Invoking twistd -y my.tac will create a log file at /tmp/my.log.
Services provided by Twisted¶
Twisted also provides pre-written IService implementations for common cases like listening on a TCP port, in the twisted.application.internet module. Here’s a simple example of constructing a service that runs an echo server on TCP port 7001:
from twisted.application import internet, service
from somemodule import EchoFactory
port = 7001
factory = EchoFactory()
echoService = internet.TCPServer(port, factory) # create the service
Each of these services (except TimerService) has a corresponding “connect” or “listen” method on the reactor, and the constructors for the services take the same arguments as the reactor methods.
The “connect” methods are for clients and the “listen” methods are for servers.
For example, TCPServer corresponds to reactor.listenTCP and TCPClient corresponds to reactor.connectTCP.
TCPServer
TCPClient
Services which allow you to make connections and listen for connections on TCP ports.
UNIXServer
UNIXClient
Services which listen and make connections over UNIX sockets.
SSLServer
SSLClient
Services which allow you to make SSL connections and run SSL servers.
UDPServer
A service which allows you to send and receive data over UDP.
See also the UDP documentation.
UNIXDatagramServer
UNIXDatagramClient
Services which send and receive data over UNIX datagram sockets.
MulticastServer
A server for UDP socket methods that support multicast.
TimerService
A service to periodically call a function.
Service Collection¶
IServiceCollection objects contain IService objects. IService objects can be added to IServiceCollection by calling setServiceParent and detached by using disownServiceParent.
The standard implementation of IServiceCollection is MultiService, which also implements IService. MultiService is useful for creating a new Service which combines two or more existing Services. For example, you could create a DNS Service as a MultiService which has a TCP and a UDP Service as children.
from twisted.application import internet, service
from twisted.names import server, dns, hosts
port = 53
# Create a MultiService, and hook up a TCPServer and a UDPServer to it as
# children.
dnsService = service.MultiService()
hostsResolver = hosts.Resolver('/etc/hosts')
tcpFactory = server.DNSServerFactory([hostsResolver])
internet.TCPServer(port, tcpFactory).setServiceParent(dnsService)
udpFactory = dns.DNSDatagramProtocol(tcpFactory)
internet.UDPServer(port, udpFactory).setServiceParent(dnsService)
# Create an application as normal
application = service.Application("DNSExample")
# Connect our MultiService to the application, just like a normal service.
dnsService.setServiceParent(application)
Writing a twistd Plugin¶
This document describes adding subcommands to
the twistd command, as a way to facilitate the deployment
of your applications. (This feature was added in Twisted 2.5)
The target audience of this document are those that have developed a Twisted application which needs a command line-based deployment mechanism.
There are a few prerequisites to understanding this document:
- A basic understanding of the Twisted Plugin System (i.e., the twisted.plugin module) is necessary, however, step-by-step instructions will be given. Reading The Twisted Plugin System is recommended, in particular the “Extending an Existing Program” section.
- The Application infrastructure
is used in
twistdplugins; in particular, you should know how to expose your program’s functionality as a Service. - In order to parse command line arguments, the
twistdplugin mechanism relies ontwisted.python.usage, which is documented in Using usage.Options .
Goals¶
After reading this document, the reader should be able to expose
their Service-using application as a subcommand
of twistd , taking into consideration whatever was passed
on the command line.
Alternatives to twistd plugins¶
The major alternative to the twistd plugin mechanism is the .tac
file, which is a simple script to be used with the
twistd -y/--python parameter. The twistd plugin mechanism
exists to offer a more extensible command-line-driven interface to
your application. For more information on .tac files, see
the document Using the Twisted Application Framework .
Creating the plugin¶
The following directory structure is assumed of your project:
- MyProject - Top level directory
- myproject - Python package
- __init__.py
- myproject - Python package
During development of your project, Twisted plugins can be loaded
from a special directory in your project, assuming your top level
directory ends up in sys.path. Create a directory
named twisted containing a directory
named plugins , and add a file
named myproject_plugin.py to it. This file will contain your
plugin. Note that you should not add any __init__.py files
to this directory structure, and the plugin file should not
be named myproject.py (because that would conflict with
your project’s module name).
In this file, define an object which provides the interfaces twisted.plugin.IPlugin and twisted.application.service.IServiceMaker .
The tapname attribute of your IServiceMaker provider
will be used as the subcommand name in a command
like twistd [subcommand] [args...] , and
the options attribute (which should be
a usage.Options
subclass) will be used to parse the given args.
from zope.interface import implements
from twisted.python import usage
from twisted.plugin import IPlugin
from twisted.application.service import IServiceMaker
from twisted.application import internet
from myproject import MyFactory
class Options(usage.Options):
optParameters = [["port", "p", 1235, "The port number to listen on."]]
class MyServiceMaker(object):
implements(IServiceMaker, IPlugin)
tapname = "myproject"
description = "Run this! It'll make your dog happy."
options = Options
def makeService(self, options):
"""
Construct a TCPServer from a factory defined in myproject.
"""
return internet.TCPServer(int(options["port"]), MyFactory())
# Now construct an object which *provides* the relevant interfaces
# The name of this variable is irrelevant, as long as there is *some*
# name bound to a provider of IPlugin and IServiceMaker.
serviceMaker = MyServiceMaker()
Now running twistd --help should
print myproject in the list of available subcommands,
followed by the description that we specified in the
plugin. twistd -n myproject would,
assuming we defined a MyFactory factory
inside myproject , start a listening server on port 1235
with that factory.
Using cred with your TAP¶
Twisted ships with a robust authentication framework to use with your application. If your server needs authentication functionality, and you haven’t read about twisted.cred yet, read up on it first.
If you are building a twistd plugin and you want to support a wide variety of authentication patterns, Twisted provides an easy-to-use mixin for your Options subclass: strcred.AuthOptionMixin . The following code is an example of using this mixin:
from twisted.cred import credentials, portal, strcred
from twisted.python import usage
from twisted.plugin import IPlugin
from twisted.application.service import IServiceMaker
from myserver import myservice
class ServerOptions(usage.Options, strcred.AuthOptionMixin):
# This part is optional; it tells AuthOptionMixin what
# kinds of credential interfaces the user can give us.
supportedInterfaces = (credentials.IUsernamePassword,)
optParameters = [
["port", "p", 1234, "Server port number"],
["host", "h", "localhost", "Server hostname"]]
class MyServerServiceMaker(object):
implements(IServiceMaker, IPlugin)
tapname = "myserver"
description = "This server does nothing productive."
options = ServerOptions
def makeService(self, options):
"""Construct a service object."""
# The realm is a custom object that your server defines.
realm = myservice.MyServerRealm(options["host"])
# The portal is something Cred can provide, as long as
# you have a list of checkers that you'll support. This
# list is provided my AuthOptionMixin.
portal = portal.Portal(realm, options["credCheckers"])
# OR, if you know you might get multiple interfaces, and
# only want to give your application one of them, you
# also have that option with AuthOptionMixin:
interface = credentials.IUsernamePassword
portal = portal.Portal(realm, options["credInterfaces"][interface])
# The protocol factory is, like the realm, something you implement.
factory = myservice.ServerFactory(realm, portal)
# Finally, return a service that will listen for connections.
return internet.TCPServer(int(options["port"]), factory)
# As in our example above, we have to construct an object that
# provides the IPlugin and IServiceMaker interfaces.
serviceMaker = MyServerServiceMaker()
Now that you have your TAP configured to support any authentication we can throw at it, you’re ready to use it. Here is an example of starting your server using the /etc/passwd file for authentication. (Clearly, this won’t work on servers with shadow passwords.)
$ twistd myserver --auth passwd:/etc/passwd
For a full list of cred plugins supported, see twisted.plugins , or use the command-line help:
$ twistd myserver --help-auth
$ twistd myserver --help-auth-type passwd
Conclusion¶
You should now be able to
- Create a twistd plugin
- Incorporate authentication into your plugin
- Use it from your development environment
- Install it correctly and use it in deployment
Deploying Twisted with systemd¶
Introduction¶
In this tutorial you will learn how to start a Twisted service using systemd.
You will also learn how to start the service using socket activation.
Note
The examples in this tutorial demonstrate how to launch a Twisted web server, but the same techniques apply to any Twisted service.
Prerequisites¶
Twisted
You will need a version of Twisted >= 12.2 for the socket activation section of this tutorial.
This tutorial was written on a Fedora 18 Linux operating system with a system wide installation of Twisted and Twisted Web.
If you have installed Twisted locally eg in your home directory or in a virtualenv, you will need to modify the paths in some of the following examples.
Test your Twisted installation by starting a
twistd webserver on TCP port 8080 with the following command:$ twistd --nodaemon web --port 8080 --path /srv/www/www.example.com/static 2013-01-28 13:21:35+0000 [-] Log opened. 2013-01-28 13:21:35+0000 [-] twistd 12.3.0 (/usr/bin/python 2.7.3) starting up. 2013-01-28 13:21:35+0000 [-] reactor class: twisted.internet.epollreactor.EPollReactor. 2013-01-28 13:21:35+0000 [-] Site starting on 8080 2013-01-28 13:21:35+0000 [-] Starting factory <twisted.web.server.Site instance at 0x7f57eb66efc8>This assumes that you have the following static web page in the following directory structure:
# tree /srv/ /srv/ └── www └── www.example.com └── static └── index.html<!doctype html> <html lang=en> <head> <meta charset=utf-8> <title>Example Site</title> </head> <body> <h1>Example Site</h1> </body> </html>Now try connecting to http://localhost:8080 in your web browser.
If you do not see your web page or if
twistddidn’t start, you should investigate and fix the problem before continuing.
Basic Systemd Service Configuration¶
The essential configuration file for a systemd service is the service file.
Later in this tutorial, you will learn about some other types of configuration file, which are used to control when and how your service is started.
But we will begin by configuring systemd to start a Twisted web server immediately on system boot.
Create a systemd.service file¶
Create the service file at /etc/systemd/system/www.example.com.service with the following content:
/etc/systemd/system/www.example.com.service
[Unit]
Description=Example Web Server
[Service]
ExecStart=/usr/bin/twistd \
--nodaemon \
--pidfile= \
web --port 8080 --path .
WorkingDirectory=/srv/www/www.example.com/static
User=nobody
Group=nobody
Restart=always
[Install]
WantedBy=multi-user.target
This configuration file contains the following note worthy directives:
ExecStart
Always include the full path to
twistdin case you have multiple versions installed.The
--nodaemonflag makestwistdrun in the foreground. Systemd works best with child processes that remain in the foreground.The
--pidfile=flag preventstwistdfrom writing a pidfile. A pidfile is not necessary when Twisted runs as a foreground process.The
--pathflag specifies the location of the website files. In this example we use ”.” which makestwistdserve files from its current working directory (see below).
WorkingDirectory
Systemd can configure the working environment of its child processes.
In this example the working directory of
twistdis set to that of the static website.
User / Group
Systemd can also control the effective user and group of its child processes.
This example uses an un-privileged user “nobody” and un-privileged group “nobody”.
This is an important security measure which ensures that the Twisted sub-process can not access restricted areas of the file system.
Restart
Systemd can automatically restart a child process if it exits or crashes unexpectedly.
In this example the
Restartoption is set toalways, which ensures thattwistdwill be restarted under all circumstances.
WantedBy
Systemd service dependencies are controlled by
WantedByandRequiredBydirectives in the[Install]section of configuration file.The special multi-user.target is used in this example so that
systemdstarts thetwistd webservice when it reaches the multi-user stage of the boot sequence.
There are many more service directives which are documented in the systemd.directives man page.
Reload systemd¶
$ sudo systemctl daemon-reload
This forces systemd to read the new configuration file.
Always run systemctl daemon-reload after changing any of the systemd configuration files.
Start the service¶
$ sudo systemctl start www.example.com
twistd should now be running and listening on TCP port 8080. You can verify this using the systemctl status command. eg
$ systemctl status www.example.com.service
www.example.com.service - Example Web Server
Loaded: loaded (/etc/systemd/system/www.example.com.service; enabled)
Active: active (running) since Mon 2013-01-28 16:16:26 GMT; 1s ago
Main PID: 10695 (twistd)
CGroup: name=systemd:/system/www.example.com.service
└─10695 /usr/bin/python /usr/bin/twistd --nodaemon --pidfile= web --port 8080 --path .
Jan 28 16:16:26 zorin.lan systemd[1]: Starting Example Web Server...
Jan 28 16:16:26 zorin.lan systemd[1]: Started Example Web Server.
Jan 28 16:16:26 zorin.lan twistd[10695]: 2013-01-28 16:16:26+0000 [-] Log opened.
Jan 28 16:16:26 zorin.lan twistd[10695]: 2013-01-28 16:16:26+0000 [-] twistd 12.1.0 (/usr/bin/python 2.7.3) starting up.
Jan 28 16:16:26 zorin.lan twistd[10695]: 2013-01-28 16:16:26+0000 [-] reactor class: twisted.internet.epollreactor.EPollReactor.
Jan 28 16:16:26 zorin.lan twistd[10695]: 2013-01-28 16:16:26+0000 [-] Site starting on 8080
Jan 28 16:16:26 zorin.lan twistd[10695]: 2013-01-28 16:16:26+0000 [-] Starting factory <twisted.web.server.Site instance at 0x159b758>
The systemctl status command is convenient because it shows you both the current status of the service and a short log of the service output.
This is especially useful for debugging and diagnosing service startup problems.
The twistd subprocess will log messages to stderr and systemd will log these messages to syslog.
You can verify this by monitoring the syslog messages or by using the new journalctl tool in Fedora.
See the systemctl man page for details of other systemctl command line options.
Enable the service¶
We’ve seen how to start the service manually, but now we need to “enable” it so that it starts automatically at boot time.
Enable the service with the following command:
$ sudo systemctl enable www.example.com.service
ln -s '/etc/systemd/system/www.example.com.service' '/etc/systemd/system/multi-user.target.wants/www.example.com.service'
This creates a symlink to the service file in the multi-user.target.wants directory.
The Twisted web server will now be started automatically at boot time.
The multi-user.target is an example of a “special” systemd unit.
Later in this tutorial you will learn how to use another special unit - the sockets.target.
Test that the service is automatically restarted¶
The Restart=always option in the systemd.service file ensures that systemd will restart the twistd process if and when it exits unexpectedly.
You can read about other Restart options in the systemd.service man page.
Try killing the twistd process and then checking its status again:
$ sudo kill 12543
$ systemctl status www.example.com.service
www.example.com.service - Example Web Server
Loaded: loaded (/etc/systemd/system/www.example.com.service; disabled)
Active: active (running) since Mon 2013-01-28 17:47:37 GMT; 1s ago
Main PID: 12611 (twistd)
The “Active” time stamp shows that the twistd process was restarted within 1 second.
Now stop the service before you proceed to the next section.
$ sudo systemctl stop www.example.com.service
$ systemctl status www.example.com.service
www.example.com.service - Example Web Server
Loaded: loaded (/etc/systemd/system/www.example.com.service; enabled)
Active: inactive (dead) since Mon 2013-01-28 16:51:12 GMT; 1s ago
Process: 10695 ExecStart=/usr/bin/twistd --nodaemon --pidfile= web --port 8080 --path . (code=exited, status=0/SUCCESS)
Socket Activation¶
First you need to understand what “socket activation” is. This extract from the systemd daemon man page explains it quite clearly.
In a socket-based activation scheme the creation and binding of the listening socket as primary communication channel of daemons to local (and sometimes remote) clients is moved out of the daemon code and into the init system.
Based on per-daemon configuration the init system installs the sockets and then hands them off to the spawned process as soon as the respective daemon is to be started.
Optionally activation of the service can be delayed until the first inbound traffic arrives at the socket, to implement on-demand activation of daemons.
However, the primary advantage of this scheme is that all providers and all consumers of the sockets can be started in parallel as soon as all sockets are established.
In addition to that daemons can be restarted with losing only a minimal number of client transactions or even any client request at all (the latter is particularly true for state-less protocols, such as DNS or syslog), because the socket stays bound and accessible during the restart, and all requests are queued while the daemon cannot process them.
Another benefit of socket activation is that systemd can listen on privileged ports and start Twisted with privileges already dropped. This allows a Twisted service to be configured and restarted by a non-root user.
Twisted (since version 12.2) includes a systemd endpoint API and a corresponding string ports syntax which allows a Twisted service to inherit a listening socket from systemd.
The following example builds on the previous example, demonstrating how to enable socket activation for a simple Twisted web server.
Note
Before continuing, stop the previous example service with the following command:
$ sudo systemctl stop www.example.com.service
Create a systemd.socket file¶
Create the systemd.socket file at /etc/systemd/system/www.example.com.socket with the following content:
/etc/systemd/system/www.example.com.socket
[Socket]
ListenStream=0.0.0.0:80
[Install]
WantedBy=sockets.target
This configuration file contains the following important directives:
ListenStream=0.0.0.0:80
This option configuressystemdto create a listening TCP socket bound to all local IPv4 addresses on port 80.
WantedBy=sockets.target
This is a special target used by all socket activated services.systemdwill automatically bind to all such socket activation ports during boot up.
You also need to modify the systemd.service file as follows:
/etc/systemd/system/www.example.com.service
[Unit]
Description=Example Web Server
[Service]
ExecStart=/usr/bin/twistd \
--nodaemon \
--pidfile= \
web --port systemd:domain=INET:index=0 --path .
NonBlocking=true
WorkingDirectory=/srv/www/www.example.com/static
User=nobody
Group=nobody
Restart=always
Note the following important directives and changes:
ExecStart
The
domain=INETendpoint argument makestwistdtreat the inherited file descriptor as an IPv4 socket.The
index=0endpoint argument makestwistdadopt the first file descriptor inherited fromsystemd.Socket activation is also technically possible with other socket families and types, but Twisted currently only accepts IPv4 and IPv6 TCP sockets. See Limitations and Known Issues below.
NonBlocking
This must be set totrueto ensure thatsystemdpasses non-blocking sockets to Twisted.
[Install]
In this example, the[Install]section has been moved to the socket configuration file.
Reload systemd so that it reads the updated configuration files.
$ sudo systemctl daemon-reload
Start and enable the socket¶
You can now start systemd listening on the socket with the following command:
$ sudo systemctl start www.example.com.socket
This command refers specifically to the socket configuration file, not the service file.
systemd should now be listening on port 80
$ systemctl status www.example.com.socket
www.example.com.socket
Loaded: loaded (/etc/systemd/system/www.example.com.socket; disabled)
Active: active (listening) since Tue 2013-01-29 14:53:17 GMT; 7s ago
Jan 29 14:53:17 zorin.lan systemd[1]: Listening on www.example.com.socket.
But twistd should not yet have started.
You can verify this using the systemctl command. eg
$ systemctl status www.example.com.service
www.example.com.service - Example Web Server
Loaded: loaded (/etc/systemd/system/www.example.com.service; static)
Active: inactive (dead) since Tue 2013-01-29 14:48:42 GMT; 6min ago
Enable the socket, so that it will be started automatically with the other socket activated services during boot up.
$ sudo systemctl enable www.example.com.socket
ln -s '/etc/systemd/system/www.example.com.socket' '/etc/systemd/system/sockets.target.wants/www.example.com.socket'
Activate the port to start the service¶
Now try connecting to http://localhost:80 in your web browser.
systemd will accept the connection and start twistd, passing it the listening socket.
You can verify this by using systemctl to report the status of the service. eg
$ systemctl status www.example.com.service
www.example.com.service - Example Web Server
Loaded: loaded (/etc/systemd/system/www.example.com.service; static)
Active: active (running) since Tue 2013-01-29 15:02:20 GMT; 3s ago
Main PID: 25605 (twistd)
CGroup: name=systemd:/system/www.example.com.service
└─25605 /usr/bin/python /usr/bin/twistd --nodaemon --pidfile= web --port systemd:domain=INET:index=0 --path .
Jan 29 15:02:20 zorin.lan systemd[1]: Started Example Web Server.
Jan 29 15:02:20 zorin.lan twistd[25605]: 2013-01-29 15:02:20+0000 [-] Log opened.
Jan 29 15:02:20 zorin.lan twistd[25605]: 2013-01-29 15:02:20+0000 [-] twistd 12.1.0 (/usr/bin/python 2.7.3) starting up.
Jan 29 15:02:20 zorin.lan twistd[25605]: 2013-01-29 15:02:20+0000 [-] reactor class: twisted.internet.epollreactor.EPollReactor.
Jan 29 15:02:20 zorin.lan twistd[25605]: 2013-01-29 15:02:20+0000 [-] Site starting on 80
Jan 29 15:02:20 zorin.lan twistd[25605]: 2013-01-29 15:02:20+0000 [-] Starting factory <twisted.web.server.Site instance at 0x24be758>
Conclusion¶
In this tutorial you have learned how to deploy a Twisted service using systemd.
You have also learned how the service can be started on demand, using socket activation.
Limitations and Known Issues¶
- Twisted can not accept UNIX or datagram sockets from
systemd. - Twisted does not support listening for SSL connections on sockets inherited from
systemd.
Further Reading¶
Logging with twisted.logger¶
The Basics¶
Logging consists of two main endpoints: applications that emit events, and observers that receive and handle those events.
An event is simply a dict object containing the relevant data that describes something interesting that has occurred in the application.
For example: a web server might emit an event after handling each request that includes the URI of requested resource, the response’s status code, a count of bytes transferred, and so on.
All of that information might be contained in a pair of objects representing the request and response, so logging this event could be as simple as:
log.info(request=request, response=response)
The above API would seem confusing to users of many logging systems, which are built around the idea of emitting strings to a file. There is, after all, no string in the above call. In such systems, one might expect the API to look like this instead:
log.info(
"{uri}: status={status}, bytes={size}, etc..."
.format(uri=request.uri, status=response.code, size=response.size)
)
Here, a string is rendered by formatting data from our request and response objects. The string can be easily appended to a log file, to be later read by an administrator, or perhaps teased out via some scripts or log gathering tools.
One disadvantage to this is that a strings meant to be human-readable are not necessarily easy (or even possible) for software to handle reliably. While text files are a common medium for storing logs, one might want to write code that notices certain types of events and does something other than store the event.
In the web server example, if many requests from a given IP address are resulting in failed authentication, someone might be trying to break in to the server. Perhaps blocking requests from that IP address would be useful. An observer receiving the above event as a string would have to resort to parsing the string to extract the relevant information, which may perform poorly and, depending on how well events are formatted, it may also be difficult to avoid bugs. Additionally, any information not encoded into the string is simply unavailable; if the IP address isn’t in the string, it cannot be obtained from the event.
However, if the request and response objects are available to the observer, as in the first example, it would be possible to write an observer that accessed any attributes of those objects.
It would also be a lot easier to write an observer that emitted structured information by serializing these objects into a format such as JSON, rows in a database, etc.
Events-as-strings do have the advantage that it’s obvious what an observer that writes strings, for example to a file, would emit. We can solve this more flexibly by providing an optional format string in events that can be used for this purpose:
log.info(
"{request.uri}: status={response.status}, bytes={response.size}, etc...",
request=request, response=response
)
Note that this looks very much like the events-as-strings version of the API, except that the string is not rendered by the caller; we leave that to the observer, if and when it is needed . The observer also has direct access to the request and response objects, as well as their attributes. Now a text-based observer can format the text in a prescribed way, and an observer that wants to handle these events in some other manner can do so as well.
Usage for emitting applications¶
The first thing that an application that emits logging events needs to do is to instantiate a Logger object, which provides the API to emit events. A Logger may be created globally for a module:
from twisted.logger import Logger
log = Logger()
def handleData(data):
log.debug("Got data: {data!r}.", data=data)
A Logger can also be associated with a class:
from twisted.logger import Logger
class Foo(object):
log = Logger()
def oops(self, data):
self.log.error(
"Oops! Invalid data from server: {data!r}",
data=data
)
When associated with a class in this manner, the "log_source" key is set in the event.
For example:
from twisted.logger import Logger
class MyObject(object):
log = Logger()
def __init__(self, value):
self.value = value
def doSomething(self, something):
self.log.info(
"Object with value {log_source.value!r} doing {something}.",
something=something
)
MyObject(7).doSomething("a task")
This example will show the string “object with value 7 doing a task” because the log_source key is automatically set to the MyObject instance that the Logger is retrieved from.
Capturing Failures¶
Logger provides a failure method, which allows one to capture a Failure object conveniently:
from twisted.logger import Logger
log = Logger()
try:
1 / 0
except:
log.failure("Math is hard!")
The emitted event will have the "log_failure" key set, which is a Failure that captures the exception.
This can be used by my observers to obtain a traceback.
For example, FileLogObserver will append the traceback to it’s output:
Math is hard!
Traceback (most recent call last):
--- <exception caught here> ---
File "/tmp/test.py", line 8, in <module>
1/0
exceptions.ZeroDivisionError: integer division or modulo by zero
Note that this API is meant to capture unexpected and unhandled errors (that is: bugs, which is why tracebacks are preserved). As such, it defaults to logging at the critical level. It is generally more appropriate to instead use log.error() when logging an expected error condition that was appropriately handled by the software.
Namespaces¶
All Logger s have a namespace, which can be used to categorize events.
Namespaces may be specified by passing in a namespace argument to Logger ‘s initializer, but if none is given, the logger will derive its namespace from the module name of the callable that instantiated it, or, in the case of a class, from the fully qualified name of the class.
A Logger will add a log_namespace key to the events it emits.
In the first example above, the namespace would be some.module , and in the second example, it would be some.module.Foo .
Log levels¶
Logger s provide a number of methods for emitting events.
These methods all have the same signature, but each will attach a specific log_level key to events.
Log levels are defined by the LogLevel constants container.
These are:
Debugging events: Information of use to a developer of the software, not generally of interest to someone running the software unless they are attempting to diagnose a software issue.
Informational events: Routine information about the status of an application, such as incoming connections, startup of a subsystem, etc.
Warning events: Events that may require greater attention than informational events but are not a systemic failure condition, such as authorization failures, bad data from a network client, etc. Such events are of potential interest to system administrators, and should ideally be phrased in such a way, or documented, so as to indicate an action that an administrator might take to mitigate the warning.
Error conditions: Events indicating a systemic failure. For example, resource exhaustion, or the loss of connectivity to an external system, such as a database or API endpoint, without which no useful work can proceed. Similar to warnings, errors related to operational parameters may be actionable to system administrators and should provide references to resources which an administrator might use to resolve them.
Critical failures: Errors indicating systemic failure (ie. service outage), data corruption, imminent data loss, etc. which must be handled immediately. This includes errors unanticipated by the software, such as unhandled exceptions, wherein the cause and consequences are unknown.
In the first example above, the call to log.debug will add a log_level key to the emitted event with a value of LogLevel.debug .
In the second example, calling self.log.error would use a value of LogLevel.error .
The above descriptions are simply guidance, but it is worth noting that log levels have a reduced value if they are used inconsistently. If one module in an application considers a message informational, and another module considers a similar message an error, then filtering based on log levels becomes harder. This is increasingly likely if the modules in question are developed by different parties, as will often be the case with externally source libraries and frameworks. (If a module tends to use higher levels than another, namespaces may be used to calibrate the relative use of log levels, but that is obviously suboptimal.) Sticking to the above guidelines will hopefully help here.
Emitter method signatures¶
The emitter methods (debug , info , warn , etc.) all take an optional format string as a first argument, followed by keyword arguments that will be included in the emitted event.
Note that all three examples in the opening section of this HOWTO fit this signature. The first omits the format, which doesn’t lend itself well to text logging. The second omits the keyword arguments, which hostile to anything other than text logging, and is therefore ill-advised. Finally, the third provides both, which is the recommended usage.
These methods are all convenience wrappers around the emit method, which takes a LogLevel as its first argument.
Format strings¶
Format strings provide observers with a standard way to format an event as text suitable for a human being to read, via the function formatEvent . When writing a format string, take care to present it in a manner which would make as much sense as possible to a human reader. Particularly, format strings need not be written with an eye towards parseability or machine-readability. If you want to save your log events along with their structure and then analyze them later, see the next section, on “saving events for later” .
Format strings should be
unicode , and use PEP 3101 syntax to describe how the event should be rendered as human-readable text.
For legacy support and convenience in python 2, UTF-8-encoded bytes are also accepted for format strings, but unicode is preferred.
There are two variations from PEP 3101 in the format strings used by this module:
Positional (numerical) field names (eg.
{0}) are not permitted. Event keys are not ordered, which means positional field names do not make sense in this context. However, this is not an accidental limitation, but an intentional design decision. As software evolves, log messages often grow to include additional information, while still logging the same conceptual event. By using meaningful names rather than opaque indexes for event keys, these identifiers are more robust against future changes in the format of messages and the information provided.Field names ending in parentheses (eg.
{foo()}) will call the referenced object with no arguments, then callstron the result, rather than callingstron the referenced object directly. This extension to PEP 3101 format syntax is provided to make it as easy as possible to defer potentially expensive work until a log message must be emitted. For example, let’s say that we wanted to log a message with some useful, but potentially expensive information from the ‘request’ object:log.info("{request.uri} useful, but expensive: {request.usefulButExpensive()}", request=request)
In the case where this log message is filtered out as uninteresting and not saved, no formatting work is done at all ; and since we can use PEP3101 attribute-access syntax in conjunction with this parenthesis extension, the caller does not even need to build a function or bound method object to pass as a separate key. There is no support for specifying arguments in the format string; the goal is to make it idiomatic to express that work be done later, not to implement a full Python expression evaluator.
Event keys added by the system¶
The logging system will add keys to emitted events.
All event keys that are inserted by the logging system with have a log_ prefix, to avoid namespace collisions with application-provided event keys.
Applications should therefore not insert event keys using the log_ prefix, as that prefix is reserved for the logging system.
System-provided event keys include:
log_logger
Logger object that the event was emitted to.
log_source
The source object that emitted the event. When a Logger is accessed as an attribute of a class, the class is the source. When accessed as an attribute of an instance, the instance is the source. In other cases, the source isNone.
log_level
The LogLevel associated with the event.
log_namespace
The namespace associated with the event.
log_format
The format string provided for use by observers that wish to render the event as text. This may beNone, if no format string was provided.
log_time
The time that the event was emitted, as returned by time .
log_failure
A Failure object captured when the event was emitted.
Avoid mutable event keys¶
Emitting applications should be cautious about inserting objects into event which may be mutated later. While observers are called synchronously, it is possible that an observer will do something like queue up the event for later serialization, in which case the serialized object may be different than intended.
Saving events for later¶
For compatibility reasons, twistd will log to a text-based format by default.
However, it’s much better to use a structured log file format which preserves information about the events being logged.
twisted.logger provides two APIs: jsonFileLogObserver and eventsFromJSONLogFile, which allow you to save and retrieve structured log events with a basic level of fidelity.
Log events are serialized as JSON dictionaries, with serialization rules that are as lenient as possible; any unknown values are replaced with simple placeholder values.
jsonFileLogObserver will create a log observer that will save events as structured data, like so:
import io
from twisted.logger import jsonFileLogObserver, Logger
log = Logger(observer=jsonFileLogObserver(io.open("log.json", "a")),
namespace="saver")
def loggit(values):
log.info("Some values: {values!r}", values=values)
loggit([1234, 5678])
loggit([9876, 5432])
And eventsFromJSONLogFile can load those events again; here, you can see that the event has preserved enough information to be formatted as human-readable again:
import sys
import io
from twisted.logger import (
eventsFromJSONLogFile, textFileLogObserver
)
output = textFileLogObserver(sys.stdout)
for event in eventsFromJSONLogFile(io.open("log.json")):
output(event)
You can also, of course, feel free to access any of the keys in the event object as you load them, as well; basic structures such as lists, numbers, dictionaries and strings (anything serializable with JSON) will be preserved:
import io
from twisted.logger import eventsFromJSONLogFile
for event in eventsFromJSONLogFile(io.open("log.json")):
print(sum(event["values"]))
Implementing an observer¶
An observer must provide the ILogObserver interface.
That interface simply describes a 1-argument callable that takes a dict , so a simple implementation may simply use the handy provider decorator on a function that takes one argument:
from zope.interface import provider
from twisted.logger import ILogObserver, formatEvent
@provider(ILogObserver)
def simpleObserver(event):
print(formatEvent(event))
The formatEvent function returns a textual (unicode ) representation of the event.
While it is recommended, in most cases it is not required that observers declare their compliance with ILogObserver .
This flexibility exists to allow for pre-existing callables and lambda expressions to be used as observers.
As an example, if one would like to accumulate events in a list , then list.append may be used as an observer.
When implementing your own log observer, however, you should always keep in mind that unlike most objects within Twisted, a log observer must be thread safe .
Specifically, a log observer:
- must be prepared to be called from threads other than the main thread (or I/O thread, or reactor thread)
- must be prepared to be called from multiple threads concurrently
- must not interact with other Twisted APIs that are not explicitly thread-safe without first taking precautions like using callFromThread
Keep in mind that this is true even if you elect not to explicitly interact with any threads from your program. Twisted itself may log messages from threads, and Twisted may internally use APIs like callInThread ; for example, Twisted uses threads to look up hostnames when making an outgoing connection.
Given this extra wrinkle, it’s usually best to see if you can find an existing log observer implementation that does what you need before implementing your own; thread safety can be tricky to implement. Luckily, twisted.logger comes with several useful observers, which are documented below.
Writing an observer for event analysis¶
Twisted includes log observers which take care of most of the “normal” uses of logging, like writing messages to a file, saving them as text, filtering them, and so on. There are two common reasons you might want to write a log observer of your own. The first is to ship log messages over to a different kind of external system which Twisted itself does not support. If you’ve read the above, you now know enough to do that; simply implement a function that converts the dictionary to something amenable to your external system or file format, and send it there or save it. The second task is to do some kind of analysis, either real-time within your process or after the fact offline.
You’re probably going to have to aggregate information from your own code and from libraries you’re using, including Twisted itself, and you may not have much in the way of control over how those messages are organized. You’re also probably trying to aggregate information from ad-hoc messages into some kind of structure.
One way to extract such semi-structured information is to feed your logs into an external system like logstash , and process them there.
Such systems are quite powerful and twisted.logger does not try to replace them.
However, such systems are necessarily external, and therefore if you want to react to the log analysis within your application - for example, denying access in response to an abusive client - you would have to write some glue code to push messages from your log-analysis system back into your application.
For such cases, it’s useful to be able to write log analysis code as a log observer.
Assuming that the libraries whose log events you’re interested in analyzing are making use of twisted.logger , you can analyze log events either live as they’re being logged, or loaded from a saved log file.
If you’re writing code to log events directly for potential analysis, then you should simply structure your messages to include all necessary information as serialization-friendly values in events, and then simply pull them out, like the loader-math.py example above.
However, let’s say you’re trying to interact with a system that logs messages like so:
from twisted.logger import Logger
class AdHoc(object):
log = Logger(namespace="ad_hoc")
def __init__(self, a, b):
self.a = a
self.b = b
def logMessage(self):
self.log.info("message from {log_source} "
"where a is {log_source.a} and b is {log_source.b}")
In this example, the AdHoc object is not itself serializable, but it has two relevant attributes, which the log message’s format string calls out as attributes of the log_source field, as described above: a and b .
To analyze this event, we can’t pursue the same strategy shown above in loader-math.py .
We don’t have any key/value pairs of the log event to examine directly; a and b are not present as keys themselves.
We could look for the log_source key within the event and access its a and b attributes, but that wouldn’t work once the event had been serialized and loaded again, since an AdHoc instance isn’t a basic type that can be saved to JSON.
Luckily, twisted.logger provides an API for doing just this: extractField . You use it like so:
from twisted.logger import extractField
fmt = (
"message from {log_source} "
"where a is {log_source.a} and b is {log_source.b}"
)
def analyze(event):
if event.get("log_format") == fmt:
a = extractField("log_source.a", event)
b = extractField("log_source.b", event)
print("A + B = " + repr(a + b))
This analyze function can then be used in two ways.
First, you can analyze your application “live”, as it’s running.
from twisted.logger import globalLogPublisher
from analyze import analyze
from ad_hoc import AdHoc
globalLogPublisher.addObserver(analyze)
AdHoc(3, 4).logMessage()
If you run this script, you will see that it extracts the values from the AdHoc object’s log message.
However, you can also analyze output from a saved log file, using the exact same code. First, you’ll need to produce a log file with a message from an AdHoc object in it:
import io
from twisted.logger import jsonFileLogObserver, globalLogPublisher
from ad_hoc import AdHoc
globalLogPublisher.addObserver(jsonFileLogObserver(io.open("log.json", "a")))
AdHoc(3, 4).logMessage()
If you run that script, it will produce a log.json file which can be analyzed in the same manner as the loader.py example above:
import io
from twisted.logger import eventsFromJSONLogFile
from analyze import analyze
for event in eventsFromJSONLogFile(io.open("log.json")):
analyze(event)
When doing analysis, it always seems like you should have saved more structured data for later. However, log messages are often written early on in development, in an ad-hoc way, before a solid understanding has developed about what information will be useful and what will be redundant. Log messages are therefore a stream-of-consciousness commentary on what is going on as a system is being developed.
extractField lets you acknowledge the messy reality of how log messages are written, but still take advantage of structured analysis later on.
Just always be sure to use event format fields, not string concatenation, to reference information about a particular event, and you’ll be able to easily pull apart hastily-written ad-hoc messages from multiple versions of a system, either as it’s running or once you’ve saved a log file.
Registering an observer¶
One way to register an observer is to construct a Logger object with it:
from twisted.logger import Logger
from myobservers import PrintingObserver
log = Logger(observer=PrintingObserver())
log.info("Hello")
This will cause all of a logger’s events to be sent to the given observer.
In the above example, all events emitted by the logger (eg. "Hello" ) will be printed.
While that is useful in some cases, it is common to register multiple observers, and to do so globally for all (or most) loggers in an application.
The global log publisher¶
The global log publisher is a log observer whose purpose is to capture log events not directed to a specific observer. In a typical application, the majority of log events will be emitted to the global log publisher. Observers can register themselves with the global log publisher in order to be forwarded these events.
When a Logger is created without specifying an observer to send events to, the logger will send its events to the global log publisher, which is accessible via the name globalLogPublisher .
The global log publisher is a singleton instance of a private subclass of LogPublisher , which is itself an ILogObserver . What this means is that the global log publisher accepts events like any other observer, and that it forwards those events to other observers. Observers can be registered to be forwarded events by calling the LogPublisher method addObserver , and unregister by calling removeObserver :
from twisted.logger import globalLogPublisher
from myobservers import PrintingObserver
log = Logger()
globalLogPublisher.addObserver(PrintingObserver())
log.info("Hello")
The result here is the same as the previous example, except that additional observers can be (and may already have been) registered.
We know that "Hello" will be printed.
We don’t know, but it’s very possible, that the same event will also be handled by other observers.
There is no supported API to discover what other observers are registered with a LogPublisher ; in general, one doesn’t need to know.
If an application is running in twistd , for example, it’s likely that an observer is streaming events to a file by the time the application code is in play.
If it is running in a twistd web container, there will probably be another observer writing to the access log.
A caveat here is that events are dict objects, which are mutable, so it is possible for an observer to modify an event that it sees.
Because doing so will modify what other observers will see, modifying a received event can be problematic and should be strongly discouraged.
It can be particularly harmful to edit or remove the content of an existing event key.
Furthermore, no guarantees are made as to the order in which observers are called, so the effect of such modifications on other observers may be non-deterministic.
Starting the global log publisher¶
When the global log publisher is created, it uses a LimitedHistoryLogObserver (see below) to store events that are logged by the application in memory until logging is started. Logging is started by registering the first set of observers with the global log publisher by calling beginLoggingTo :
from twisted.logger import globalLogBeginner
from myobservers import PrintingObserver
log = Logger()
log.info("Hello")
observers = [PrintingObserver()]
globalLogBeginner.beginLoggingTo(observers)
log.info("Hello, again")
What this does is add the given observers (in this example, the PrintingObserver ) with the global log observer, then forwards all of the events that were stored in memory prior to calling beginLoggingTo to these observers, and gets rid of the LimitedHistoryLogObserver , as it is no longer needed.
It is an error to call beginLoggingTo more than once.
Provided log observers¶
This module provides a number of pre-built observers for applications to use:
Forwards events to other publishers. This allows one to create a graph of observers.
Stores a limited number of received events, and can re-play those stored events to another observer later. This is useful for keeping recent logging history in memory for inspection when other log outputs are not available.
Formats events as text, prefixed with a time stamp and a “system identifier”, and writes them to a file. The system identifier defaults to a combination of the event’s namespace and level.
Forwards events to another observer after applying a set of filter predicates (providers of ILogFilterPredicate ). LogLevelFilterPredicate is a predicate that be configured to keep track of which log levels to filter for different namespaces, and will filter out events that are not at the appropriate level or higher.
Compatibility with standard library logging¶
STDLibLogObserver is provided for compatibility with the standard library’s logging module. Log levels are mapped between the two systems, and the various attributes of standard library log records are filled in properly.
Note that standard library logging is a blocking API, and logging can be configured to block for long periods (eg. it may write to the network). No protection is provided to prevent blocking, so such configurations may cause Twisted applications to perform poorly.
Compatibility with twisted.python.log¶
This module provides some facilities to enable the existing twisted.python.log module to compatibly forward it’s messages to this module. As such, existing clients of twisted.python.log will begin using this module indirectly, with no changes to the older module’s API.
Incrementally porting observers¶
Observers have an incremental path for porting to the new module. LegacyLogObserverWrapper is an ILogObserver that wraps a log observer written for the older module. This allows an old-style observer to be registered with a new-style logger or log publisher compatibly.
Twisted’s Legacy Logging System: twisted.python.log¶
Note
There is now a new logging system in Twisted (you can read about how to use it here and its API reference here) which is a replacement for twisted.python.log.
The old logging API, described here, remains for compatibility, and is now implemented as a client of the new logging system.
New code should adopt the new API.
Basic usage¶
Twisted provides a simple and flexible logging system in the twisted.python.log module. It has three commonly used functions:
Logs a new message. For example:
from twisted.python import log log.msg('Hello, world.')
Writes a failure to the log, including traceback information (if any). You can pass it a Failure or Exception instance, or nothing. If you pass something else, it will be converted to a string with
reprand logged.If you pass nothing, it will construct a Failure from the currently active exception, which makes it convenient to use in an
exceptclause:try: x = 1 / 0 except: log.err() # will log the ZeroDivisionError
Starts logging to a given file-like object. For example:
log.startLogging(open('/var/log/foo.log', 'w'))or:
log.startLogging(sys.stdout)or:
from twisted.python.logfile import DailyLogFile log.startLogging(DailyLogFile.fromFullPath("/var/log/foo.log"))By default,
startLoggingwill also redirect anything written tosys.stdoutandsys.stderrto the log. You can disable this by passingsetStdout=FalsetostartLogging.
Before startLogging is called, log messages will be
discarded and errors will be written to stderr.
Logging and twistd¶
If you are using twistd to run your daemon, it
will take care of calling startLogging for you, and will also
rotate log files. See twistd and tac
and the twistd man page for details of using
twistd.
Log files¶
The twisted.python.logfile module provides
some standard classes suitable for use with startLogging , such
as DailyLogFile ,
which will rotate the log to a new file once per day.
Using the standard library logging module¶
If your application uses the Python standard library logging module or you want to use its easy configuration but don’t want to lose twisted-produced messages, the observer PythonLoggingObserver should be useful to you.
You just start it like any other observer:
observer = log.PythonLoggingObserver()
observer.start()
Then configure the standard library logging module to behave as you want.
This method allows you to customize the log level received by the
standard library logging module using the logLevel keyword:
log.msg("This is important!", logLevel=logging.CRITICAL)
log.msg("Don't mind", logLevel=logging.DEBUG)
Unless logLevel is provided, logging.INFO is used for log.msg
and logging.ERROR is used for log.err .
One special care should be made when you use special configuration of
the standard library logging module: some handlers (e.g. SMTP, HTTP) use the network and
so can block inside the reactor loop. Nothing in PythonLoggingObserver is
done to prevent that.
Writing log observers¶
Log observers are the basis of the Twisted logging system.
Whenever log.msg (or log.err ) is called, an
event is emitted. The event is passed to each observer which has been
registered. There can be any number of observers, and each can treat
the event in any way desired.
An example of
a log observer in Twisted is the emit method of FileLogObserver .
FileLogObserver , used by
startLogging , writes events to a log file. A log observer
is just a callable that accepts a dictionary as its only argument. You can
then register it to receive all log events (in addition to any other
observers):
twisted.python.log.addObserver(yourCallable)
The dictionary will have at least two items:
message
The message (a list, usually of strings) for this log event, as passed tolog.msgor the message in the failure passed tolog.err.
isError
This is a boolean that will be true if this event came from a call tolog.err. If this is set, there may be afailureitem in the dictionary as will, with a Failure object in it.
Other items the built in logging functionality may add include:
printed
This message was captured fromsys.stdout, i.e. this message came from aisErroris also true, it came fromsys.stderr.
You can pass additional items to the event dictionary by passing keyword
arguments to log.msg and log.err . The standard
log observers will ignore dictionary items they don’t use.
Important notes:
- Never block in a log observer, as it may run in main Twisted thread. This means you can’t use socket or syslog standard library logging backends.
- The observer needs to be thread safe if you anticipate using threads in your program.
Customizing twistd logging¶
The behavior of the logging that twistd does can be
customized either with the --logger option or by setting the
ILogObserver component on the application object. See the Application document for more information.
Symbolic Constants¶
Overview¶
It is often useful to define names which will be treated as constants. twisted.python.constants provides APIs for defining such symbolic constants with minimal overhead and some useful features beyond those afforded by the common Python idioms for this task.
This document will explain how to use these APIs and what circumstances they might be helpful in.
Constant Names¶
Constants which have no value apart from their name and identity can be defined by subclassing Names . Consider this example, in which some HTTP request method constants are defined.
from twisted.python.constants import NamedConstant, Names
class METHOD(Names):
"""
Constants representing various HTTP request methods.
"""
GET = NamedConstant()
PUT = NamedConstant()
POST = NamedConstant()
DELETE = NamedConstant()
Only direct subclasses of Names are supported (i.e., you cannot subclass METHOD to add new constants the collection).
Given this definition, constants can be looked up by name using attribute access on the METHOD object:
>>> METHOD.GET
<METHOD=GET>
>>> METHOD.PUT
<METHOD=PUT>
>>>
If it’s necessary to look up constants from a string (e.g. based on user input of some sort), a safe way to do it is using lookupByName :
>>> METHOD.lookupByName('GET')
<METHOD=GET>
>>> METHOD.lookupByName('__doc__')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "twisted/python/constants.py", line 145, in lookupByName
raise ValueError(name)
ValueError: __doc__
>>>
As demonstrated, it is safe because any name not associated with a constant (even those special names initialized by Python itself) will result in ValueError being raised, not some other object not intended to be used the way the constants are used.
The constants can also be enumerated using the iterconstants method:
>>> list(METHOD.iterconstants())
[<METHOD=GET>, <METHOD=PUT>, <METHOD=POST>, <METHOD=DELETE>]
>>>
Constants can be compared for equality or identity:
>>> METHOD.GET is METHOD.GET
True
>>> METHOD.GET == METHOD.GET
True
>>> METHOD.GET is METHOD.PUT
False
>>> METHOD.GET == METHOD.PUT
False
>>>
Ordered comparisons (and therefore sorting) also work. The order is defined to be the same as the instantiation order of the constants:
>>> from twisted.python.constants import NamedConstant, Names
>>> class Letters(Names):
... a = NamedConstant()
... b = NamedConstant()
... c = NamedConstant()
...
>>> Letters.a < Letters.b < Letters.c
True
>>> Letters.a > Letters.b
False
>>> sorted([Letters.b, Letters.a, Letters.c])
[<Letters=a>, <Letters=b>, <Letters=c>]
>>>
A subclass of Names may define class methods to implement custom functionality.
Consider this definition of METHOD :
from twisted.python.constants import NamedConstant, Names
class METHOD(Names):
"""
Constants representing various HTTP request methods.
"""
GET = NamedConstant()
PUT = NamedConstant()
POST = NamedConstant()
DELETE = NamedConstant()
@classmethod
def isIdempotent(cls, method):
"""
Return True if the given method is side-effect free, False otherwise.
"""
return method is cls.GET
This functionality can be used as any class methods are used:
>>> METHOD.isIdempotent(METHOD.GET)
True
>>> METHOD.isIdempotent(METHOD.POST)
False
>>>
Constants With Values¶
Constants with a particular associated value are supported by the Values base class. Consider this example, in which some HTTP status code constants are defined.
from twisted.python.constants import ValueConstant, Values
class STATUS(Values):
"""
Constants representing various HTTP status codes.
"""
OK = ValueConstant("200")
FOUND = ValueConstant("302")
NOT_FOUND = ValueConstant("404")
As with Names , constants are accessed as attributes of the class object:
>>> STATUS.OK
<STATUS=OK>
>>> STATUS.FOUND
<STATUS=FOUND>
>>>
Additionally, the values of the constants can be accessed using the value attribute of one these objects:
>>> STATUS.OK.value
'200'
>>>
As with Names , constants can be looked up by name:
>>> STATUS.lookupByName('NOT_FOUND')
<STATUS=NOT_FOUND>
>>>
Constants on a Values subclass can also be looked up by value:
>>> STATUS.lookupByValue('404')
<STATUS=NOT_FOUND>
>>> STATUS.lookupByValue('500')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "twisted/python/constants.py", line 244, in lookupByValue
raise ValueError(value)
ValueError: 500
>>>
Multiple constants may have the same value.
If they do, lookupByValue will find the one which is defined first.
Iteration is also supported:
>>> list(STATUS.iterconstants())
[<STATUS=OK>, <STATUS=FOUND>, <STATUS=NOT_FOUND>]
>>>
Constants can be compared for equality, identity and ordering:
>>> STATUS.OK == STATUS.OK
True
>>> STATUS.OK is STATUS.OK
True
>>> STATUS.OK is STATUS.NOT_FOUND
False
>>> STATUS.OK == STATUS.NOT_FOUND
False
>>> STATUS.NOT_FOUND > STATUS.OK
True
>>> STATUS.FOUND < STATUS.OK
False
>>>
Note that like Names , Values are ordered by instantiation order, not by value, though either order is the same in the above example.
As with Names , a subclass of Values can define custom methods:
from twisted.python.constants import ValueConstant, Values
class STATUS(Values):
"""
Constants representing various HTTP status codes.
"""
OK = ValueConstant("200")
NO_CONTENT = ValueConstant("204")
NOT_MODIFIED = ValueConstant("304")
NOT_FOUND = ValueConstant("404")
@classmethod
def hasBody(cls, status):
"""
Return True if the given status is associated with a response body,
False otherwise.
"""
return status not in (cls.NO_CONTENT, cls.NOT_MODIFIED)
This functionality can be used as any class methods are used:
>>> STATUS.hasBody(STATUS.OK)
True
>>> STATUS.hasBody(STATUS.NO_CONTENT)
False
>>>
Constants As Flags¶
Integers are often used as a simple set for constants. The values for these constants are assigned as powers of two so that bits in the integer can be set to represent them. Individual bits are often called flags . Flags supports this use-case, including allowing constants with particular bits to be set, for interoperability with other tools.
POSIX filesystem access control is traditionally done using a bitvector defining which users and groups may perform which operations on a file.
This state might be represented using Flags as follows:
from twisted.python.constants import FlagConstant, Flags
class Permission(Flags):
"""
Constants representing user, group, and other access bits for reading,
writing, and execution.
"""
OTHER_EXECUTE = FlagConstant()
OTHER_WRITE = FlagConstant()
OTHER_READ = FlagConstant()
GROUP_EXECUTE = FlagConstant()
GROUP_WRITE = FlagConstant()
GROUP_READ = FlagConstant()
USER_EXECUTE = FlagConstant()
USER_WRITE = FlagConstant()
USER_READ = FlagConstant()
As for the previous types of constants, these can be accessed as attributes of the class object:
>>> Permission.USER_READ
<Permission=USER_READ>
>>> Permission.USER_WRITE
<Permission=USER_WRITE>
>>> Permission.USER_EXECUTE
<Permission=USER_EXECUTE>
>>>
These constant objects also have a value attribute giving their integer value:
>>> Permission.USER_READ.value
256
>>>
These constants can be looked up by name or value:
>>> Permission.lookupByName('USER_READ') is Permission.USER_READ
True
>>> Permission.lookupByValue(256) is Permission.USER_READ
True
>>>
Constants can also be combined using the logical operators & (and ), | (or ), and ^ (exclusive or ).
>>> Permission.USER_READ | Permission.USER_WRITE
<Permission={USER_READ,USER_WRITE}>
>>> (Permission.USER_READ | Permission.USER_WRITE) & Permission.USER_WRITE
<Permission=USER_WRITE>
>>> (Permission.USER_READ | Permission.USER_WRITE) ^ Permission.USER_WRITE
<Permission=USER_READ>
>>>
These combined constants can be deconstructed via iteration:
>>> mode = Permission.USER_READ | Permission.USER_WRITE
>>> list(mode)
[<Permission=USER_READ>, <Permission=USER_WRITE>]
>>> Permission.USER_READ in mode
True
>>> Permission.USER_EXECUTE in mode
False
>>>
They can also be inspected via boolean operations:
>>> Permission.USER_READ & mode
<Permission=USER_READ>
>>> bool(Permission.USER_READ & mode)
True
>>> Permission.USER_EXECUTE & mode
<Permission={}>
>>> bool(Permission.USER_EXECUTE & mode)
False
>>>
The unary operator ~ (not ) is also defined:
>>> ~Permission.USER_READ
<Permission={GROUP_EXECUTE,GROUP_READ,GROUP_WRITE,OTHER_EXECUTE,OTHER_READ,OTHER_WRITE,USER_EXECUTE,USER_WRITE}>
>>>
Constants created using these operators also have a value attribute.
>>> (~Permission.USER_WRITE).value
383
>>>
Note the care taken to ensure the ~ operator is applied first and the value attribute is looked up second.
A Flags subclass can also define methods, just as a Names or Values subclass may.
For example, Permission might benefit from a method to format a flag as a string in the traditional style.
Consider this addition to that class:
from twisted.python import filepath
from twisted.python.constants import FlagConstant, Flags
class Permission(Flags):
...
@classmethod
def format(cls, permissions):
"""
Format permissions flags in the traditional 'rwxr-xr-x' style.
"""
return filepath.Permissions(permissions.value).shorthand()
Use this like any other class method:
>>> Permission.format(Permission.USER_READ | Permission.USER_WRITE | Permission.GROUP_READ | Permission.OTHER_READ)
'rw-r--r--'
>>>
twisted.enterprise.adbapi: Twisted RDBMS support¶
Abstract¶
Twisted is an asynchronous networking framework, but most database API implementations unfortunately have blocking interfaces – for this reason, twisted.enterprise.adbapi was created. It is a non-blocking interface to the standardized DB-API 2.0 API, which allows you to access a number of different RDBMSes.
What you should already know¶
- Python :-)
- How to write a simple Twisted Server (see this tutorial to learn how)
- Familiarity with using database interfaces (see the documentation for DBAPI 2.0)
Quick Overview¶
Twisted is an asynchronous framework. This means standard database modules cannot be used directly, as they typically work something like:
# Create connection...
db = dbmodule.connect('mydb', 'andrew', 'password')
# ...which blocks for an unknown amount of time
# Create a cursor
cursor = db.cursor()
# Do a query...
resultset = cursor.query('SELECT * FROM table WHERE ...')
# ...which could take a long time, perhaps even minutes.
Those delays are unacceptable when using an asynchronous framework such as Twisted. For this reason, Twisted provides twisted.enterprise.adbapi, an asynchronous wrapper for any DB-API 2.0-compliant module.
adbapi will do blocking database operations in separate threads, which trigger callbacks in the originating thread when they complete. In the meantime, the original thread can continue doing normal work, like servicing other requests.
How do I use adbapi?¶
Rather than creating a database connection directly, use the adbapi.ConnectionPool class to manage a connections for you. This allows adbapi to use multiple connections, one per thread. This is easy:
# Using the "dbmodule" from the previous example, create a ConnectionPool
from twisted.enterprise import adbapi
dbpool = adbapi.ConnectionPool("dbmodule", 'mydb', 'andrew', 'password')
Things to note about doing this:
- There is no need to import dbmodule directly. You just pass the name to adbapi.ConnectionPool‘s constructor.
- The parameters you would pass to dbmodule.connect are passed as extra arguments to adbapi.ConnectionPool‘s constructor. Keyword parameters work as well.
Now we can do a database query:
# equivalent of cursor.execute(statement), return cursor.fetchall():
def getAge(user):
return dbpool.runQuery("SELECT age FROM users WHERE name = ?", user)
def printResult(l):
if l:
print l[0][0], "years old"
else:
print "No such user"
getAge("joe").addCallback(printResult)
This is straightforward, except perhaps for the return value of getAge.
It returns a Deferred, which allows arbitrary callbacks to be called upon completion (or upon failure).
More documentation on Deferred is available here.
In addition to runQuery, there is also runOperation and runInteraction that gets called with a callable (e.g. a function).
The function will be called in the thread with a adbapi.Transaction, which basically mimics a DB-API cursor.
In all cases a database transaction will be committed after your database usage is finished, unless an exception is raised in which case it will be rolled back.
def _getAge(txn, user):
# this will run in a thread, we can use blocking calls
txn.execute("SELECT * FROM foo")
# ... other cursor commands called on txn ...
txn.execute("SELECT age FROM users WHERE name = ?", user)
result = txn.fetchall()
if result:
return result[0][0]
else:
return None
def getAge(user):
return dbpool.runInteraction(_getAge, user)
def printResult(age):
if age != None:
print age, "years old"
else:
print "No such user"
getAge("joe").addCallback(printResult)
Also worth noting is that these examples assumes that dbmodule uses the “qmarks” paramstyle (see the DB-API specification).
If your dbmodule uses a different paramstyle (e.g. pyformat) then use that.
Twisted doesn’t attempt to offer any sort of magic parameter munging – runQuery(query, params, ...) maps directly onto cursor.execute(query, params, ...).
Examples of various database adapters¶
Notice that the first argument is the module name you would usually import and get connect(...) from, and that following arguments are whatever arguments you’d call connect(...) with.
from twisted.enterprise import adbapi
# PostgreSQL PyPgSQL
cp = adbapi.ConnectionPool("pyPgSQL.PgSQL", database="test")
# MySQL
cp = adbapi.ConnectionPool("MySQLdb", db="test")
And that’s it!¶
That’s all you need to know to use a database from within Twisted. You probably should read the adbapi module’s documentation to get an idea of the other functions it has, but hopefully this document presents the core ideas.
Parsing command-lines with usage.Options¶
Introduction¶
There is frequently a need for programs to parse a UNIX-like
command line program: options preceded by - or
-- , sometimes followed by a parameter, followed by
a list of arguments. The twisted.python.usage provides a class,
Options , to facilitate such parsing.
While Python has the getopt module for doing
this, it provides a very low level of abstraction for options.
Twisted has a higher level of abstraction, in the class twisted.python.usage.Options . It uses
Python’s reflection facilities to provide an easy to use yet
flexible interface to the command line. While most command line
processors either force the application writer to write their own
loops, or have arbitrary limitations on the command line (the
most common one being not being able to have more than one
instance of a specific option, thus rendering the idiom
program -v -v -v impossible), Twisted allows the
programmer to decide how much control they want.
The Options class is used by subclassing. Since
a lot of time it will be used in the twisted.tap package, where the local
conventions require the specific options parsing class to also
be called Options , it is usually imported with
from twisted.python import usage
Boolean Options¶
For simple boolean options, define the attribute
optFlags like this:
class Options(usage.Options):
optFlags = [["fast", "f", "Act quickly"], ["safe", "s", "Act safely"]]
optFlags should be a list of 3-lists. The first element
is the long name, and will be used on the command line as
--fast . The second one is the short name, and will be used
on the command line as -f . The last element is a
description of the flag and will be used to generate the usage
information text. The long name also determines the name of the key
that will be set on the Options instance. Its value will be 1 if the
option was seen, 0 otherwise. Here is an example for usage:
class Options(usage.Options):
optFlags = [
["fast", "f", "Act quickly"],
["good", "g", "Act well"],
["cheap", "c", "Act cheaply"]
]
command_line = ["-g", "--fast"]
options = Options()
try:
options.parseOptions(command_line)
except usage.UsageError, errortext:
print '%s: %s' % (sys.argv[0], errortext)
print '%s: Try --help for usage details.' % (sys.argv[0])
sys.exit(1)
if options['fast']:
print "fast",
if options['good']:
print "good",
if options['cheap']:
print "cheap",
print
The above will print fast good .
Note here that Options fully supports the mapping interface. You can access it mostly just like you can access any other dict. Options are stored as mapping items in the Options instance: parameters as ‘paramname’: ‘value’ and flags as ‘flagname’: 1 or 0.
Inheritance, Or: How I Learned to Stop Worrying and Love the Superclass¶
Sometimes there is a need for several option processors with a unifying core.
Perhaps you want all your commands to understand -q /--quiet means to be quiet, or something similar.
On the face of it, this looks impossible: in Python, the subclass’s optFlags would shadow the superclass’s.
However, usage.Options uses special reflection code to get all of the optFlags defined in the hierarchy. So the following:
class BaseOptions(usage.Options):
optFlags = [["quiet", "q", None]]
class SpecificOptions(BaseOptions):
optFlags = [
["fast", "f", None], ["good", "g", None], ["cheap", "c", None]
]
Is the same as:
class SpecificOptions(usage.Options):
optFlags = [
["quiet", "q", "Silence output"],
["fast", "f", "Run quickly"],
["good", "g", "Don't validate input"],
["cheap", "c", "Use cheap resources"]
]
Parameters¶
Parameters are specified using the attribute
optParameters . They must be given a
default. If you want to make sure you got the parameter from
the command line, give a non-string default. Since the command
line only has strings, this is completely reliable.
Here is an example:
from twisted.python import usage
class Options(usage.Options):
optFlags = [
["fast", "f", "Run quickly"],
["good", "g", "Don't validate input"],
["cheap", "c", "Use cheap resources"]
]
optParameters = [["user", "u", None, "The user name"]]
config = Options()
try:
config.parseOptions() # When given no argument, parses sys.argv[1:]
except usage.UsageError, errortext:
print '%s: %s' % (sys.argv[0], errortext)
print '%s: Try --help for usage details.' % (sys.argv[0])
sys.exit(1)
if config['user'] is not None:
print "Hello", config['user']
print "So, you want it:"
if config['fast']:
print "fast",
if config['good']:
print "good",
if config['cheap']:
print "cheap",
print
Like optFlags , optParameters works
smoothly with inheritance.
Option Subcommands¶
It is useful, on occasion, to group a set of options together based
on the logical “action” to which they belong. For this, the
usage.Options class allows you to define a set of
“subcommands” , each of which can provide its own
usage.Options instance to handle its particular
options.
Here is an example for an Options class that might parse options like those the cvs program takes
from twisted.python import usage
class ImportOptions(usage.Options):
optParameters = [
['module', 'm', None, None], ['vendor', 'v', None, None],
['release', 'r', None]
]
class CheckoutOptions(usage.Options):
optParameters = [['module', 'm', None, None], ['tag', 'r', None, None]]
class Options(usage.Options):
subCommands = [['import', None, ImportOptions, "Do an Import"],
['checkout', None, CheckoutOptions, "Do a Checkout"]]
optParameters = [
['compression', 'z', 0, 'Use compression'],
['repository', 'r', None, 'Specify an alternate repository']
]
config = Options(); config.parseOptions()
if config.subCommand == 'import':
doImport(config.subOptions)
elif config.subCommand == 'checkout':
doCheckout(config.subOptions)
The subCommands attribute of Options
directs the parser to the two other Options subclasses
when the strings "import" or "checkout" are
present on the command
line. All options after the given command string are passed to the
specified Options subclass for further parsing. Only one subcommand
may be specified at a time. After parsing has completed, the Options
instance has two new attributes - subCommand and subOptions - which hold the command string and the Options
instance used to parse the remaining options.
Generic Code For Options¶
Sometimes, just setting an attribute on the basis of the options is not flexible enough. In those cases, Twisted does not even attempt to provide abstractions such as “counts” or “lists” , but rather lets you call your own method, which will be called whenever the option is encountered.
Here is an example of counting verbosity
from twisted.python import usage
class Options(usage.Options):
def __init__(self):
usage.Options.__init__(self)
self['verbosity'] = 0 # default
def opt_verbose(self):
self['verbosity'] = self['verbosity']+1
def opt_quiet(self):
self['verbosity'] = self['verbosity']-1
opt_v = opt_verbose
opt_q = opt_quiet
Command lines that look like
command -v -v -v -v will
increase verbosity to 4, while
command -q -q -q will decrease
verbosity to -3.
The usage.Options class knows that these are parameter-less options, since the methods do not receive an argument. Here is an example for a method with a parameter:
from twisted.python import usage
class Options(usage.Options):
def __init__(self):
usage.Options.__init__(self)
self['symbols'] = []
def opt_define(self, symbol):
self['symbols'].append(symbol)
opt_D = opt_define
This example is useful for the common idiom of having
command -DFOO -DBAR to define symbols.
Parsing Arguments¶
usage.Options does not stop helping when the
last parameter is gone. All the other arguments are sent into a
function which should deal with them. Here is an example for a
cmp like command.
from twisted.python import usage
class Options(usage.Options):
optParameters = [["max_differences", "d", 1, None]]
def parseArgs(self, origin, changed):
self['origin'] = origin
self['changed'] = changed
The command should look like command origin changed .
If you want to have a variable number of left-over
arguments, just use def parseArgs(self, *args): .
This is useful for commands like the UNIX
cat(1) .
Post Processing¶
Sometimes, you want to perform post processing of options to patch up inconsistencies, and the like. Here is an example:
from twisted.python import usage
class Options(usage.Options):
optFlags = [
["fast", "f", "Run quickly"],
["good", "g", "Don't validate input"],
["cheap", "c", "Use cheap resources"]
]
def postOptions(self):
if self['fast'] and self['good'] and self['cheap']:
raise usage.UsageError, "can't have it all, brother"
Type enforcement¶
By default, all options are handled as strings. You may want to
enforce the type of your option in some specific case, the classic example
being port number. Any callable can be specified in the fifth row of
optParameters and will be called with the string value passed
in parameter.
from twisted.python import usage
class Options(usage.Options):
optParameters = [
["shiny_integer", "s", 1, None, int],
["dummy_float", "d", 3.14159, None, float],
]
Note that default values are not coerced, so you should either declare it with the good type (as above) or handle it when you use your options.
The coerce function may have a coerceDoc attribute, the content of which will be printed after the documentation of the option. It’s particularly useful for reusing the function at multiple places.
def oneTwoThree(val):
val = int(val)
if val not in range(1, 4):
raise ValueError("Not in range")
return val
oneTwoThree.coerceDoc = "Must be 1, 2 or 3."
from twisted.python import usage
class Options(usage.Options):
optParameters = [["one_choice", "o", 1, None, oneTwoThree]]
This example code will print the following help when added to your program:
$ python myprogram.py --help
Usage: myprogram [options]
Options:
-o, --one_choice= [default: 0]. Must be 1, 2 or 3.
Shell tab-completion¶
The Options class may provide tab-completion to interactive
command shells. Only zsh is supported at present, but there is
some interest in supporting bash in the future.
Support is automatic for all of the commands shipped with Twisted. Zsh
has shipped, for a number of years, a completion function which ties in to
the support provided by the Options class.
If you are writing a twistd plugin, then tab-completion
for your twistd sub-command is also automatic.
For other commands you may easily provide zsh tab-completion support. Copy the file “twisted/python/twisted-completion.zsh” and name it something like “_mycommand”. A leading underscore with no extension is zsh’s convention for completion function files.
Edit the new file and change the first line to refer only to your new command(s), like so:
#compdef mycommand
Then ensure this file is made available to the shell by placing it in
one of the directories appearing in zsh’s $fpath. Restart zsh, and ensure
advanced completion is enabled
(autoload -U compinit; compinit) . You should then be able to
type the name of your command and press Tab to have your command-line
options completed.
Completion metadata¶
Optionally, a special attribute, compData , may be defined
on your Options subclass in order to provide more information
to the shell-completion system. The attribute should be an instance of
I DON’T KNOW WHAT TO DO WITH THIS LINK!
In addition, compData may be defined on parent classes in
your inheritance hiearchy. The information from each
I DON’T KNOW WHAT TO DO WITH THIS LINK!
DirDBM: Directory-based Storage¶
dirdbm.DirDBM¶
twisted.persisted.dirdbm.DirDBM is a DBM-like storage system. That is, it stores mappings between keys and values, like a Python dictionary, except that it stores the values in files in a directory - each entry is a different file. The keys must always be strings, as are the values. Other than that, DirDBM objects act just like Python dictionaries.
DirDBM is useful for cases when you want to store small amounts of data in an organized fashion, without having to deal with the complexity of a RDBMS or other sophisticated database. It is simple, easy to use, cross-platform, and doesn’t require any external C libraries, unlike Python’s built-in DBM modules.
>>> from twisted.persisted import dirdbm
>>> d = dirdbm.DirDBM("/tmp/dir")
>>> d["librarian"] = "ook"
>>> d["librarian"]
'ook'
>>> d.keys()
['librarian']
>>> del d["librarian"]
>>> d.items()
[]
dirdbm.Shelf¶
Sometimes it is neccessary to persist more complicated objects than strings.
With some care, dirdbm.Shelf
can transparently persist
them. Shelf works exactly like DirDBM , except that
the values (but not the keys) can be arbitrary picklable objects. However,
notice that mutating an object after it has been stored in the Shelf has no effect on the Shelf.
When mutating objects, it is neccessary to explicitly store them back in the Shelf
afterwards:
>>> from twisted.persisted import dirdbm
>>> d = dirdbm.Shelf("/tmp/dir2")
>>> d["key"] = [1, 2]
>>> d["key"]
[1, 2]
>>> l = d["key"]
>>> l.append(3)
>>> d["key"]
[1, 2]
>>> d["key"] = l
>>> d["key"]
[1, 2, 3]
Writing tests for Twisted code using Trial¶
Trial basics¶
Trial is Twisted’s testing framework. It provides a
library for writing test cases and utility functions for working with the
Twisted environment in your tests, and a command-line utility for running your
tests. Trial is built on the Python standard library’s unittest
module. For more information on how Trial finds tests, see the
loadModule documentation.
To run all the Twisted tests, do:
$ trial twisted
Refer to the Trial man page for other command-line options.
Trial directories¶
You might notice a new _trial_temp folder in the
current working directory after Trial completes the tests. This folder is the
working directory for the Trial process. It can be used by unit tests and
allows them to write whatever data they like to disk, and not worry
about polluting the current working directory.
Folders named _trial_temp-<counter> are
created if two instances of Trial are run in parallel from the same directory,
so as to avoid giving two different test-runs the same temporary directory.
The twisted.python.lockfile utility is used to lock
the _trial_temp directories. On Linux, this results
in symlinks to pids. On Windows, directories are created with a single file with
a pid as the contents. These lock files will be cleaned up if Trial exits normally
and otherwise they will be left behind. They should be cleaned up the next time
Trial tries to use the directory they lock, but it’s also safe to delete them
manually if desired.
Twisted-specific quirks: reactor, Deferreds, callLater¶
The standard Python unittest framework, from which Trial is
derived, is ideal for testing code with a fairly linear flow of control.
Twisted is an asynchronous networking framework which provides a clean,
sensible way to establish functions that are run in response to events (like
timers and incoming data), which creates a highly non-linear flow of control.
Trial has a few extensions which help to test this kind of code. This section
provides some hints on how to use these extensions and how to best structure
your tests.
Leave the Reactor as you found it¶
Trial runs the entire test suite (over four thousand tests) in a single process, with a single reactor. Therefore it is important that your test leave the reactor in the same state as it found it. Leftover timers may expire during somebody else’s unsuspecting test. Leftover connection attempts may complete (and fail) during a later test. These lead to intermittent failures that wander from test to test and are very time-consuming to track down.
If your test leaves event sources in the reactor, Trial will fail the test.
The tearDown method is a good place to put cleanup code: it is
always run regardless of whether your test passes or fails (like a finally
clause in a try-except-finally construct). Exceptions in tearDown
are flagged as errors and flunk the test.
TestCase.addCleanup is
another useful tool for cleaning up. With it, you can register callables to
clean up resources as the test allocates them. Generally, code should be
written so that only resources allocated in the tests need to be cleaned up in
the tests. Resources which are allocated internally by the implementation
should be cleaned up by the implementation.
If your code uses Deferreds or depends on the reactor running, you can
return a Deferred from your test method, setUp, or tearDown and Trial will
do the right thing. That is, it will run the reactor for you until the
Deferred has triggered and its callbacks have been run. Don’t use
reactor.run() , reactor.stop() , reactor.crash() or reactor.iterate() in your tests.
Calls to reactor.callLater create IDelayedCall s. These need to be run
or cancelled during a test, otherwise they will outlive the test. This would
be bad, because they could interfere with a later test, causing confusing
failures in unrelated tests! For this reason, Trial checks the reactor to make
sure there are no leftover IDelayedCall s in the reactor after a
test, and will fail the test if there are. The cleanest and simplest way to
make sure this all works is to return a Deferred from your test.
Similarly, sockets created during a test should be closed by the end of the
test. This applies to both listening ports and client connections. So, calls
to reactor.listenTCP (and listenUNIX , and so on)
return IListeningPort s, and these should be
cleaned up before a test ends by calling their stopListening method.
Calls to reactor.connectTCP return IConnector s, which should be cleaned
up by calling their disconnect method. Trial
will warn about unclosed sockets.
The golden rule is: If your tests call a function which returns a Deferred, your test should return a Deferred.
Using Timers to Detect Failing Tests¶
It is common for tests to establish some kind of fail-safe timeout that will terminate the test in case something unexpected has happened and none of the normal test-failure paths are followed. This timeout puts an upper bound on the time that a test can consume, and prevents the entire test suite from stalling because of a single test. This is especially important for the Twisted test suite, because it is run automatically by the buildbot whenever changes are committed to the Subversion repository.
The way to do this in Trial is to set the .timeout attribute
on your unit test method. Set the attribute to the number of seconds you wish
to elapse before the test raises a timeout error. Trial has a default timeout
which will be applied even if the timeout attribute is not set.
The Trial default timeout is usually sufficient and should be overridden only
in unusual cases.
Interacting with warnings in tests¶
Trial includes specific support for interacting with Python’s
warnings module. This support allows warning-emitting code to
be written test-driven, just as any other code would be. It also improves
the way in which warnings reporting when a test suite is running.
TestCase.flushWarnings
allows tests to be written which make assertions about what warnings have
been emitted during a particular test method. In order to test a warning with
flushWarnings , write a test which first invokes the code which
will emit a warning and then calls flushWarnings and makes
assertions about the result. For example:
class SomeWarningsTests(TestCase):
def test_warning(self):
warnings.warn("foo is bad")
self.assertEqual(len(self.flushWarnings()), 1)
Warnings emitted in tests which are not flushed will be included by the
default reporter in its output after the result of the test. If Python’s
warnings filter system (see the-W command option to Python ) is configured to treat a warning as an error,
then unflushed warnings will causes tests to fail and will be included in
the summary section of the default reporter. Note that unlike usual
operation, when warnings.warn is called as part of a test
method, it will not raise an exception when warnings have been configured as
errors. However, if called outside of a test method (for example, at module
scope in a test module or a module imported by a test module) then it
will raise an exception.
Parallel test¶
In many situations, your unit tests may run faster if they are allowed to
run in parallel, such that blocking I/O calls allow other tests to continue.
Trial, like unittest, supports the -j parameter. Run trial -j 3
to run 3 test runners at the same time.
This requires care in your test creation. Obviously, you need to ensure that your code is otherwise content to work in a parallel fashion while working within Twisted... and if you are using weird global variables in places, parallel tests might reveal this.
However, if you have a test that fires up a schema on an external database
in the setUp function, does some operations on it in the test, and
then deletes that schema in the tearDown function, your tests will behave in an
unpredictable fashion as they tromp upon each other if they have their own
schema. And this won’t actually indicate a real error in your code, merely a
testing-specific race-condition.
Extremely Low-Level Socket Operations¶
Introduction¶
Beyond supporting streams of data (SOCK_STREAM) or datagrams (SOCK_DGRAM), POSIX sockets have additional features not accessible via send(2) and recv(2). These features include things like scatter/gather I/O, duplicating file descriptors into other processes, and accessing out-of-band data.
Twisted includes a wrapper around the two C APIs which make these things possible, sendmsg and recvmsg . This document covers their usage. It is intended for Twisted maintainers. Application developers looking for this functionality should look for the high-level APIs Twisted provides on top of these wrappers.
sendmsg¶
sendmsg(2) exposes nearly all sender-side functionality of a socket.
For a SOCK_STREAM socket, it can send bytes that become part of the stream of data being carried over the connection.
For a SOCK_DGRAM socket, it can send bytes that become datagrams sent from the socket.
It can send data from multiple memory locations (gather I/O).
Over AF_UNIX sockets, it can copy file descriptors into whichever process is receiving on the other side.
The wrapper included in Twisted, sendmsg, exposes many (but not all) of these features.
This document covers the usage of the features it does expose.
The primary limitation of this wrapper is that the interface supports sending only one iovec at a time.
recvmsg¶
Likewise, recvmsg(2) exposes nearly all the receiver-side functionality of a socket.
It can receive stream data over from a SOCK_STREAM socket or datagrams from a SOCK_DGRAM socket.
It can receive that data into multiple memory locations (scatter I/O), and it can receive those copied file descriptors.
The wrapper included in Twisted, recvmsg, exposes many (but not all) of these features.
This document covers the usage of the features it does expose.
The primary limitation of this wrapper is that the interface supports receiving only one iovec at a time.
Sending And Receiving Regular Data¶
sendmsg can be used in a way which makes it equivalent to using the send call. The first argument to sendmsg is (in this case and all others) a socket over which to send the data. The second argument is a bytestring giving the data to send.
On the other end, recvmsg can be used to replace a recv call. The first argument to recvmsg is (again, in all cases) a socket over which to receive the data. The second argument is an integer giving the maximum number of bytes of data to receive.
# Copyright (c) Twisted Matrix Laboratories.
# See LICENSE for details.
"""
Demonstration of sending bytes over a TCP connection using sendmsg.
"""
from __future__ import print_function
from socket import socketpair
from twisted.python.sendmsg import sendmsg, recvmsg
def main():
foo, bar = socketpair()
sent = sendmsg(foo, b"Hello, world")
print("Sent", sent, "bytes")
(received, ancillary, flags) = recvmsg(bar, 1024)
print("Received", repr(received))
print("Extra stuff, boring in this case", flags, ancillary)
if __name__ == '__main__':
main()
Copying File Descriptors¶
Used with an AF_UNIX socket, sendmsg send a copy of a file descriptor into whatever process is receiving on the other end of the socket. This is done using the ancillary data argument. Ancillary data consists of a list of three-tuples. A three-tuple constructed with SOL_SOCKET, SCM_RIGHTS, and a platform-endian packed file descriptor number will copy that file descriptor.
File descriptors copied this way must be received using a recvmsg call. No special arguments are required to receive these descriptors. They will appear, encoded as a native-order string, in the ancillary data list returned by recvmsg.
# Copyright (c) Twisted Matrix Laboratories.
# See LICENSE for details.
"""
Demonstration of copying a file descriptor over an AF_UNIX connection using
sendmsg.
"""
from __future__ import print_function
from os import pipe, read, write
from socket import SOL_SOCKET, socketpair
from struct import unpack, pack
from twisted.python.sendmsg import SCM_RIGHTS, sendmsg, recvmsg
def main():
foo, bar = socketpair()
reader, writer = pipe()
# Send a copy of the descriptor. Notice that there must be at least one
# byte of normal data passed in.
sent = sendmsg(
foo, b"\x00", [(SOL_SOCKET, SCM_RIGHTS, pack("i", reader))])
# Receive the copy, including that one byte of normal data.
data, ancillary, flags = recvmsg(bar, 1024)
duplicate = unpack("i", ancillary[0][2])[0]
# Demonstrate that the copy works just like the original
write(writer, b"Hello, world")
print("Read from original (%d): %r" % (reader, read(reader, 6)))
print("Read from duplicate (%d): %r" % (duplicate, read(duplicate, 6)))
if __name__ == '__main__':
main()
A synchronous M essaging P rotocol Overview
A synchronous M essaging P rotocol Overview The purpose of this guide is to describe the uses for and usage of twisted.protocols.amp beyond what is explained in the API documentation. It will show you how to implement an AMP server which can respond to commands or interact directly with individual messages. It will also show you how to implement an AMP client which can issue commands to a server.
AMP is a bidirectional command/response-oriented protocol intended to be extended with application-specific request types and handlers. Various simple data types are supported and support for new data types can be added by applications.
Setting Up¶
AMP runs over a stream-oriented connection-based protocol, such as TCP or SSL. Before you can use any features of the AMP protocol, you need a connection. The protocol class to use to establish an AMP connection is AMP . Connection setup works as it does for almost all protocols in Twisted. For example, you can set up a listening AMP server using a server endpoint:
from twisted.protocols.amp import AMP
from twisted.internet import reactor
from twisted.internet.protocol import Factory
from twisted.internet.endpoints import TCP4ServerEndpoint
from twisted.application.service import Application
from twisted.application.internet import StreamServerEndpointService
application = Application("basic AMP server")
endpoint = TCP4ServerEndpoint(reactor, 8750)
factory = Factory()
factory.protocol = AMP
service = StreamServerEndpointService(endpoint, factory)
service.setServiceParent(application)
And you can connect to an AMP server using a client endpoint:
if __name__ == '__main__':
import basic_client
raise SystemExit(basic_client.main())
from sys import stdout
from twisted.python.log import startLogging, err
from twisted.protocols.amp import AMP
from twisted.internet import reactor
from twisted.internet.protocol import Factory
from twisted.internet.endpoints import TCP4ClientEndpoint
def connect():
endpoint = TCP4ClientEndpoint(reactor, "127.0.0.1", 8750)
return endpoint.connect(Factory.forProtocol(AMP))
def main():
startLogging(stdout)
d = connect()
d.addErrback(err, "Connection failed")
def done(ignored):
reactor.stop()
d.addCallback(done)
reactor.run()
Commands¶
Either side of an AMP connection can issue a command to the other side. Each kind of command is represented as a subclass of Command . A Command defines arguments, response values, and error conditions.
from twisted.protocols.amp import Integer, String, Unicode, Command
class UsernameUnavailable(Exception):
pass
class RegisterUser(Command):
arguments = [('username', Unicode()),
('publickey', String())]
response = [('uid', Integer())]
errors = {UsernameUnavailable: 'username-unavailable'}
The definition of the command’s signature - its arguments, response, and possible error conditions - is separate from the implementation of the behavior to execute when the command is received. The Command subclass only defines the former.
Commands are issued by calling callRemote on either side of the connection. This method returns a Deferred which eventually fires with the result of the command.
if __name__ == '__main__':
import command_client
raise SystemExit(command_client.main())
from sys import stdout
from twisted.python.log import startLogging, err
from twisted.protocols.amp import Integer, String, Unicode, Command
from twisted.internet import reactor
from basic_client import connect
class UsernameUnavailable(Exception):
pass
class RegisterUser(Command):
arguments = [('username', Unicode()),
('publickey', String())]
response = [('uid', Integer())]
errors = {UsernameUnavailable: 'username-unavailable'}
def main():
startLogging(stdout)
d = connect()
def connected(protocol):
return protocol.callRemote(
RegisterUser,
username=u'alice',
publickey='ssh-rsa AAAAB3NzaC1yc2 alice@actinium')
d.addCallback(connected)
def registered(result):
print 'Registration result:', result
d.addCallback(registered)
d.addErrback(err, "Failed to register")
def finished(ignored):
reactor.stop()
d.addCallback(finished)
reactor.run()
Locators¶
The logic for handling a command can be specified as an object separate from the AMP instance which interprets and formats bytes over the network.
from twisted.protocols.amp import CommandLocator
from twisted.python.filepath import FilePath
class UsernameUnavailable(Exception):
pass
class UserRegistration(CommandLocator):
uidCounter = 0
@RegisterUser.responder
def register(self, username, publickey):
path = FilePath(username)
if path.exists():
raise UsernameUnavailable()
self.uidCounter += 1
path.setContent('%d %s\n' % (self.uidCounter, publickey))
return self.uidCounter
When you define a separate CommandLocator subclass, use it by passing an instance of it to the AMP initializer.
factory = Factory()
factory.protocol = lambda: AMP(locator=UserRegistration())
If no locator is passed in, AMP acts as its own locator. Command responders can be defined on an AMP subclass, just as the responder was defined on the UserRegistration example above.
Box Receivers¶
AMP conversations consist of an exchange of messages called boxes . A box consists of a sequence of pairs of key and value (for example, the pair username and alice ). Boxes are generally represented as dict instances. Normally boxes are passed back and forth to implement the command request/response features described above. The logic for handling each box can be specified as an object separate from the AMP instance.
from zope.interface import implements
from twisted.protocols.amp import IBoxReceiver
class BoxReflector(object):
implements(IBoxReceiver)
def startReceivingBoxes(self, boxSender):
self.boxSender = boxSender
def ampBoxReceived(self, box):
self.boxSender.sendBox(box)
def stopReceivingBoxes(self, reason):
self.boxSender = None
These methods parallel those of IProtocol . Startup notification is given by startReceivingBoxes . The argument passed to it is an IBoxSender provider, which can be used to send boxes back out over the network. ampBoxReceived delivers notification for a complete box having been received. And last, stopReceivingBoxes notifies the object that no more boxes will be received and no more can be sent. The argument passed to it is a Failure which may contain details about what caused the conversation to end.
To use a custom IBoxReceiver , pass it to the AMP initializer.
factory = Factory()
factory.protocol = lambda: AMP(boxReceiver=BoxReflector())
If no box receiver is passed in, AMP acts as its own box receiver. It handles boxes by treating them as command requests or responses and delivering them to the appropriate responder or as a result to a callRemote Deferred .
Overview of Twisted Spread¶
Perspective Broker (affectionately known as “PB” ) is an asynchronous, symmetric [1] network protocol for secure, remote method calls and transferring of objects. PB is “translucent, not transparent” , meaning that it is very visible and obvious to see the difference between local method calls and potentially remote method calls, but remote method calls are still extremely convenient to make, and it is easy to emulate them to have objects which work both locally and remotely.
PB supports user-defined serialized data in return values, which can be either copied each time the value is returned, or “cached” : only copied once and updated by notifications.
PB gets its name from the fact that access to objects is through a “perspective” . This means that when you are responding to a remote method call, you can establish who is making the call.
Rationale¶
No other currently existing protocols have all the properties of PB at the same time. The particularly interesting combination of attributes, though, is that PB is flexible and lightweight, allowing for rapid development, while still powerful enough to do two-way method calls and user-defined data types.
It is important to have these attributes in order to allow for a protocol which is extensible. One of the facets of this flexibility is that PB can integrate an arbitrary number of services could be aggregated over a single connection, as well as publish and call new methods on existing objects without restarting the server or client.
Footnotes
| [1] | There is a negotiation phase
for the banana serialization protocol with particular roles for listener and initiator, so it’s not
completely symmetric, but after the connection is fully established,
the protocol is completely symmetrical. |
Introduction to Perspective Broker¶
Introduction¶
Suppose you find yourself in control of both ends of the wire: you have two programs that need to talk to each other, and you get to use any protocol you want. If you can think of your problem in terms of objects that need to make method calls on each other, then chances are good that you can use Twisted’s Perspective Broker protocol rather than trying to shoehorn your needs into something like HTTP, or implementing yet another RPC mechanism [1] .
The Perspective Broker system (abbreviated “PB” , spawning numerous sandwich-related puns) is based upon a few central concepts:
- serialization : taking fairly arbitrary objects and types, turning them into a chunk of bytes, sending them over a wire, then reconstituting them on the other end. By keeping careful track of object ids, the serialized objects can contain references to other objects and the remote copy will still be useful.
- remote method calls : doing something to a local object and
causing a method to get run on a distant one. The local object is called a
RemoteReference , and you
“do something” by running its
.callRemotemethod.
This document will contain several examples that will (hopefully) appear redundant and verbose once you’ve figured out what’s going on. To begin with, much of the code will just be labelled “magic” : don’t worry about how these parts work yet. It will be explained more fully later.
Object Roadmap¶
To start with, here are the major classes, interfaces, and functions involved in PB, with links to the file where they are defined (all of which are under twisted/, of course). Don’t worry about understanding what they all do yet: it’s easier to figure them out through their interaction than explaining them one at a time.
- Factory
:
internet/protocol.py - PBServerFactory
:
spread/pb.py - Broker
:
spread/pb.py
Other classes that are involved at some point:
- RemoteReference
:
spread/pb.py - pb.Root
:
spread/pb.py, actually defined astwisted.spread.flavors.Rootinspread/flavors.py - pb.Referenceable
:
spread/pb.py, actually defined astwisted.spread.flavors.Referenceableinspread/flavors.py
Classes and interfaces that get involved when you start to care about authorization and security:
- Portal
:
cred/portal.py - IRealm
:
cred/portal.py - IPerspective
:
spread/pb.py, which you will usually be interacting with via pb.Avatar (a basic implementor of the interface).
Subclassing and Implementing¶
Technically you can subclass anything you want, but technically you could also write a whole new framework, which would just waste a lot of time. Knowing which classes are useful to subclass or which interfaces to implement is one of the bits of knowledge that’s crucial to using PB (and all of Twisted) successfully. Here are some hints to get started:
- pb.Root , pb.Referenceable : you’ll subclass these to make remotely-referenceable objects (i.e., objects which you can call methods on remotely) using PB. You don’t need to change any of the existing behavior, just inherit all of it and add the remotely-accessible methods that you want to export.
- pb.Avatar : You’ll be subclassing this when you get into PB programming with authorization. This is an implementor of IPerspective.
- ICredentialsChecker : Implement this if you want to authenticate your users against some sort of data store: i.e., an LDAP database, an RDBMS, etc. There are already a few implementations of this for various back-ends in twisted.cred.checkers.
Things you can Call Remotely¶
At this writing, there are three “flavors” of objects that can
be accessed remotely through RemoteReference objects. Each of these
flavors has a rule for how the callRemote
message is transformed into a local method call on the server. In
order to use one of these “flavors” , subclass them and name your
published methods with the appropriate prefix.
twisted.spread.pb.IPerspective implementors
This is the first interface we deal with. It is a “perspective” onto your PB application. Perspectives are slightly special because they are usually the first object that a given user can access in your application (after they log on). A user should only receive a reference to their own perspective. PB works hard to verify, as best it can, that any method that can be called on a perspective directly is being called on behalf of the user who is represented by that perspective. (Services with unusual requirements for “on behalf of” , such as simulations with the ability to posses another player’s avatar, are accomplished by providing indirected access to another user’s perspective.)
Perspectives are not usually serialized as remote references, so do not return an IPerspective-implementor directly.
The way most people will want to implement IPerspective is by subclassing pb.Avatar. Remotely accessible methods on pb.Avatar instances are named with the
perspective_prefix.twisted.spread.pb.Referenceable
Referenceable objects are the simplest kind of PB object. You can call methods on them and return them from methods to provide access to other objects’ methods.
However, when a method is called on a Referenceable, it’s not possible to tell who called it.
Remotely accessible methods on Referenceables are named with the
remote_prefix.-
Viewable objects are remotely referenceable objects which have the additional requirement that it must be possible to tell who is calling them. The argument list to a Viewable’s remote methods is modified in order to include the Perspective representing the calling user.
Remotely accessible methods on Viewables are named with the
view_prefix.
Things you can Copy Remotely¶
In addition to returning objects that you can call remote methods on, you can return structured copies of local objects.
There are 2 basic flavors that allow for copying objects remotely. Again,
you can use these by subclassing them. In order to specify what state you want
to have copied when these are serialized, you can either use the Python default
__getstate__ or specialized method calls for that
flavor.
-
This is the simpler kind of object that can be copied. Every time this object is returned from a method or passed as an argument, it is serialized and unserialized.
Copyable provides a method you can override,
getStateToCopyFor(perspective), which allows you to decide what an object will look like for the perspective who is requesting it. Theperspectiveargument will be the perspective which is either passing an argument or returning a result an instance of your Copyable class.For security reasons, in order to allow a particular Copyable class to actually be copied, you must declare a
RemoteCopyhandler for that Copyable subclass. The easiest way to do this is to declare both in the same module, like so:from twisted.spread import flavors class Foo(flavors.Copyable): pass class RemoteFoo(flavors.RemoteCopy): pass flavors.setUnjellyableForClass(Foo, RemoteFoo)
In this case, each time a Foo is copied between peers, a RemoteFoo will be instantiated and populated with the Foo’s state. If you do not do this, PB will complain that there have been security violations, and it may close the connection.
-
Let me preface this with a warning: Cacheable may be hard to understand. The motivation for it may be unclear if you don’t have some experience with real-world applications that use remote method calling of some kind. Once you understand why you need it, what it does will likely seem simple and obvious, but if you get confused by this, forget about it and come back later. It’s possible to use PB without understanding Cacheable at all.
Cacheable is a flavor which is designed to be copied only when necessary, and updated on the fly as changes are made to it. When passed as an argument or a return value, if a Cacheable exists on the side of the connection it is being copied to, it will be referred to by ID and not copied.
Cacheable is designed to minimize errors involved in replicating an object between multiple servers, especially those related to having stale information. In order to do this, Cacheable automatically registers observers and queries state atomically, together. You can override the method
getStateToCacheAndObserveFor(self, perspective, observer)in order to specify how your observers will be stored and updated.Similar to
getStateToCopyFor,getStateToCacheAndObserveForgets passed a perspective. It also gets passed anobserver, which is a remote reference to a “secret” fourth referenceable flavor: RemoteCache .A RemoteCache is simply the object that represents your Cacheable on the other side of the connection. It is registered using the same method as RemoteCopy , above. RemoteCache is different, however, in that it will be referenced by its peer. It acts as a Referenceable, where all methods prefixed with
observe_will be callable remotely. It is recommended that your object maintain a list (note: library support for this is forthcoming!) of observers, and update them usingcallRemotewhen the Cacheable changes in a way that should be noticeable to its clients.Finally, when all references to a Cacheable from a given perspective are lost,
stoppedObserving(perspective, observer)will be called on the Cacheable , with the same perspective/observer pair thatgetStateToCacheAndObserveForwas originally called with. Any cleanup remote calls can be made there, as well as removing the observer object from any lists which it was previously in. Any further calls to this observer object will be invalid.
Footnotes
| [1] | Most of Twisted is like this. Hell, most of Unix is like this: if you think it would be useful, someone else has probably thought that way in the past, and acted on it, and you can take advantage of the tool they created to solve the same problem you’re facing now. |
Using Perspective Broker¶
Basic Example¶
The first example to look at is a complete (although somewhat trivial)
application. It uses PBServerFactory() on the server side, and
PBClientFactory() on the client side.
# Copyright (c) Twisted Matrix Laboratories.
# See LICENSE for details.
from twisted.spread import pb
from twisted.internet import reactor
class Echoer(pb.Root):
def remote_echo(self, st):
print 'echoing:', st
return st
if __name__ == '__main__':
reactor.listenTCP(8789, pb.PBServerFactory(Echoer()))
reactor.run()
# Copyright (c) Twisted Matrix Laboratories.
# See LICENSE for details.
from twisted.spread import pb
from twisted.internet import reactor
from twisted.python import util
factory = pb.PBClientFactory()
reactor.connectTCP("localhost", 8789, factory)
d = factory.getRootObject()
d.addCallback(lambda object: object.callRemote("echo", "hello network"))
d.addCallback(lambda echo: 'server echoed: '+echo)
d.addErrback(lambda reason: 'error: '+str(reason.value))
d.addCallback(util.println)
d.addCallback(lambda _: reactor.stop())
reactor.run()
First we look at the server. This defines an Echoer class (derived from
pb.Root ), with a method called
remote_echo() .
pb.Root objects (because of
their inheritance of
pb.Referenceable , described
later) can define methods with names of the form remote_* ; a
client which obtains a remote reference to that
pb.Root object will be able to
invoke those methods.
The pb.Root -ish object is
given to a pb.PBServerFactory () . This is a
Factory object like
any other: the Protocol objects it creates for new
connections know how to speak the PB protocol. The object you give to
pb.PBServerFactory() becomes the “root object” , which
simply makes it available for the client to retrieve. The client may only
request references to the objects you want to provide it: this helps you
implement your security model. Because it is so common to export just a
single object (and because a remote_* method on that one can
return a reference to any other object you might want to give out), the
simplest example is one where the PBServerFactory is given the root object, and
the client retrieves it.
The client side uses
pb.PBClientFactory to make a
connection to a given port. This is a two-step process involving opening
a TCP connection to a given host and port and requesting the root object
using .getRootObject() .
Because .getRootObject() has to wait until a network
connection has been made and exchange some data, it may take a while,
so it returns a Deferred, to which the gotObject() callback is
attached. (See the documentation on Deferring Execution for a complete explanation of Deferred s). If and when the
connection succeeds and a reference to the remote root object is
obtained, this callback is run. The first argument passed to the
callback is a remote reference to the distant root object. (you can
give other arguments to the callback too, see the other parameters for
.addCallback() and .addCallbacks() ).
The callback does:
object.callRemote("echo", "hello network")
which causes the server’s .remote_echo() method to be invoked.
(running .callRemote("boom") would cause
.remote_boom() to be run, etc). Again because of the delay
involved, callRemote() returns a
Deferred . Assuming the
remote method was run without causing an exception (including an attempt to
invoke an unknown method), the callback attached to that
Deferred will be
invoked with any objects that were returned by the remote method call.
In this example, the server’s Echoer object has a method
invoked, exactly as if some code on the server side had done:
echoer_object.remote_echo("hello network")
and from the definition of remote_echo() we see that this just
returns the same string it was given: “hello network” .
From the client’s point of view, the remote call gets another Deferred object instead of
that string. callRemote() always returns a Deferred . This is why PB is
described as a system for “translucent” remote method calls instead of “transparent” ones: you cannot pretend that the remote object is really
local. Trying to do so (as some other RPC mechanisms do, coughCORBAcough)
breaks down when faced with the asynchronous nature of the network. Using
Deferreds turns out to be a very clean way to deal with the whole thing.
The remote reference object (the one given to
getRootObject() ‘s success callback) is an instance the RemoteReference class. This means
you can use it to invoke methods on the remote object that it refers to. Only
instances of RemoteReference are eligible for
.callRemote() . The RemoteReference object is the one that lives
on the remote side (the client, in this case), not the local side (where the
actual object is defined).
In our example, the local object is that Echoer() instance,
which inherits from pb.Root ,
which inherits from
pb.Referenceable . It is that
Referenceable class that makes the object eligible to be available
for remote method calls [1] . If you have
an object that is Referenceable, then any client that manages to get a
reference to it can invoke any remote_* methods they please.
Note
The only thing they can do is invoke those
methods. In particular, they cannot access attributes. From a security point
of view, you control what they can do by limiting what the remote_* methods can do.
Also note: the other classes like
Referenceable allow access to
other methods, in particular perspective_* and view_*
may be accessed. Don’t write local-only methods with these names, because then
remote callers will be able to do more than you intended.
Also also note: the other classes like pb.Copyable do allow access to attributes, but you control which ones they can see.
You don’t have to be a pb.Root to be remotely callable, but you do have to be pb.Referenceable . (Objects that inherit from pb.Referenceable but not from pb.Root can be remotely called, but only pb.Root -ish objects can be given to the PBServerFactory .)
Complete Example¶
Here is an example client and server which uses pb.Referenceable as a root object and as the result of a remotely exposed method. In each context, methods can be invoked on the exposed Referenceable instance. In this example, the initial root object has a method that returns a reference to the second object.
#!/usr/bin/env python
# Copyright (c) Twisted Matrix Laboratories.
# See LICENSE for details.
from twisted.spread import pb
class Two(pb.Referenceable):
def remote_three(self, arg):
print "Two.three was given", arg
class One(pb.Root):
def remote_getTwo(self):
two = Two()
print "returning a Two called", two
return two
from twisted.internet import reactor
reactor.listenTCP(8800, pb.PBServerFactory(One()))
reactor.run()
#!/usr/bin/env python
# Copyright (c) Twisted Matrix Laboratories.
# See LICENSE for details.
from twisted.spread import pb
from twisted.internet import reactor
def main():
factory = pb.PBClientFactory()
reactor.connectTCP("localhost", 8800, factory)
def1 = factory.getRootObject()
def1.addCallbacks(got_obj1, err_obj1)
reactor.run()
def err_obj1(reason):
print "error getting first object", reason
reactor.stop()
def got_obj1(obj1):
print "got first object:", obj1
print "asking it to getTwo"
def2 = obj1.callRemote("getTwo")
def2.addCallbacks(got_obj2)
def got_obj2(obj2):
print "got second object:", obj2
print "telling it to do three(12)"
obj2.callRemote("three", 12)
main()
pb.PBClientFactory.getRootObject will
handle all the details of waiting for the creation of a connection.
It returns a Deferred , which will have its
callback called when the reactor connects to the remote server and
pb.PBClientFactory gets the
root, and have its errback called when the
object-connection fails for any reason, whether it was host lookup
failure, connection refusal, or some server-side error.
The root object has a method called remote_getTwo , which
returns the Two() instance. On the client end, the callback gets
a RemoteReference to that
instance. The client can then invoke two’s .remote_three()
method.
RemoteReference
objects have one method which is their purpose for being: callRemote . This method allows you to call a
remote method on the object being referred to by the Reference. RemoteReference.callRemote , like pb.PBClientFactory.getRootObject , returns
a Deferred .
When a response to the method-call being sent arrives, the Deferred ‘s callback or errback
will be made, depending on whether an error occurred in processing the
method call.
You can use this technique to provide access to arbitrary sets of objects.
Just remember that any object that might get passed “over the wire” must
inherit from Referenceable
(or one of the other flavors). If you try to pass a non-Referenceable object
(say, by returning one from a remote_* method), you’ll get an
InsecureJelly
exception [2] .
References can come back to you¶
If your server gives a reference to a client, and then that client gives
the reference back to the server, the server will wind up with the same
object it gave out originally. The serialization layer watches for returning
reference identifiers and turns them into actual objects. You need to stay
aware of where the object lives: if it is on your side, you do actual method
calls. If it is on the other side, you do
.callRemote() [3] .
#!/usr/bin/env python
# Copyright (c) Twisted Matrix Laboratories.
# See LICENSE for details.
from twisted.spread import pb
from twisted.internet import reactor
class Two(pb.Referenceable):
def remote_print(self, arg):
print "two.print was given", arg
class One(pb.Root):
def __init__(self, two):
#pb.Root.__init__(self) # pb.Root doesn't implement __init__
self.two = two
def remote_getTwo(self):
print "One.getTwo(), returning my two called", self.two
return self.two
def remote_checkTwo(self, newtwo):
print "One.checkTwo(): comparing my two", self.two
print "One.checkTwo(): against your two", newtwo
if self.two == newtwo:
print "One.checkTwo(): our twos are the same"
two = Two()
root_obj = One(two)
reactor.listenTCP(8800, pb.PBServerFactory(root_obj))
reactor.run()
#!/usr/bin/env python
# Copyright (c) Twisted Matrix Laboratories.
# See LICENSE for details.
from twisted.spread import pb
from twisted.internet import reactor
def main():
foo = Foo()
factory = pb.PBClientFactory()
reactor.connectTCP("localhost", 8800, factory)
factory.getRootObject().addCallback(foo.step1)
reactor.run()
# keeping globals around is starting to get ugly, so we use a simple class
# instead. Instead of hooking one function to the next, we hook one method
# to the next.
class Foo:
def __init__(self):
self.oneRef = None
def step1(self, obj):
print "got one object:", obj
self.oneRef = obj
print "asking it to getTwo"
self.oneRef.callRemote("getTwo").addCallback(self.step2)
def step2(self, two):
print "got two object:", two
print "giving it back to one"
print "one is", self.oneRef
self.oneRef.callRemote("checkTwo", two)
main()
The server gives a Two() instance to the client, who then
returns the reference back to the server. The server compares the “two”
given with the “two” received and shows that they are the same, and that
both are real objects instead of remote references.
A few other techniques are demonstrated in pb2client.py . One
is that the callbacks are added with .addCallback instead
of .addCallbacks . As you can tell from the Deferred documentation, .addCallback is a
simplified form which only adds a success callback. The other is that to
keep track of state from one callback to the next (the remote reference to
the main One() object), we create a simple class, store the reference in an
instance thereof, and point the callbacks at a sequence of bound methods.
This is a convenient way to encapsulate a state machine. Each response kicks
off the next method, and any data that needs to be carried from one state to
the next can simply be saved as an attribute of the object.
Remember that the client can give you back any remote reference you’ve given them. Don’t base your zillion-dollar stock-trading clearinghouse server on the idea that you trust the client to give you back the right reference. The security model inherent in PB means that they can only give you back a reference that you’ve given them for the current connection (not one you’ve given to someone else instead, nor one you gave them last time before the TCP session went down, nor one you haven’t yet given to the client), but just like with URLs and HTTP cookies, the particular reference they give you is entirely under their control.
References to client-side objects¶
Anything that’s Referenceable can get passed across the wire, in either direction . The “client” can give a reference to the “server” , and then the server can use .callRemote() to invoke methods on the client end. This fuzzes the distinction between “client” and “server” : the only real difference is who initiates the original TCP connection; after that it’s all symmetric.
#!/usr/bin/env python
# Copyright (c) Twisted Matrix Laboratories.
# See LICENSE for details.
from twisted.spread import pb
from twisted.internet import reactor
class One(pb.Root):
def remote_takeTwo(self, two):
print "received a Two called", two
print "telling it to print(12)"
two.callRemote("print", 12)
reactor.listenTCP(8800, pb.PBServerFactory(One()))
reactor.run()
#!/usr/bin/env python
# Copyright (c) Twisted Matrix Laboratories.
# See LICENSE for details.
from twisted.spread import pb
from twisted.internet import reactor
class Two(pb.Referenceable):
def remote_print(self, arg):
print "Two.print() called with", arg
def main():
two = Two()
factory = pb.PBClientFactory()
reactor.connectTCP("localhost", 8800, factory)
def1 = factory.getRootObject()
def1.addCallback(got_obj, two) # hands our 'two' to the callback
reactor.run()
def got_obj(obj, two):
print "got One:", obj
print "giving it our two"
obj.callRemote("takeTwo", two)
main()
In this example, the client gives a reference to its own object to the server. The server then invokes a remote method on the client-side object.
Raising Remote Exceptions¶
Everything so far has covered what happens when things go right. What about when they go wrong? The Python Way is to raise an exception of some sort. The Twisted Way is the same.
The only special thing you do is to define your Exception
subclass by deriving it from pb.Error . When any remotely-invokable method
(like remote_* or perspective_* ) raises a
pb.Error -derived exception, a serialized form of that Exception
object will be sent back over the wire [4] . The other side (which
did callRemote ) will have the “errback”
callback run with a Failure object that contains a copy of
the exception object. This Failure object can be queried to
retrieve the error message and a stack traceback.
Failure is a
special class, defined in twisted/python/failure.py , created to
make it easier to handle asynchronous exceptions. Just as exception handlers
can be nested, errback functions can be chained. If one errback
can’t handle the particular type of failure, it can be “passed along” to a
errback handler further down the chain.
For simple purposes, think of the Failure as just a container
for remotely-thrown Exception objects. To extract the string that
was put into the exception, use its .getErrorMessage() method.
To get the type of the exception (as a string), look at its
.type attribute. The stack traceback is available too. The
intent is to let the errback function get just as much information about the
exception as Python’s normal try: clauses do, even though the
exception occurred in somebody else’s memory space at some unknown time in
the past.
#!/usr/bin/env python
# Copyright (c) Twisted Matrix Laboratories.
# See LICENSE for details.
from twisted.spread import pb
from twisted.internet import reactor
class MyError(pb.Error):
"""This is an Expected Exception. Something bad happened."""
pass
class MyError2(Exception):
"""This is an Unexpected Exception. Something really bad happened."""
pass
class One(pb.Root):
def remote_broken(self):
msg = "fall down go boom"
print "raising a MyError exception with data '%s'" % msg
raise MyError(msg)
def remote_broken2(self):
msg = "hadda owie"
print "raising a MyError2 exception with data '%s'" % msg
raise MyError2(msg)
def main():
reactor.listenTCP(8800, pb.PBServerFactory(One()))
reactor.run()
if __name__ == '__main__':
main()
#!/usr/bin/env python
# Copyright (c) Twisted Matrix Laboratories.
# See LICENSE for details.
from twisted.spread import pb
from twisted.internet import reactor
def main():
factory = pb.PBClientFactory()
reactor.connectTCP("localhost", 8800, factory)
d = factory.getRootObject()
d.addCallbacks(got_obj)
reactor.run()
def got_obj(obj):
# change "broken" into "broken2" to demonstrate an unhandled exception
d2 = obj.callRemote("broken")
d2.addCallback(working)
d2.addErrback(broken)
def working():
print "erm, it wasn't *supposed* to work.."
def broken(reason):
print "got remote Exception"
# reason should be a Failure (or subclass) holding the MyError exception
print " .__class__ =", reason.__class__
print " .getErrorMessage() =", reason.getErrorMessage()
print " .type =", reason.type
reactor.stop()
main()
$ ./exc_client.py
got remote Exception
.__class__ = twisted.spread.pb.CopiedFailure
.getErrorMessage() = fall down go boom
.type = __main__.MyError
Main loop terminated.
Oh, and what happens if you raise some other kind of exception? Something
that isn’t subclassed from pb.Error ? Well, those are
called “unexpected exceptions” , which make Twisted think that something
has really gone wrong. These will raise an exception on the
server side. This won’t break the connection (the exception is
trapped, just like most exceptions that occur in response to network
traffic), but it will print out an unsightly stack trace on the server’s
stderr with a message that says “Peer Will Receive PB Traceback” , just
as if the exception had happened outside a remotely-invokable method. (This
message will go the current log target, if log.startLogging was used to redirect it). The
client will get the same Failure object in either case, but
subclassing your exception from pb.Error is the way to tell
Twisted that you expect this sort of exception, and that it is ok to just
let the client handle it instead of also asking the server to complain. Look
at exc_client.py and change it to invoke broken2()
instead of broken() to see the change in the server’s
behavior.
If you don’t add an errback function to the Deferred , then a remote
exception will still send a Failure object back over, but it
will get lodged in the Deferred with nowhere to go. When that
Deferred finally goes out of scope, the side that did
callRemote will emit a message about an “Unhandled error in Deferred” , along with an ugly stack trace. It can’t raise an exception at
that point (after all, the callRemote that triggered the
problem is long gone), but it will emit a traceback. So be a good programmer
and always add errback handlers, even if they are just
calls to log.err .
Try/Except blocks and Failure.trap¶
To implement the equivalent of the Python try/except blocks (which can
trap particular kinds of exceptions and pass others “up” to
higher-level try/except blocks), you can use the
.trap() method in conjunction with multiple
errback handlers on the Deferred . Re-raising an
exception in an errback handler serves to pass that new
exception to the next handler in the chain. The trap method is
given a list of exceptions to look for, and will re-raise anything that
isn’t on the list. Instead of passing unhandled exceptions “up” to an
enclosing try block, this has the effect of passing the
exception “off” to later errback handlers on the same Deferred . The trap calls are used in chained
errbacks to test for each kind of exception in sequence.
#!/usr/bin/env python
# Copyright (c) Twisted Matrix Laboratories.
# See LICENSE for details.
from twisted.internet import reactor
from twisted.spread import pb
class MyException(pb.Error):
pass
class One(pb.Root):
def remote_fooMethod(self, arg):
if arg == "panic!":
raise MyException
return "response"
def remote_shutdown(self):
reactor.stop()
reactor.listenTCP(8800, pb.PBServerFactory(One()))
reactor.run()
#!/usr/bin/env python
# Copyright (c) Twisted Matrix Laboratories.
# See LICENSE for details.
from twisted.spread import pb, jelly
from twisted.python import log
from twisted.internet import reactor
class MyException(pb.Error): pass
class MyOtherException(pb.Error): pass
class ScaryObject:
# not safe for serialization
pass
def worksLike(obj):
# the callback/errback sequence in class One works just like an
# asynchronous version of the following:
try:
response = obj.callMethod(name, arg)
except pb.DeadReferenceError:
print " stale reference: the client disconnected or crashed"
except jelly.InsecureJelly:
print " InsecureJelly: you tried to send something unsafe to them"
except (MyException, MyOtherException):
print " remote raised a MyException" # or MyOtherException
except:
print " something else happened"
else:
print " method successful, response:", response
class One:
def worked(self, response):
print " method successful, response:", response
def check_InsecureJelly(self, failure):
failure.trap(jelly.InsecureJelly)
print " InsecureJelly: you tried to send something unsafe to them"
return None
def check_MyException(self, failure):
which = failure.trap(MyException, MyOtherException)
if which == MyException:
print " remote raised a MyException"
else:
print " remote raised a MyOtherException"
return None
def catch_everythingElse(self, failure):
print " something else happened"
log.err(failure)
return None
def doCall(self, explanation, arg):
print explanation
try:
deferred = self.remote.callRemote("fooMethod", arg)
deferred.addCallback(self.worked)
deferred.addErrback(self.check_InsecureJelly)
deferred.addErrback(self.check_MyException)
deferred.addErrback(self.catch_everythingElse)
except pb.DeadReferenceError:
print " stale reference: the client disconnected or crashed"
def callOne(self):
self.doCall("callOne: call with safe object", "safe string")
def callTwo(self):
self.doCall("callTwo: call with dangerous object", ScaryObject())
def callThree(self):
self.doCall("callThree: call that raises remote exception", "panic!")
def callShutdown(self):
print "telling them to shut down"
self.remote.callRemote("shutdown")
def callFour(self):
self.doCall("callFour: call on stale reference", "dummy")
def got_obj(self, obj):
self.remote = obj
reactor.callLater(1, self.callOne)
reactor.callLater(2, self.callTwo)
reactor.callLater(3, self.callThree)
reactor.callLater(4, self.callShutdown)
reactor.callLater(5, self.callFour)
reactor.callLater(6, reactor.stop)
factory = pb.PBClientFactory()
reactor.connectTCP("localhost", 8800, factory)
deferred = factory.getRootObject()
deferred.addCallback(One().got_obj)
reactor.run()
$ ./trap_client.py
callOne: call with safe object
method successful, response: response
callTwo: call with dangerous object
InsecureJelly: you tried to send something unsafe to them
callThree: call that raises remote exception
remote raised a MyException
telling them to shut down
callFour: call on stale reference
stale reference: the client disconnected or crashed
In this example, callTwo tries to send an instance of a
locally-defined class through callRemote . The default security
model implemented by jelly
on the remote end will not allow unknown classes to be unserialized (i.e.
taken off the wire as a stream of bytes and turned back into an object: a
living, breathing instance of some class): one reason is that it does not
know which local class ought to be used to create an instance that
corresponds to the remote object [5] .
The receiving end of the connection gets to decide what to accept and what
to reject. It indicates its disapproval by raising a jelly.InsecureJelly exception. Because it occurs
at the remote end, the exception is returned to the caller asynchronously,
so an errback handler for the associated Deferred
is run. That errback receives a Failure which wraps the
InsecureJelly .
Remember that trap re-raises exceptions that it wasn’t asked
to look for. You can only check for one set of exceptions per errback
handler: all others must be checked in a subsequent handler.
check_MyException shows how multiple kinds of exceptions can be
checked in a single errback: give a list of exception types to
trap , and it will return the matching member. In this case, the
kinds of exceptions we are checking for (MyException and
MyOtherException ) may be raised by the remote end: they inherit
from pb.Error .
The handler can return None to terminate processing of the
errback chain (to be precise, it switches to the callback that follows the
errback; if there is no callback then processing terminates). It is a good
idea to put an errback that will catch everything (no trap
tests, no possible chance of raising more exceptions, always returns
None ) at the end of the chain. Just as with regular try: except: handlers, you need to think carefully about ways in which
your errback handlers could themselves raise exceptions. The extra
importance in an asynchronous environment is that an exception that falls
off the end of the Deferred will not be signalled until that
Deferred goes out of scope, and at that point may only cause a
log message (which could even be thrown away if log.startLogging is not used to point it at
stdout or a log file). In contrast, a synchronous exception that is not
handled by any other except: block will very visibly terminate
the program immediately with a noisy stack trace.
callFour shows another kind of exception that can occur
while using callRemote : pb.DeadReferenceError . This one occurs when the
remote end has disconnected or crashed, leaving the local side with a stale
reference. This kind of exception happens to be reported right away (XXX: is
this guaranteed? probably not), so must be caught in a traditional
synchronous try: except pb.DeadReferenceError block.
Yet another kind that can occur is a pb.PBConnectionLost exception. This occurs
(asynchronously) if the connection was lost while you were waiting for a callRemote call to complete. When the line goes dead, all
pending requests are terminated with this exception. Note that you have no
way of knowing whether the request made it to the other end or not, nor how
far along in processing it they had managed before the connection was
lost. XXX: explain transaction semantics, find a decent reference.
Footnotes
| [1] | There are a few other classes that can bestow this ability, but pb.Referenceable is the easiest to understand; see ‘flavors’ below for details on the others. |
| [2] | This can be overridden, by subclassing one of the Serializable flavors and defining custom serialization code for your class. See Passing Complex Types for details. |
| [3] | The binary nature of this local vs. remote scheme works because you cannot give RemoteReferences to a third party. If you could, then your object A could go to B, B could give it to C, C might give it back to you, and you would be hard pressed to tell if the object lived in C’s memory space, in B’s, or if it was really your own object, tarnished and sullied after being handed down like a really ugly picture that your great aunt owned and which nobody wants but which nobody can bear to throw out. Ok, not really like that, but you get the idea. |
| [4] | To be precise, the Failure will be sent if any exception is raised, not just pb.Error-derived ones. But the server will print ugly error messages if you raise ones that aren’t derived from pb.Error. |
| [5] | The naive approach
of simply doing The InsecureJelly
exception arises because the class being sent over the wire has not been
registered with the serialization layer (known as jelly ). The easiest way to make it possible to
copy entire class instances over the wire is to have them inherit from pb.Copyable , and then to use
|
Managing Clients of Perspectives¶
| Author: | Kevin Turner |
|---|
Overview¶
In all the IPerspective uses
we have shown so far, we ignored the mind argument and created
a new Avatar for every connection. This is usually an easy
design choice, and it works well for simple cases.
In more complicated cases, for example an Avatar that
represents a player object which is persistent in the game universe,
we will want connections from the same player to use the same Avatar .
Another thing which is necessary in more complicated scenarios
is notifying a player asynchronously. While it is possible, of
course, to allow a player to call perspective_remoteListener(referencable) that would
mean both duplication of code and a higher latency in logging in,
both bad.
In previous sections all realms looked to be identical. In this one we will show the usefulness of realms in accomplishing those two objectives.
Managing Avatars¶
The simplest way to manage persistent avatars is to use a straight-forward caching mechanism:
from zope.interface import implements
class SimpleAvatar(pb.Avatar):
greetings = 0
def __init__(self, name):
self.name = name
def perspective_greet(self):
self.greetings += 1
return "<%d>hello %s" % (self.greetings, self.name)
class CachingRealm:
implements(portal.IRealm)
def __init__(self):
self.avatars = {}
def requestAvatar(self, avatarId, mind, *interfaces):
if pb.IPerspective not in interfaces: raise NotImplementedError
if avatarId in self.avatars:
p = self.avatars[avatarId]
else:
p = self.avatars[avatarId] = SimpleAvatar(avatarId)
return pb.IPerspective, p, lambda:None
This gives us a perspective which counts the number of greetings it
sent its client. Implementing a caching strategy, as opposed to generating
a realm with the correct avatars already in it, is usually easier. This
makes adding new checkers to the portal, or adding new users to a checker
database, transparent. Otherwise, careful synchronization is needed between
the checker and avatar is needed (much like the synchronization between
UNIX’s /etc/shadow and /etc/passwd ).
Sometimes, however, an avatar will need enough per-connection state that it would be easier to generate a new avatar and cache something else. Here is an example of that:
from zope.interface import implements
class Greeter:
greetings = 0
def hello(self):
self.greetings += 1
return "<%d>hello" % (self.greetings, self.name)
class SimpleAvatar(pb.Avatar):
def __init__(self, name, greeter):
self.name = name
self.greeter = greeter
def perspective_greet(self):
return self.greeter.hello()+' '+self.name
class CachingRealm:
implements(portal.IRealm)
def __init__(self):
self.greeters = {}
def requestAvatar(self, avatarId, mind, *interfaces):
if pb.IPerspective not in interfaces: raise NotImplementedError
if avatarId in self.greeters:
p = self.greeters[avatarId]
else:
p = self.greeters[avatarId] = Greeter()
return pb.IPerspective, SimpleAvatar(avatarId, p), lambda:None
It might seem tempting to use this pattern to have an avatar which is notified of new connections. However, the problems here are twofold: it would lead to a thin class which needs to forward all of its methods, and it would be impossible to know when disconnections occur. Luckily, there is a better pattern:
from zope.interface import implements
class SimpleAvatar(pb.Avatar):
greetings = 0
connections = 0
def __init__(self, name):
self.name = name
def connect(self):
self.connections += 1
def disconnect(self):
self.connections -= 1
def perspective_greet(self):
self.greetings += 1
return "<%d>hello %s" % (self.greetings, self.name)
class CachingRealm:
implements(portal.IRealm)
def __init__(self):
self.avatars = {}
def requestAvatar(self, avatarId, mind, *interfaces):
if pb.IPerspective not in interfaces: raise NotImplementedError
if avatarId in self.avatars:
p = self.avatars[avatarId]
else:
p = self.avatars[avatarId] = SimpleAvatar(avatarId)
p.connect()
return pb.IPerspective, p, p.disconnect
It is possible to use such a pattern to define an arbitrary limit for the number of concurrent connections:
from zope.interface import implements
class SimpleAvatar(pb.Avatar):
greetings = 0
connections = 0
def __init__(self, name):
self.name = name
def connect(self):
self.connections += 1
def disconnect(self):
self.connections -= 1
def perspective_greet(self):
self.greetings += 1
return "<%d>hello %s" % (self.greetings, self.name)
class CachingRealm:
implements(portal.IRealm)
def __init__(self, max=1):
self.avatars = {}
self.max = max
def requestAvatar(self, avatarId, mind, *interfaces):
if pb.IPerspective not in interfaces: raise NotImplementedError
if avatarId in self.avatars:
p = self.avatars[avatarId]
else:
p = self.avatars[avatarId] = SimpleAvatar(avatarId)
if p.connections >= self.max:
raise ValueError("too many connections")
p.connect()
return pb.IPerspective, p, p.disconnect
Managing Clients¶
So far, all our realms have ignored the mind argument.
In the case of PB, the mind is an object supplied by
the remote login method – usually, when it passes over the wire,
it becomes a pb.RemoteReference . This object allows
sending messages to the client as soon as the connection is established
and authenticated.
Here is a simple remote-clock application which shows the usefulness
of the mind argument:
from zope.interface import implements
class SimpleAvatar(pb.Avatar):
def __init__(self, client):
self.s = internet.TimerService(1, self.telltime)
self.s.startService()
self.client = client
def telltime(self):
self.client.callRemote("notifyTime", time.time())
def perspective_setperiod(self, period):
self.s.stopService()
self.s = internet.TimerService(period, self.telltime)
self.s.startService()
def logout(self):
self.s.stopService()
class Realm:
implements(portal.IRealm)
def requestAvatar(self, avatarId, mind, *interfaces):
if pb.IPerspective not in interfaces: raise NotImplementedError
p = SimpleAvatar(mind)
return pb.IPerspective, p, p.logout
In more complicated situations, you might want to cache the avatars and give each one a set of “current clients” or something similar.
PB Copyable: Passing Complex Types¶
Overview¶
This chapter focuses on how to use PB to pass complex types (specifically class instances) to and from a remote process. The first section is on simply copying the contents of an object to a remote process (pb.Copyable ). The second covers how to copy those contents once, then update them later when they change (Cacheable ).
Motivation¶
From the previous chapter , you’ve seen how to pass basic types to a remote process, by using them in the arguments or return values of a callRemote function. However, if you’ve experimented with it, you may have discovered problems when trying to pass anything more complicated than a primitive int/list/dict/string type, or another pb.Referenceable object. At some point you want to pass entire objects between processes, instead of having to reduce them down to dictionaries on one end and then re-instantiating them on the other.
Passing Objects¶
The most obvious and straightforward way to send an object to a remote process is with something like the following code. It also happens that this code doesn’t work, as will be explained below.
class LilyPond:
def __init__(self, frogs):
self.frogs = frogs
pond = LilyPond(12)
ref.callRemote("sendPond", pond)
If you try to run this, you might hope that a suitable remote end which
implements the remote_sendPond method would see that method get
invoked with an instance from the LilyPond class. But instead,
you’ll encounter the dreaded InsecureJelly exception. This is
Twisted’s way of telling you that you’ve violated a security restriction,
and that the receiving end refuses to accept your object.
Security Options¶
What’s the big deal? What’s wrong with just copying a class into another process’ namespace?
Reversing the question might make it easier to see the issue: what is the problem with accepting a stranger’s request to create an arbitrary object in your local namespace? The real question is how much power you are granting them: what actions can they convince you to take on the basis of the bytes they are sending you over that remote connection.
Objects generally represent more power than basic types like strings and dictionaries because they also contain (or reference) code, which can modify other data structures when executed. Once previously-trusted data is subverted, the rest of the program is compromised.
The built-in Python “batteries included” classes are relatively
tame, but you still wouldn’t want to let a foreign program use them to
create arbitrary objects in your namespace or on your computer. Imagine a
protocol that involved sending a file-like object with a read()
method that was supposed to used later to retrieve a document. Then imagine
what if that object were created with
os.fdopen("~/.gnupg/secring.gpg") . Or an instance of
telnetlib.Telnet("localhost", "chargen") .
Classes you’ve written for your own program are likely to have far more
power. They may run code during __init__ , or even have special
meaning simply because of their existence. A program might have
User objects to represent user accounts, and have a rule that
says all User objects in the system are referenced when
authorizing a login session. (In this system, User.__init__
would probably add the object to a global list of known users). The simple
act of creating an object would give access to somebody. If you could be
tricked into creating a bad object, an unauthorized user would get
access.
So object creation needs to be part of a system’s security design. The dotted line between “trusted inside” and “untrusted outside” needs to describe what may be done in response to outside events. One of those events is the receipt of an object through a PB remote procedure call, which is a request to create an object in your “inside” namespace. The question is what to do in response to it. For this reason, you must explicitly specify what remote classes will be accepted, and how their local representatives are to be created.
What class to use?¶
Another basic question to answer before we can do anything useful with an incoming serialized object is: what class should we create? The simplistic answer is to create the “same kind” that was serialized on the sender’s end of the wire, but this is not as easy or as straightforward as you might think. Remember that the request is coming from a different program, using a potentially different set of class libraries. In fact, since PB has also been implemented in Java, Emacs-Lisp, and other languages, there’s no guarantee that the sender is even running Python! All we know on the receiving end is a list of two things which describe the instance they are trying to send us: the name of the class, and a representation of the contents of the object.
PB lets you specify the mapping from remote class names to local classes with the setUnjellyableForClass function [1] .
This function takes a remote/sender class reference (either the
fully-qualified name as used by the sending end, or a class object from
which the name can be extracted), and a local/recipient class (used to
create the local representation for incoming serialized objects). Whenever
the remote end sends an object, the class name that they transmit is looked
up in the table controlled by this function. If a matching class is found,
it is used to create the local object. If not, you get the
InsecureJelly exception.
In general you expect both ends to share the same codebase: either you
control the program that is running on both ends of the wire, or both
programs share some kind of common language that is implemented in code
which exists on both ends. You wouldn’t expect them to send you an object of
the MyFooziWhatZit class unless you also had a definition for that class. So
it is reasonable for the Jelly layer to reject all incoming classes except
the ones that you have explicitly marked with
setUnjellyableForClass . But keep in mind that the sender’s idea
of a User object might differ from the recipient’s, either
through namespace collisions between unrelated packages, version skew
between nodes that haven’t been updated at the same rate, or a malicious
intruder trying to cause your code to fail in some interesting or
potentially vulnerable way.
pb.Copyable¶
Ok, enough of this theory. How do you send a fully-fledged object from one side to the other?
#!/usr/bin/env python
# Copyright (c) Twisted Matrix Laboratories.
# See LICENSE for details.
from twisted.spread import pb, jelly
from twisted.python import log
from twisted.internet import reactor
class LilyPond:
def setStuff(self, color, numFrogs):
self.color = color
self.numFrogs = numFrogs
def countFrogs(self):
print "%d frogs" % self.numFrogs
class CopyPond(LilyPond, pb.Copyable):
pass
class Sender:
def __init__(self, pond):
self.pond = pond
def got_obj(self, remote):
self.remote = remote
d = remote.callRemote("takePond", self.pond)
d.addCallback(self.ok).addErrback(self.notOk)
def ok(self, response):
print "pond arrived", response
reactor.stop()
def notOk(self, failure):
print "error during takePond:"
if failure.type == jelly.InsecureJelly:
print " InsecureJelly"
else:
print failure
reactor.stop()
return None
def main():
from copy_sender import CopyPond # so it's not __main__.CopyPond
pond = CopyPond()
pond.setStuff("green", 7)
pond.countFrogs()
# class name:
print ".".join([pond.__class__.__module__, pond.__class__.__name__])
sender = Sender(pond)
factory = pb.PBClientFactory()
reactor.connectTCP("localhost", 8800, factory)
deferred = factory.getRootObject()
deferred.addCallback(sender.got_obj)
reactor.run()
if __name__ == '__main__':
main()
# Copyright (c) Twisted Matrix Laboratories.
# See LICENSE for details.
"""
PB copy receiver example.
This is a Twisted Application Configuration (tac) file. Run with e.g.
twistd -ny copy_receiver.tac
See the twistd(1) man page or
http://twistedmatrix.com/documents/current/howto/application for details.
"""
import sys
if __name__ == '__main__':
print __doc__
sys.exit(1)
from twisted.application import service, internet
from twisted.internet import reactor
from twisted.spread import pb
from copy_sender import LilyPond, CopyPond
from twisted.python import log
#log.startLogging(sys.stdout)
class ReceiverPond(pb.RemoteCopy, LilyPond):
pass
pb.setUnjellyableForClass(CopyPond, ReceiverPond)
class Receiver(pb.Root):
def remote_takePond(self, pond):
print " got pond:", pond
pond.countFrogs()
return "safe and sound" # positive acknowledgement
def remote_shutdown(self):
reactor.stop()
application = service.Application("copy_receiver")
internet.TCPServer(8800, pb.PBServerFactory(Receiver())).setServiceParent(
service.IServiceCollection(application))
The sending side has a class called LilyPond . To make this
eligible for transport through callRemote (either as an
argument, a return value, or something referenced by either of those [like a
dictionary value]), it must inherit from one of the four Serializable classes. In this section,
we focus on Copyable .
The copyable subclass of LilyPond is called
CopyPond . We create an instance of it and send it through
callRemote as an argument to the receiver’s
remote_takePond method. The Jelly layer will serialize
(“jelly” ) that object as an instance with a class name of”copy_sender.CopyPond” and some chunk of data that represents the
object’s state. pond.__class__.__module__ and
pond.__class__.__name__ are used to derive the class name
string. The object’s getStateToCopy method is
used to get the state: this is provided by pb.Copyable , and the default just retrieves
self.__dict__ . This works just like the optional
__getstate__ method used by pickle . The pair of
name and state are sent over the wire to the receiver.
The receiving end defines a local class named ReceiverPond
to represent incoming LilyPond instances. This class derives
from the sender’s LilyPond class (with a fully-qualified name
of copy_sender.LilyPond ), which specifies how we expect it to
behave. We trust that this is the same LilyPond class as the
sender used. (At the very least, we hope ours will be able to accept a state
created by theirs). It also inherits from pb.RemoteCopy , which is a requirement for all
classes that act in this local-representative role (those which are given to
the second argument of setUnjellyableForClass ).
RemoteCopy provides the methods that tell the Jelly layer how
to create the local object from the incoming serialized state.
Then setUnjellyableForClass is used to register the two
classes. This has two effects: instances of the remote class (the first
argument) will be allowed in through the security layer, and instances of
the local class (the second argument) will be used to contain the state that
is transmitted when the sender serializes the remote object.
When the receiver unserializes (“unjellies” ) the object, it will
create an instance of the local ReceiverPond class, and hand
the transmitted state (usually in the form of a dictionary) to that object’s
setCopyableState method.
This acts just like the __setstate__ method that
pickle uses when unserializing an object.
getStateToCopy /setCopyableState are distinct from
__getstate__ /__setstate__ to allow objects to be
persisted (across time) differently than they are transmitted (across
[memory]space).
When this is run, it produces the following output:
[-] twisted.spread.pb.PBServerFactory starting on 8800
[-] Starting factory <twisted.spread.pb.PBServerFactory instance at
0x406159cc>
[Broker,0,127.0.0.1] got pond: <__builtin__.ReceiverPond instance at
0x406ec5ec>
[Broker,0,127.0.0.1] 7 frogs
$ ./copy_sender.py
7 frogs
copy_sender.CopyPond
pond arrived safe and sound
Main loop terminated.
$
Controlling the Copied State¶
By overriding getStateToCopy and
setCopyableState , you can control how the object is transmitted
over the wire. For example, you might want perform some data-reduction:
pre-compute some results instead of sending all the raw data over the wire.
Or you could replace references to a local object on the sender’s side with
markers before sending, then upon receipt replace those markers with
references to a receiver-side proxy that could perform the same operations
against a local cache of data.
Another good use for getStateToCopy is to implement “local-only” attributes: data that is only accessible by the local
process, not to any remote users. For example, a .password
attribute could be removed from the object state before sending to a remote
system. Combined with the fact that Copyable objects return
unchanged from a round trip, this could be used to build a
challenge-response system (in fact PB does this with
pb.Referenceable objects to implement authorization as
described here ).
Whatever getStateToCopy returns from the sending object will
be serialized and sent over the wire; setCopyableState gets
whatever comes over the wire and is responsible for setting up the state of
the object it lives in.
#!/usr/bin/env python
# Copyright (c) Twisted Matrix Laboratories.
# See LICENSE for details.
from twisted.spread import pb
class FrogPond:
def __init__(self, numFrogs, numToads):
self.numFrogs = numFrogs
self.numToads = numToads
def count(self):
return self.numFrogs + self.numToads
class SenderPond(FrogPond, pb.Copyable):
def getStateToCopy(self):
d = self.__dict__.copy()
d['frogsAndToads'] = d['numFrogs'] + d['numToads']
del d['numFrogs']
del d['numToads']
return d
class ReceiverPond(pb.RemoteCopy):
def setCopyableState(self, state):
self.__dict__ = state
def count(self):
return self.frogsAndToads
pb.setUnjellyableForClass(SenderPond, ReceiverPond)
#!/usr/bin/env python
# Copyright (c) Twisted Matrix Laboratories.
# See LICENSE for details.
from twisted.spread import pb, jelly
from twisted.python import log
from twisted.internet import reactor
from copy2_classes import SenderPond
class Sender:
def __init__(self, pond):
self.pond = pond
def got_obj(self, obj):
d = obj.callRemote("takePond", self.pond)
d.addCallback(self.ok).addErrback(self.notOk)
def ok(self, response):
print "pond arrived", response
reactor.stop()
def notOk(self, failure):
print "error during takePond:"
if failure.type == jelly.InsecureJelly:
print " InsecureJelly"
else:
print failure
reactor.stop()
return None
def main():
pond = SenderPond(3, 4)
print "count %d" % pond.count()
sender = Sender(pond)
factory = pb.PBClientFactory()
reactor.connectTCP("localhost", 8800, factory)
deferred = factory.getRootObject()
deferred.addCallback(sender.got_obj)
reactor.run()
if __name__ == '__main__':
main()
#!/usr/bin/env python
# Copyright (c) Twisted Matrix Laboratories.
# See LICENSE for details.
from twisted.application import service, internet
from twisted.internet import reactor
from twisted.spread import pb
import copy2_classes # needed to get ReceiverPond registered with Jelly
class Receiver(pb.Root):
def remote_takePond(self, pond):
print " got pond:", pond
print " count %d" % pond.count()
return "safe and sound" # positive acknowledgement
def remote_shutdown(self):
reactor.stop()
application = service.Application("copy_receiver")
internet.TCPServer(8800, pb.PBServerFactory(Receiver())).setServiceParent(
service.IServiceCollection(application))
In this example, the classes are defined in a separate source file, which
also sets up the binding between them. The SenderPond and ReceiverPond are unrelated save for this binding: they happen
to implement the same methods, but use different internal instance variables
to accomplish them.
The recipient of the object doesn’t even have to import the class
definition into their namespace. It is sufficient that they import the class
definition (and thus execute the setUnjellyableForClass
statement). The Jelly layer remembers the class definition until a matching
object is received. The sender of the object needs the definition, of
course, to create the object in the first place.
When run, the copy2 example emits the following:
$ twistd -n -y copy2_receiver.py
[-] twisted.spread.pb.PBServerFactory starting on 8800
[-] Starting factory <twisted.spread.pb.PBServerFactory instance at
0x40604b4c>
[Broker,0,127.0.0.1] got pond: <copy2_classes.ReceiverPond instance at
0x406eb2ac>
[Broker,0,127.0.0.1] count 7
$ ./copy2_sender.py
count 7
pond arrived safe and sound
Main loop terminated.
Things To Watch Out For¶
- The first argument to
setUnjellyableForClassmust refer to the class as known by the sender . The sender has no way of knowing about how your localimportstatements are set up, and Python’s flexible namespace semantics allow you to access the same class through a variety of different names. You must match whatever the sender does. Having both ends import the class from a separate file, using a canonical module name (no “sibling imports” ), is a good way to get this right, especially when both the sending and the receiving classes are defined together, with thesetUnjellyableForClassimmediately following them. - The class that is sent must inherit from pb.Copyable . The class that is registered to receive it must inherit from pb.RemoteCopy [2] .
- The same class can be used to send and receive. Just have it inherit
from both
pb.Copyableandpb.RemoteCopy. This will also make it possible to send the same class symmetrically back and forth over the wire. But don’t get confused about when it is coming (and usingsetCopyableState) versus when it is going (usinggetStateToCopy). - InsecureJelly
exceptions are raised by the receiving end. They will be delivered
asynchronously to an
errbackhandler. If you do not add one to theDeferredreturned bycallRemote, then you will never receive notification of the problem. - The class that is derived from pb.RemoteCopy will be created using a
constructor
__init__method that takes no arguments. All setup must be performed in thesetCopyableStatemethod. As the docstring on RemoteCopy says, don’t implement a constructor that requires arguments in a subclass ofRemoteCopy.
More Information¶
pb.Copyableis mostly implemented intwisted.spread.flavors, and the docstrings there are the best source of additional information.Copyableis also used in twisted.web.distrib to deliver HTTP requests to other programs for rendering, allowing subtrees of URL space to be delegated to multiple programs (on multiple machines).- twisted.manhole.explorer also uses
Copyableto distribute debugging information from the program under test to the debugging tool.
pb.Cacheable¶
Sometimes the object you want to send to the remote process is big and slow. “big” means it takes a lot of data (storage, network bandwidth, processing) to represent its state. “slow” means that state doesn’t change very frequently. It may be more efficient to send the full state only once, the first time it is needed, then afterwards only send the differences or changes in state whenever it is modified. The pb.Cacheable class provides a framework to implement this.
pb.Cacheable is derived from pb.Copyable , so it is based upon the idea of an object’s state being captured on the sending side, and then turned into a new object on the receiving side. This is extended to have an object “publishing” on the sending side (derived from pb.Cacheable ), matched with one”observing” on the receiving side (derived from pb.RemoteCache ).
To effectively use pb.Cacheable , you need to isolate changes
to your object into accessor functions (specifically “setter”
functions). Your object needs to get control every single time some
attribute is changed [3] .
You derive your sender-side class from pb.Cacheable , and you
add two methods: getStateToCacheAndObserveFor
and stoppedObserving . The first
is called when a remote caching reference is first created, and retrieves
the data with which the cache is first filled. It also provides an
object called the “observer” [4] that points at that receiver-side cache. Every time the state of the object
is changed, you give a message to the observer, informing them of the
change. The other method, stoppedObserving , is called when the
remote cache goes away, so that you can stop sending updates.
On the receiver end, you make your cache class inherit from pb.RemoteCache , and implement the
setCopyableState as you would for a pb.RemoteCopy
object. In addition, you must implement methods to receive the updates sent
to the observer by the pb.Cacheable : these methods should have
names that start with observe_ , and match the
callRemote invocations from the sender side just as the usual
remote_* and perspective_* methods match normal
callRemote calls.
The first time a reference to the pb.Cacheable object is
sent to any particular recipient, a sender-side Observer will be created for
it, and the getStateToCacheAndObserveFor method will be called
to get the current state and register the Observer. The state which that
returns is sent to the remote end and turned into a local representation
using setCopyableState just like pb.RemoteCopy ,
described above (in fact it inherits from that class).
After that, your “setter” functions on the sender side should call
callRemote on the Observer, which causes observe_*
methods to run on the receiver, which are then supposed to update the
receiver-local (cached) state.
When the receiver stops following the cached object and the last
reference goes away, the pb.RemoteCache object can be freed.
Just before it dies, it tells the sender side it no longer cares about the
original object. When that reference count goes to zero, the
Observer goes away and the pb.Cacheable object can stop
announcing every change that takes place. The stoppedObserving method is
used to tell the pb.Cacheable that the Observer has gone
away.
With the pb.Cacheable and pb.RemoteCache
classes in place, bound together by a call to
pb.setUnjellyableForClass , all that remains is to pass a
reference to your pb.Cacheable over the wire to the remote end.
The corresponding pb.RemoteCache object will automatically be
created, and the matching methods will be used to keep the receiver-side
slave object in sync with the sender-side master object.
Example¶
Here is a complete example, in which the MasterDuckPond is
controlled by the sending side, and the SlaveDuckPond is a
cache that tracks changes to the master:
#!/usr/bin/env python
# Copyright (c) Twisted Matrix Laboratories.
# See LICENSE for details.
from twisted.spread import pb
class MasterDuckPond(pb.Cacheable):
def __init__(self, ducks):
self.observers = []
self.ducks = ducks
def count(self):
print "I have [%d] ducks" % len(self.ducks)
def addDuck(self, duck):
self.ducks.append(duck)
for o in self.observers: o.callRemote('addDuck', duck)
def removeDuck(self, duck):
self.ducks.remove(duck)
for o in self.observers: o.callRemote('removeDuck', duck)
def getStateToCacheAndObserveFor(self, perspective, observer):
self.observers.append(observer)
# you should ignore pb.Cacheable-specific state, like self.observers
return self.ducks # in this case, just a list of ducks
def stoppedObserving(self, perspective, observer):
self.observers.remove(observer)
class SlaveDuckPond(pb.RemoteCache):
# This is a cache of a remote MasterDuckPond
def count(self):
return len(self.cacheducks)
def getDucks(self):
return self.cacheducks
def setCopyableState(self, state):
print " cache - sitting, er, setting ducks"
self.cacheducks = state
def observe_addDuck(self, newDuck):
print " cache - addDuck"
self.cacheducks.append(newDuck)
def observe_removeDuck(self, deadDuck):
print " cache - removeDuck"
self.cacheducks.remove(deadDuck)
pb.setUnjellyableForClass(MasterDuckPond, SlaveDuckPond)
#!/usr/bin/env python
# Copyright (c) Twisted Matrix Laboratories.
# See LICENSE for details.
from twisted.spread import pb, jelly
from twisted.python import log
from twisted.internet import reactor
from cache_classes import MasterDuckPond
class Sender:
def __init__(self, pond):
self.pond = pond
def phase1(self, remote):
self.remote = remote
d = remote.callRemote("takePond", self.pond)
d.addCallback(self.phase2).addErrback(log.err)
def phase2(self, response):
self.pond.addDuck("ugly duckling")
self.pond.count()
reactor.callLater(1, self.phase3)
def phase3(self):
d = self.remote.callRemote("checkDucks")
d.addCallback(self.phase4).addErrback(log.err)
def phase4(self, dummy):
self.pond.removeDuck("one duck")
self.pond.count()
self.remote.callRemote("checkDucks")
d = self.remote.callRemote("ignorePond")
d.addCallback(self.phase5)
def phase5(self, dummy):
d = self.remote.callRemote("shutdown")
d.addCallback(self.phase6)
def phase6(self, dummy):
reactor.stop()
def main():
master = MasterDuckPond(["one duck", "two duck"])
master.count()
sender = Sender(master)
factory = pb.PBClientFactory()
reactor.connectTCP("localhost", 8800, factory)
deferred = factory.getRootObject()
deferred.addCallback(sender.phase1)
reactor.run()
if __name__ == '__main__':
main()
#!/usr/bin/env python
# Copyright (c) Twisted Matrix Laboratories.
# See LICENSE for details.
from twisted.application import service, internet
from twisted.internet import reactor
from twisted.spread import pb
import cache_classes
class Receiver(pb.Root):
def remote_takePond(self, pond):
self.pond = pond
print "got pond:", pond # a DuckPondCache
self.remote_checkDucks()
def remote_checkDucks(self):
print "[%d] ducks: " % self.pond.count(), self.pond.getDucks()
def remote_ignorePond(self):
# stop watching the pond
print "dropping pond"
# gc causes __del__ causes 'decache' msg causes stoppedObserving
self.pond = None
def remote_shutdown(self):
reactor.stop()
application = service.Application("copy_receiver")
internet.TCPServer(8800, pb.PBServerFactory(Receiver())).setServiceParent(
service.IServiceCollection(application))
When run, this example emits the following:
$ twistd -n -y cache_receiver.py
[-] twisted.spread.pb.PBServerFactory starting on 8800
[-] Starting factory <twisted.spread.pb.PBServerFactory instance at
0x40615acc>
[Broker,0,127.0.0.1] cache - sitting, er, setting ducks
[Broker,0,127.0.0.1] got pond: <cache_classes.SlaveDuckPond instance at
0x406eb5ec>
[Broker,0,127.0.0.1] [2] ducks: ['one duck', 'two duck']
[Broker,0,127.0.0.1] cache - addDuck
[Broker,0,127.0.0.1] [3] ducks: ['one duck', 'two duck', 'ugly duckling']
[Broker,0,127.0.0.1] cache - removeDuck
[Broker,0,127.0.0.1] [2] ducks: ['two duck', 'ugly duckling']
[Broker,0,127.0.0.1] dropping pond
$ ./cache_sender.py
I have [2] ducks
I have [3] ducks
I have [2] ducks
Main loop terminated.
Points to notice:
There is one
Observerfor each remote program that holds an active reference. Multiple references inside the same program don’t matter: the serialization layer notices the duplicates and does the appropriate reference counting [5] .Multiple Observers need to be kept in a list, and all of them need to be updated when something changes. By sending the initial state at the same time as you add the observer to the list, in a single atomic action that cannot be interrupted by a state change, you insure that you can send the same status update to all the observers.
The
observer.callRemotecalls can still fail. If the remote side has disconnected very recently andstoppedObservinghas not yet been called, you may get aDeadReferenceError. It is a good idea to add an errback to thosecallRemotes to throw away such an error. This is a useful idiom:observer.callRemote('foo', arg).addErrback(lambda f: None)
getStateToCacheAndObserverFormust return some object that represents the current state of the object. This may simply be the object’s__dict__attribute. It is a good idea to remove thepb.Cacheable-specific members of it before sending it to the remote end. The list of Observers, in particular, should be left out, to avoid dizzying recursive Cacheable references. The mind boggles as to the potential consequences of leaving in such an item.- A
perspectiveargument is available togetStateToCacheAndObserveFor, as well asstoppedObserving. I think the purpose of this is to allow viewer-specific changes to the way the cache is updated. If all remote viewers are supposed to see the same data, it can be ignored.
More Information¶
- The best source for information comes from the docstrings
in twisted.spread.flavors ,
where
pb.Cacheableis implemented. - twisted.manhole.explorer uses
Cacheable, and does some fairly interesting things with it. - The spread.publish module also
uses
Cacheable, and might be a source of further information.
Footnotes
| [1] | Note that, in this context, “unjelly” is
a verb with the opposite meaning of “jelly” . The verb “to jelly”
means to serialize an object or data structure into a sequence of bytes (or
other primitive transmittable/storable representation), while “to unjelly” means to unserialize the bytestream into a live object in the
receiver’s memory space. “Unjellyable” is a noun, (not an
adjective), referring to the class that serves as a destination or
recipient of the unjellying process. “A is unjellyable into B” means
that a serialized representation A (of some remote object) can be
unserialized into a local object of type B. It is these objects “B”
that are the “Unjellyable” second argument of the
setUnjellyableForClass function.
In particular, “unjellyable” does not mean “cannot be jellied” . Unpersistable means “not persistable” , but “unjelly” , “unserialize” , and “unpickle”
mean to reverse the operations of “jellying” , “serializing” , and
“pickling” . |
| [2] | pb.RemoteCopy is actually defined
in twisted.spread.flavors , but
pb.RemoteCopy is the preferred way to access it |
| [3] | Of course you could be clever and
add a hook to __setattr__ , along with magical change-announcing
subclasses of the usual builtin types, to detect changes that result from
normal “=” set operations. The semi-magical “property attributes”
that were introduced in Python 2.2 could be useful too. The result might be
hard to maintain or extend, though. |
| [4] | This is actually a RemoteCacheObserver , but it isn’t very
useful to subclass or modify, so simply treat it as a little demon that sits
in your pb.Cacheable class and helps you distribute change
notifications. The only useful thing to do with it is to run its
callRemote method, which acts just like a normal
pb.Referenceable ‘s method of the same name. |
| [5] | This applies to multiple references through the same Broker . If you’ve managed to make multiple TCP connections to the same program, you deserve whatever you get. |
Authentication with Perspective Broker¶
Overview¶
The examples shown in Using Perspective Broker demonstrate how to do basic remote method calls, but provided no facilities for authentication. In this context, authentication is about who gets which remote references, and how to restrict access to the “right” set of people or programs.
As soon as you have a program which offers services to multiple users, where those users should not be allowed to interfere with each other, you need to think about authentication. Many services use the idea of an “account” , and rely upon fact that each user has access to only one account. Twisted uses a system called cred to handle authentication issues, and Perspective Broker has code to make it easy to implement the most common use cases.
Compartmentalizing Services¶
Imagine how you would write a chat server using PB. The first step might
be a ChatServer object which had a bunch of
pb.RemoteReference s that point at user clients. Pretend that
those clients offered a remote_print method which lets the
server print a message on the user’s console. In that case, the server might
look something like this:
class ChatServer(pb.Referenceable):
def __init__(self):
self.groups = {} # indexed by name
self.users = {} # indexed by name
def remote_joinGroup(self, username, groupname):
if not self.groups.has_key(groupname):
self.groups[groupname] = []
self.groups[groupname].append(self.users[username])
def remote_sendMessage(self, from_username, groupname, message):
group = self.groups[groupname]
if group:
# send the message to all members of the group
for user in group:
user.callRemote("print",
"<%s> says: %s" % (from_username,
message))
For now, assume that all clients have somehow acquired a
pb.RemoteReference to this ChatServer object,
perhaps using pb.Root and getRootObject as
described in the previous chapter . In this
scheme, when a user sends a message to the group, their client runs
something like the following:
remotegroup.callRemote("sendMessage", "alice", "Hi, my name is alice.")
Incorrect Arguments¶
You’ve probably seen the first problem: users can trivially spoof each other. We depend upon the user to pass a correct value in their “username” argument, and have no way to tell if they’re lying or not. There is nothing to prevent Alice from modifying her client to do:
remotegroup.callRemote("sendMessage", "bob", "i like pork")
much to the horror of Bob’s vegetarian friends. [1]
(In general, learn to get suspicious if you see any argument of a remotely-invokable method described as “must be X” )
The best way to fix this is to keep track of the user’s name locally,
rather than asking them to send it to the server with each message. The best
place to keep state is in an object, so this suggests we need a per-user
object. Rather than choosing an obvious name [2] , let’s call this the
User class.
class User(pb.Referenceable):
def __init__(self, username, server, clientref):
self.name = username
self.server = server
self.remote = clientref
def remote_joinGroup(self, groupname):
self.server.joinGroup(groupname, self)
def remote_sendMessage(self, groupname, message):
self.server.sendMessage(self.name, groupname, message)
def send(self, message):
self.remote.callRemote("print", message)
class ChatServer:
def __init__(self):
self.groups = {} # indexed by name
def joinGroup(self, groupname, user):
if not self.groups.has_key(groupname):
self.groups[groupname] = []
self.groups[groupname].append(user)
def sendMessage(self, from_username, groupname, message):
group = self.groups[groupname]
if group:
# send the message to all members of the group
for user in group:
user.send("<%s> says: %s" % (from_username, message))
Again, assume that each remote client gets access to a single
User object, which is created with the proper username.
Note how the ChatServer object has no remote access: it
isn’t even pb.Referenceable anymore. This means that all access
to it must be mediated through other objects, with code that is under your
control.
As long as Alice only has access to her own User object, she
can no longer spoof Bob. The only way for her to invoke
ChatServer.sendMessage is to call her User
object’s remote_sendMessage method, and that method uses its
own state to provide the from_username argument. It doesn’t
give her any way to change that state.
This restriction is important. The User object is able to
maintain its own integrity because there is a wall between the object and
the client: the client cannot inspect or modify internal state, like the
.name attribute. The only way through this wall is via remote
method invocations, and the only control Alice has over those invocations is
when they get invoked and what arguments they are given.
Note
No object can maintain its integrity against local threats: by design,
Python offers no mechanism for class instances to hide their attributes, and
once an intruder has a copy of self.__dict__ , they can do
everything the original object was able to do.
Unforgeable References¶
Now suppose you wanted to implement group parameters, for example a mode
in which nobody was allowed to talk about mattresses because some users were
sensitive and calming them down after someone said “mattress” is a
hassle that’s best avoided altogether. Again, per-group state implies a
per-group object. We’ll go out on a limb and call this the
Group object:
class User(pb.Referenceable):
def __init__(self, username, server, clientref):
self.name = username
self.server = server
self.remote = clientref
def remote_joinGroup(self, groupname, allowMattress=True):
return self.server.joinGroup(groupname, self, allowMattress)
def send(self, message):
self.remote.callRemote("print", message)
class Group(pb.Referenceable):
def __init__(self, groupname, allowMattress):
self.name = groupname
self.allowMattress = allowMattress
self.users = []
def remote_send(self, from_user, message):
if not self.allowMattress and "mattress" in message:
raise ValueError, "Don't say that word"
for user in self.users:
user.send("<%s> says: %s" % (from_user.name, message))
def addUser(self, user):
self.users.append(user)
class ChatServer:
def __init__(self):
self.groups = {} # indexed by name
def joinGroup(self, groupname, user, allowMattress):
if groupname not in self.groups:
self.groups[groupname] = Group(groupname, allowMattress)
self.groups[groupname].addUser(user)
return self.groups[groupname]
This example takes advantage of the fact that
pb.Referenceable objects sent over a wire can be returned to
you, and they will be turned into references to the same object that you
originally sent. The client cannot modify the object in any way: all they
can do is point at it and invoke its remote_* methods. Thus,
you can be sure that the .name attribute remains the same as
you left it. In this case, the client code would look something like
this:
class ClientThing(pb.Referenceable):
def remote_print(self, message):
print message
def join(self):
d = self.remoteUser.callRemote("joinGroup", "#twisted",
allowMattress=False)
d.addCallback(self.gotGroup)
def gotGroup(self, group):
group.callRemote("send", self.remoteUser, "hi everybody")
The User object is sent from the server side, and is turned
into a pb.RemoteReference when it arrives at the client. The
client sends it back to Group.remote_send , and PB turns it back
into a reference to the original User when it gets there.
Group.remote_send can then use its .name attribute
as the sender of the message.
Note
Third party references (there aren’t any)
This technique also relies upon the fact that the
pb.Referenceable reference can only come from someone
who holds a corresponding pb.RemoteReference . The design of the
serialization mechanism (implemented in twisted.spread.jelly : pb, jelly, spread.. get it? Look for “banana” , too. What other networking framework
can claim API names based on sandwich ingredients?) makes it impossible for
a client to obtain a reference that they weren’t explicitly given.
References passed over the wire are given id numbers and recorded in a
per-connection dictionary. If you didn’t give them the reference, the id
number won’t be in the dict, and no amount of guessing by a malicious client
will give them anything else. The dict goes away when the connection is
dropped, further limiting the scope of those references.
Furthermore, it is not possible for Bob to send his
User reference to Alice (perhaps over some other PB channel
just between the two of them). Outside the context of Bob’s connection to
the server, that reference is just a meaningless number. To prevent
confusion, PB will tell you if you try to give it away: when you try to hand
a pb.RemoteReference to a third party, you’ll get an exception
(implemented with an assert in pb.py:364 RemoteReference.jellyFor).
This helps the security model somewhat: only the client you gave the
reference to can cause any damage with it. Of course, the client might be a
brainless zombie, simply doing anything some third party wants. When it’s
not proxying callRemote invocations, it’s probably terrorizing
the living and searching out human brains for sustenance. In short, if you
don’t trust them, don’t give them that reference.
And remember that everything you’ve ever given them over that connection
can come back to you. If expect the client to invoke your method with some
object A that you sent to them earlier, and instead they send you object B
(that you also sent to them earlier), and you don’t check it somehow, then
you’ve just opened up a security hole (we’ll see an example of this
shortly). It may be better to keep such objects in a dictionary on the
server side, and have the client send you an index string instead. Doing it
that way makes it obvious that they can send you anything they want, and
improves the chances that you’ll remember to implement the right checks.
(This is exactly what PB is doing underneath, with a per-connection
dictionary of Referenceable objects, indexed by a number).
And, of course, you have to make sure you don’t accidentally hand out a reference to the wrong object.
But again, note the vulnerability. If Alice holds a
RemoteReference to any object on the server side that
has a .name attribute, she can use that name as a spoofed”from” parameter. As a simple example, what if her client code looked
like:
class ClientThing(pb.Referenceable):
def join(self):
d = self.remoteUser.callRemote("joinGroup", "#twisted")
d.addCallback(self.gotGroup)
def gotGroup(self, group):
group.callRemote("send", from_user=group, "hi everybody")
This would let her send a message that appeared to come from “#twisted” rather than “Alice” . If she joined a group that happened to be named “bob” (perhaps it is the “How To Be Bob” channel, populated by Alice and countless others, a place where they can share stories about their best impersonating-Bob moments), then she would be able to emit a message that looked like “<bob> says: hi there” , and she has accomplished her lifelong goal.
Argument Typechecking¶
There are two techniques to close this hole. The first is to have your
remotely-invokable methods do type-checking on their arguments: if
Group.remote_send asserted isinstance(from_user, User) then Alice couldn’t use non-User objects to do her spoofing,
and hopefully the rest of the system is designed well enough to prevent her
from obtaining access to somebody else’s User object.
Objects as Capabilities¶
The second technique is to avoid having the client send you the objects
altogether. If they don’t send you anything, there is nothing to verify. In
this case, you would have to have a per-user-per-group object, in which the
remote_send method would only take a single
message argument. The UserGroup object is created
with references to the only User and Group objects
that it will ever use, so no lookups are needed:
class UserGroup(pb.Referenceable):
def __init__(self, user, group):
self.user = user
self.group = group
def remote_send(self, message):
self.group.send(self.user.name, message)
class Group:
def __init__(self, groupname, allowMattress):
self.name = groupname
self.allowMattress = allowMattress
self.users = []
def send(self, from_user, message):
if not self.allowMattress and "mattress" in message:
raise ValueError, "Don't say that word"
for user in self.users:
user.send("<%s> says: %s" % (from_user.name, message))
def addUser(self, user):
self.users.append(user)
The only message-sending method Alice has left is
UserGroup.remote_send , and it only accepts a message: there are
no remaining ways to influence the “from” name.
In this model, each remotely-accessible object represents a very small set of capabilities. Security is achieved by only granting a minimal set of abilities to each remote user.
PB provides a shortcut which makes this technique easier to use. The
Viewable class will be discussed below .
Avatars and Perspectives¶
In Twisted’s cred system, an “Avatar” is an object that lives on the “server” side (defined here as the side farthest from the human who is trying to get something done) which lets the remote user get something done. The avatar isn’t really a particular class, it’s more like a description of a role that some object plays, as in “the Foo object here is acting as the user’s avatar for this particular service” . Generally, the remote user has some way of getting their avatar to run some code. The avatar object may enforce some security checks, and provide additional data, then call other methods which get things done.
The two pieces in the cred puzzle (for any protocol, not just PB) are: “what serves as the Avatar?” , and “how does the user get access to it?” .
For PB, the first question is easy. The Avatar is a remotely-accessible
object which can run code: this is a perfect description of
pb.Referenceable and its subclasses. We shall defer the second
question until the next section.
In the example above, you can think of the ChatServer and
Group objects as a service. The User object is the
user’s server-side representative: everything the user is capable of doing
is done by running one of its methods. Anything that the server wants to do
to the user (change their group membership, change their name, delete their
pet cat, whatever) is done by manipulating the User object.
There are multiple User objects living in peace and harmony around the ChatServer. Each has a different point of view on the services provided by the ChatServer and the Groups: each may belong to different groups, some might have more permissions than others (like the ability to create groups). These different points of view are called “Perspectives” . This is the origin of the term “Perspective” in “Perspective Broker” : PB provides and controls (i.e. “brokers” ) access to Perspectives.
Once upon a time, these local-representative objects were actually called
pb.Perspective . But this has changed with the advent of the
rewritten cred system, and now the more generic term for a local
representative object is an Avatar. But you will still see reference to
“Perspective” in the code, the docs, and the module names [3] . Just remember
that perspectives and avatars are basically the same thing.
Despite all we’ve been telling you about how
Avatars are more of a concept than an actual class, the base class from
which you can create your server-side avatar-ish objects is, in fact, named
pb.Avatar [4] . These objects behave very much like
pb.Referenceable . The only difference is that instead of
offering “remote_FOO” methods, they offer “perspective_FOO”
methods.
The other way in which pb.Avatar differs from
pb.Referenceable is that the avatar objects are designed to be
the first thing retrieved by a cred-using remote client. Just as
PBClientFactory.getRootObject gives the client access to a
pb.Root object (which can then provide access to all kinds of
other objects), PBClientFactory.login gives client access to a
pb.Avatar object (which can return other references).
So, the first half of using cred in your PB application is to create an
Avatar object which implements perspective_ methods and is
careful to do useful things for the remote user while remaining vigilant
against being tricked with unexpected argument values. It must also be
careful to never give access to objects that the user should not have access
to, whether by returning them directly, returning objects which contain
them, or returning objects which can be asked (remotely) to provide
them.
The second half is how the user gets a pb.RemoteReference to
your Avatar. As explained elsewhere , Avatars are
obtained from a Realm. The Realm doesn’t deal with authentication at all
(usernames, passwords, public keys, challenge-response systems, retinal
scanners, real-time DNA sequencers, etc). It simply takes an “avatarID”
(which is effectively a username) and returns an Avatar object. The Portal
and its Checkers deal with authenticating the user: by the time they are
done, the remote user has proved their right to access the avatarID that is
given to the Realm, so the Realm can return a remotely-controllable object
that has whatever powers you wish to grant to this particular user.
For PB, the realm is expected to return a pb.Avatar (or
anything which implements pb.IPerspective , really, but there’s
no reason to not return a pb.Avatar subclass). This object will
be given to the client just like a pb.Root would be without
cred, and the user can get access to other objects through it (if you let
them).
The basic idea is that there is a separate IPerspective-implementing object (i.e. the Avatar subclass) (i.e. the “perspective” ) for each user, and only the authorized user gets a remote reference to that object. You can store whatever permissions or capabilities the user possesses in that object, and then use them when the user invokes a remote method. You give the user access to the perspective object instead of the objects that do the real work.
Perspective Examples¶
Here is a brief example of using a pb.Avatar. Most of the support code is magic for now: we’ll explain it later.
One Client¶
#!/usr/bin/env python
# Copyright (c) Twisted Matrix Laboratories.
# See LICENSE for details.
from zope.interface import implements
from twisted.spread import pb
from twisted.cred import checkers, portal
from twisted.internet import reactor
class MyPerspective(pb.Avatar):
def __init__(self, name):
self.name = name
def perspective_foo(self, arg):
print "I am", self.name, "perspective_foo(",arg,") called on", self
class MyRealm:
implements(portal.IRealm)
def requestAvatar(self, avatarId, mind, *interfaces):
if pb.IPerspective not in interfaces:
raise NotImplementedError
return pb.IPerspective, MyPerspective(avatarId), lambda:None
p = portal.Portal(MyRealm())
p.registerChecker(
checkers.InMemoryUsernamePasswordDatabaseDontUse(user1="pass1"))
reactor.listenTCP(8800, pb.PBServerFactory(p))
reactor.run()
#!/usr/bin/env python
# Copyright (c) Twisted Matrix Laboratories.
# See LICENSE for details.
from twisted.spread import pb
from twisted.internet import reactor
from twisted.cred import credentials
def main():
factory = pb.PBClientFactory()
reactor.connectTCP("localhost", 8800, factory)
def1 = factory.login(credentials.UsernamePassword("user1", "pass1"))
def1.addCallback(connected)
reactor.run()
def connected(perspective):
print "got perspective ref:", perspective
print "asking it to foo(12)"
perspective.callRemote("foo", 12)
main()
Ok, so that wasn’t really very exciting. It doesn’t accomplish much more than the first PB example, and used a lot more code to do it. Let’s try it again with two users this time.
Note
When the client runs login to request the Perspective,
they can provide it with an optional client argument (which
must be a pb.Referenceable object). If they do, then a
reference to that object will be handed to the realm’s
requestAvatar in the mind argument.
The server-side Perspective can use it to invoke remote methods on something in the client, so that the client doesn’t always have to drive the interaction. In a chat server, the client object would be the one to which”display text” messages were sent. In a board game server, this would provide a way to tell the clients that someone has made a move, so they can update their game boards.
Two Clients¶
#!/usr/bin/env python
# Copyright (c) Twisted Matrix Laboratories.
# See LICENSE for details.
from zope.interface import implements
from twisted.spread import pb
from twisted.cred import checkers, portal
from twisted.internet import reactor
class MyPerspective(pb.Avatar):
def __init__(self, name):
self.name = name
def perspective_foo(self, arg):
print "I am", self.name, "perspective_foo(",arg,") called on", self
class MyRealm:
implements(portal.IRealm)
def requestAvatar(self, avatarId, mind, *interfaces):
if pb.IPerspective not in interfaces:
raise NotImplementedError
return pb.IPerspective, MyPerspective(avatarId), lambda:None
p = portal.Portal(MyRealm())
c = checkers.InMemoryUsernamePasswordDatabaseDontUse(user1="pass1",
user2="pass2")
p.registerChecker(c)
reactor.listenTCP(8800, pb.PBServerFactory(p))
reactor.run()
#!/usr/bin/env python
# Copyright (c) Twisted Matrix Laboratories.
# See LICENSE for details.
from twisted.spread import pb
from twisted.internet import reactor
from twisted.cred import credentials
def main():
factory = pb.PBClientFactory()
reactor.connectTCP("localhost", 8800, factory)
def1 = factory.login(credentials.UsernamePassword("user1", "pass1"))
def1.addCallback(connected)
reactor.run()
def connected(perspective):
print "got perspective1 ref:", perspective
print "asking it to foo(13)"
perspective.callRemote("foo", 13)
main()
#!/usr/bin/env python
# Copyright (c) Twisted Matrix Laboratories.
# See LICENSE for details.
from twisted.spread import pb
from twisted.internet import reactor
from twisted.spread import pb
from twisted.internet import reactor
from twisted.cred import credentials
def main():
factory = pb.PBClientFactory()
reactor.connectTCP("localhost", 8800, factory)
def1 = factory.login(credentials.UsernamePassword("user2", "pass2"))
def1.addCallback(connected)
reactor.run()
def connected(perspective):
print "got perspective2 ref:", perspective
print "asking it to foo(14)"
perspective.callRemote("foo", 14)
main()
While pb6server.py is running, try starting pb6client1, then pb6client2.
Compare the argument passed by the .callRemote() in each
client. You can see how each client gets connected to a different
Perspective.
How that example worked¶
Let’s walk through the previous example and see what was going on.
First, we created a subclass called MyPerspective which is
our server-side Avatar. It implements a perspective_foo method
that is exposed to the remote client.
Second, we created a realm (an object which implements
IRealm , and therefore implements requestAvatar ).
This realm manufactures MyPerspective objects. It makes as many
as we want, and names each one with the avatarID (a username) that comes out
of the checkers. This MyRealm object returns two other objects as well,
which we will describe later.
Third, we created a portal to hold this realm. The portal’s job is to dispatch incoming clients to the credential checkers, and then to request Avatars for any which survive the authentication process.
Fourth, we made a simple checker (an object which implements
IChecker ) to hold valid user/password pairs. The checker
gets registered with the portal, so it knows who to ask when new
clients connect. We use a checker named
InMemoryUsernamePasswordDatabaseDontUse , which suggests
that 1: all the username/password pairs are kept in memory instead of
being saved to a database or something, and 2: you shouldn’t use
it. The admonition against using it is because there are better
schemes: keeping everything in memory will not work when you have
thousands or millions of users to keep track of, the passwords will be
stored in the .tap file when the application shuts down (possibly a
security risk), and finally it is a nuisance to add or remove users
after the checker is constructed.
Fifth, we create a pb.PBServerFactory to listen on a TCP
port. This factory knows how to connect the remote client to the Portal, so
incoming connections will be handed to the authentication process. Other
protocols (non-PB) would do something similar: the factory that creates
Protocol objects will give those objects access to the Portal so
authentication can take place.
On the client side, a pb.PBClientFactory is created (as before ) and attached to a TCP connection. When the
connection completes, the factory will be asked to produce a Protocol, and
it will create a PB object. Unlike the previous chapter, where we used
.getRootObject , here we use factory.login to
initiate the cred authentication process. We provide a
credentials object, which is the client-side agent for doing
our half of the authentication process. This process may involve several
messages: challenges, responses, encrypted passwords, secure hashes, etc. We
give our credentials object everything it will need to respond correctly (in
this case, a username and password, but you could write a credential that
used public-key encryption or even fancier techniques).
login returns a Deferred which, when it fires, will return a
pb.RemoteReference to the remote avatar. We can then do
callRemote to invoke a perspective_foo method on
that Avatar.
Anonymous Clients¶
#!/usr/bin/env python
# Copyright (c) Twisted Matrix Laboratories.
# See LICENSE for details.
"""
Implement the realm for and run on port 8800 a PB service which allows both
anonymous and username/password based access.
Successful username/password-based login requests given an instance of
MyPerspective with a name which matches the username with which they
authenticated. Success anonymous login requests are given an instance of
MyPerspective with the name "Anonymous".
"""
from sys import stdout
from zope.interface import implements
from twisted.python.log import startLogging
from twisted.cred.checkers import ANONYMOUS, AllowAnonymousAccess
from twisted.cred.checkers import InMemoryUsernamePasswordDatabaseDontUse
from twisted.cred.portal import IRealm, Portal
from twisted.internet import reactor
from twisted.spread.pb import Avatar, IPerspective, PBServerFactory
class MyPerspective(Avatar):
"""
Trivial avatar exposing a single remote method for demonstrative
purposes. All successful login attempts in this example will result in
an avatar which is an instance of this class.
@type name: C{str}
@ivar name: The username which was used during login or C{"Anonymous"}
if the login was anonymous (a real service might want to avoid the
collision this introduces between anonoymous users and authenticated
users named "Anonymous").
"""
def __init__(self, name):
self.name = name
def perspective_foo(self, arg):
"""
Print a simple message which gives the argument this method was
called with and this avatar's name.
"""
print "I am %s. perspective_foo(%s) called on %s." % (
self.name, arg, self)
class MyRealm(object):
"""
Trivial realm which supports anonymous and named users by creating
avatars which are instances of MyPerspective for either.
"""
implements(IRealm)
def requestAvatar(self, avatarId, mind, *interfaces):
if IPerspective not in interfaces:
raise NotImplementedError("MyRealm only handles IPerspective")
if avatarId is ANONYMOUS:
avatarId = "Anonymous"
return IPerspective, MyPerspective(avatarId), lambda: None
def main():
"""
Create a PB server using MyRealm and run it on port 8800.
"""
startLogging(stdout)
p = Portal(MyRealm())
# Here the username/password checker is registered.
c1 = InMemoryUsernamePasswordDatabaseDontUse(user1="pass1", user2="pass2")
p.registerChecker(c1)
# Here the anonymous checker is registered.
c2 = AllowAnonymousAccess()
p.registerChecker(c2)
reactor.listenTCP(8800, PBServerFactory(p))
reactor.run()
if __name__ == '__main__':
main()
#!/usr/bin/env python
# Copyright (c) Twisted Matrix Laboratories.
# See LICENSE for details.
"""
Client which will talk to the server run by pbAnonServer.py, logging in
either anonymously or with username/password credentials.
"""
from sys import stdout
from twisted.python.log import err, startLogging
from twisted.cred.credentials import Anonymous, UsernamePassword
from twisted.internet import reactor
from twisted.internet.defer import gatherResults
from twisted.spread.pb import PBClientFactory
def error(why, msg):
"""
Catch-all errback which simply logs the failure. This isn't expected to
be invoked in the normal case for this example.
"""
err(why, msg)
def connected(perspective):
"""
Login callback which invokes the remote "foo" method on the perspective
which the server returned.
"""
print "got perspective1 ref:", perspective
print "asking it to foo(13)"
return perspective.callRemote("foo", 13)
def finished(ignored):
"""
Callback invoked when both logins and method calls have finished to shut
down the reactor so the example exits.
"""
reactor.stop()
def main():
"""
Connect to a PB server running on port 8800 on localhost and log in to
it, both anonymously and using a username/password it will recognize.
"""
startLogging(stdout)
factory = PBClientFactory()
reactor.connectTCP("localhost", 8800, factory)
anonymousLogin = factory.login(Anonymous())
anonymousLogin.addCallback(connected)
anonymousLogin.addErrback(error, "Anonymous login failed")
usernameLogin = factory.login(UsernamePassword("user1", "pass1"))
usernameLogin.addCallback(connected)
usernameLogin.addErrback(error, "Username/password login failed")
bothDeferreds = gatherResults([anonymousLogin, usernameLogin])
bothDeferreds.addCallback(finished)
reactor.run()
if __name__ == '__main__':
main()
pbAnonServer.py implements a server based on pb6server.py, extending it to
permit anonymous logins in addition to authenticated logins. An
AllowAnonymousAccess
checker and an InMemoryUsernamePasswordDatabaseDontUse
checker are registered and the
client’s choice of credentials object determines which is used to authenticate
the login. In either case, the realm will be called on to create an avatar for
the login. AllowAnonymousAccess always produces an avatarId of twisted.cred.checkers.ANONYMOUS .
On the client side, the only change is the use of an instance of Anonymous when calling PBClientFactory.login .
Using Avatars¶
Avatar Interfaces¶
The first element of the 3-tuple returned by requestAvatar
indicates which Interface this Avatar implements. For PB avatars, it will
always be pb.IPerspective , because that’s the only interface
these avatars implement.
This element is present because requestAvatar is actually
presented with a list of possible Interfaces. The question being posed to
the Realm is: “do you have an avatar for (avatarID) that can implement one of the following set of Interfaces?” . Some portals and checkers might
give a list of Interfaces and the Realm could pick; the PB code only knows
how to do one, so we cannot take advantage of this feature.
Logging Out¶
The third element of the 3-tuple is a zero-argument callable, which will be invoked by the protocol when the connection has been lost. We can use this to notify the Avatar when the client has lost its connection. This will be described in more detail below.
Making Avatars¶
In the example above, we create Avatars upon request, during
requestAvatar . Depending upon the service, these Avatars might
already exist before the connection is received, and might outlive the
connection. The Avatars might also accept multiple connections.
Another possibility is that the Avatars might exist ahead of time, but in
a different form (frozen in a pickle and/or saved in a database). In this
case, requestAvatar may need to perform a database lookup and
then do something with the result before it can provide an avatar. In this
case, it would probably return a Deferred so it could provide the real
Avatar later, once the lookup had completed.
Here are some possible implementations of
MyRealm.requestAvatar :
# pre-existing, static avatars
def requestAvatar(self, avatarID, mind, *interfaces):
assert pb.IPerspective in interfaces
avatar = self.avatars[avatarID]
return pb.IPerspective, avatar, lambda:None
# database lookup and unpickling
def requestAvatar(self, avatarID, mind, *interfaces):
assert pb.IPerspective in interfaces
d = self.database.fetchAvatar(avatarID)
d.addCallback(self.doUnpickle)
return pb.IPerspective, d, lambda:None
def doUnpickle(self, pickled):
avatar = pickle.loads(pickled)
return avatar
# everybody shares the same Avatar
def requestAvatar(self, avatarID, mind, *interfaces):
assert pb.IPerspective in interfaces
return pb.IPerspective, self.theOneAvatar, lambda:None
# anonymous users share one Avatar, named users each get their own
def requestAvatar(self, avatarID, mind, *interfaces):
assert pb.IPerspective in interfaces
if avatarID == checkers.ANONYMOUS:
return pb.IPerspective, self.anonAvatar, lambda:None
else:
return pb.IPerspective, self.avatars[avatarID], lambda:None
# anonymous users get independent (but temporary) Avatars
# named users get their own persistent one
def requestAvatar(self, avatarID, mind, *interfaces):
assert pb.IPerspective in interfaces
if avatarID == checkers.ANONYMOUS:
return pb.IPerspective, MyAvatar(), lambda:None
else:
return pb.IPerspective, self.avatars[avatarID], lambda:None
The last example, note that the new MyAvatar instance is not
saved anywhere: it will vanish when the connection is dropped. By contrast,
the avatars that live in the self.avatars dictionary will
probably get persisted into the .tap file along with the Realm, the Portal,
and anything else that is referenced by the top-level Application object.
This is an easy way to manage saved user profiles.
Connecting and Disconnecting¶
It may be useful for your Avatars to be told when remote clients gain (and lose) access to them. For example, and Avatar might be updated by something in the server, and if there are clients attached, it should update them (through the “mind” argument which lets the Avatar do callRemote on the client).
One common idiom which accomplishes this is to have the Realm tell the
avatar that a remote client has just attached. The Realm can also ask the
protocol to let it know when the connection goes away, so it can then inform
the Avatar that the client has detached. The third member of the
requestAvatar return tuple is a callable which will be invoked
when the connection is lost.
class MyPerspective(pb.Avatar):
def __init__(self):
self.clients = []
def attached(self, mind):
self.clients.append(mind)
print "attached to", mind
def detached(self, mind):
self.clients.remove(mind)
print "detached from", mind
def update(self, message):
for c in self.clients:
c.callRemote("update", message)
class MyRealm:
def requestAvatar(self, avatarID, mind, *interfaces):
assert pb.IPerspective in interfaces
avatar = self.avatars[avatarID]
avatar.attached(mind)
return pb.IPerspective, avatar, lambda a=avatar:a.detached(mind)
Viewable¶
Once you have IPerspective objects (i.e. the Avatar) to
represent users, the Viewable class can come into play. This
class behaves a lot like Referenceable : it turns into a
RemoteReference when sent over the wire, and certain methods
can be invoked by the holder of that reference. However, the methods that
can be called have names that start with view_ instead of remote_ , and those methods are always called with an extra perspective argument that points to the Avatar through which
the reference was sent:
class Foo(pb.Viewable):
def view_doFoo(self, perspective, arg1, arg2):
pass
This is useful if you want to let multiple clients share a reference to
the same object. The view_ methods can use the
“perspective” argument to figure out which client is calling them. This
gives them a way to do additional permission checks, do per-user accounting,
etc.
This is the shortcut which makes per-user-per-group capability objects
much easier to use. Instead of creating such per-(user,group) objects, you
just have per-group objects which inherit from pb.Viewable , and
give the user references to them. The local pb.Avatar object
will automatically show up as the “perspective” argument in the
view_* method calls, give you a chance to involve the Avatar in
the process.
Chat Server with Avatars¶
Combining all the above techniques, here is an example chat server which uses a fixed set of identities (say, for the three members of your bridge club, who hang out in “#NeedAFourth” hoping that someone will discover your server, guess somebody’s password, break in, join the group, and also be available for a game next Saturday afternoon).
#!/usr/bin/env python
# Copyright (c) Twisted Matrix Laboratories.
# See LICENSE for details.
from zope.interface import implements
from twisted.cred import portal, checkers
from twisted.spread import pb
from twisted.internet import reactor
class ChatServer:
def __init__(self):
self.groups = {} # indexed by name
def joinGroup(self, groupname, user, allowMattress):
if groupname not in self.groups:
self.groups[groupname] = Group(groupname, allowMattress)
self.groups[groupname].addUser(user)
return self.groups[groupname]
class ChatRealm:
implements(portal.IRealm)
def requestAvatar(self, avatarID, mind, *interfaces):
assert pb.IPerspective in interfaces
avatar = User(avatarID)
avatar.server = self.server
avatar.attached(mind)
return pb.IPerspective, avatar, lambda a=avatar:a.detached(mind)
class User(pb.Avatar):
def __init__(self, name):
self.name = name
def attached(self, mind):
self.remote = mind
def detached(self, mind):
self.remote = None
def perspective_joinGroup(self, groupname, allowMattress=True):
return self.server.joinGroup(groupname, self, allowMattress)
def send(self, message):
self.remote.callRemote("print", message)
class Group(pb.Viewable):
def __init__(self, groupname, allowMattress):
self.name = groupname
self.allowMattress = allowMattress
self.users = []
def addUser(self, user):
self.users.append(user)
def view_send(self, from_user, message):
if not self.allowMattress and "mattress" in message:
raise ValueError, "Don't say that word"
for user in self.users:
user.send("<%s> says: %s" % (from_user.name, message))
realm = ChatRealm()
realm.server = ChatServer()
checker = checkers.InMemoryUsernamePasswordDatabaseDontUse()
checker.addUser("alice", "1234")
checker.addUser("bob", "secret")
checker.addUser("carol", "fido")
p = portal.Portal(realm, [checker])
reactor.listenTCP(8800, pb.PBServerFactory(p))
reactor.run()
Notice that the client uses perspective_joinGroup to both
join a group and retrieve a RemoteReference to the
Group object. However, the reference they get is actually to a
special intermediate object called a pb.ViewPoint . When they do
group.callRemote("send", "message") , their avatar is inserted
into the argument list that Group.view_send actually sees. This
lets the group get their username out of the Avatar without giving the
client an opportunity to spoof someone else.
The client side code that joins a group and sends a message would look like this:
#!/usr/bin/env python
# Copyright (c) Twisted Matrix Laboratories.
# See LICENSE for details.
from twisted.spread import pb
from twisted.internet import reactor
from twisted.cred import credentials
class Client(pb.Referenceable):
def remote_print(self, message):
print message
def connect(self):
factory = pb.PBClientFactory()
reactor.connectTCP("localhost", 8800, factory)
def1 = factory.login(credentials.UsernamePassword("alice", "1234"),
client=self)
def1.addCallback(self.connected)
reactor.run()
def connected(self, perspective):
print "connected, joining group #NeedAFourth"
# this perspective is a reference to our User object. Save a reference
# to it here, otherwise it will get garbage collected after this call,
# and the server will think we logged out.
self.perspective = perspective
d = perspective.callRemote("joinGroup", "#NeedAFourth")
d.addCallback(self.gotGroup)
def gotGroup(self, group):
print "joined group, now sending a message to all members"
# 'group' is a reference to the Group object (through a ViewPoint)
d = group.callRemote("send", "You can call me Al.")
d.addCallback(self.shutdown)
def shutdown(self, result):
reactor.stop()
Client().connect()
Footnotes
| [1] | Apparently Alice is one of those weirdos who has nothing better to do than to try and impersonate Bob. She will lie to her chat client, send incorrect objects to remote methods, even rewrite her local client code entirely to accomplish this juvenile prank. Given this adversarial relationship, one must wonder why she and Bob seem to spend so much time together: their adventures are clearly documented by the cryptographic literature. |
| [2] | The
obvious name is clearly
ServerSidePerUserObjectWhichNobodyElseHasAccessTo , but because
Python makes everything else so easy to read, it only seems fair to make
your audience work for something . |
| [3] | We could just go ahead and rename Perspective Broker to be Avatar Broker, but 1) that would cause massive compatibility problems, and 2) “AB” doesn’t fit into the whole sandwich-themed naming scheme nearly as well as “PB” does. If we changed it to AB, we’d probably have to change Banana to be CD (CoderDecoder), and Jelly to be EF (EncapsulatorFragmentor). twisted.spread would then have to be renamed twisted.alphabetsoup, and then the whole food-pun thing would start all over again. |
| [4] | The avatar-ish class is named
pb.Avatar because pb.Perspective was already
taken, by the (now obsolete) oldcred perspective-ish class. It is a pity,
but it simply wasn’t possible both replace pb.Perspective
in-place and maintain a reasonable level of
backwards-compatibility. |
PB Limits¶
There are a number of limits you might encounter when using Perspective Broker. This document is an attempt to prepare you for as many of them as possible so you can avoid them or at least recognize them when you do run into them.
Banana Limits¶
Perspective Broker is implemented in terms of a simpler, less
functional protocol called Banana. Twisted’s implementation of Banana
imposes a limit on the length of any sequence-like data type. This applies
directly to lists and strings and indirectly to dictionaries, instances and
other types. The purpose of this limit is to put an upper bound on the
amount of memory which will be allocated to handle a message received over
the network. Without, a malicious peer could easily perform a denial of
service attack resulting in exhaustion of the receiver’s memory. The basic
limit is 640 * 1024 bytes, defined by twisted.spread.banana.SIZE_LIMIT .
It’s possible to raise this limit by changing this value (but take care to
change it on both sides of the connection).
Another limit imposed by Twisted’s Banana implementation is a limit on
the size of long integers. The purpose of this limit is the same as the
SIZE_LIMIT . By default, only integers between -2 ** 448 and 2
** 448 (exclusive) can be transferred. This limit can be changed using
twisted.spread.banana.setPrefixLimit .
Perspective Broker Limits¶
Perspective Broker imposes an additional limit on top of these lower
level limits. The number of local objects for which remote references may
exist at a single time over a single connection, by default, is limited to
1024, defined by twisted.spread.pb.MAX_BROKER_REFS . This limit
also exists to prevent memory exhaustion attacks.
Porting to Python 3¶
Introduction¶
Twisted is currently being ported to work with Python 3.3. This document covers Twisted-specific issues in porting your code to Python 3.
Only some parts of Twisted have been ported, and therefore only a subset of modules are installed under Python 3. You can see the full list of ported modules in twisted.python.dist3.
API Differences¶
twisted.python.failure¶
Failure.trap raises itself (i.e. a Failure ) in Python 2. In Python 3, the wrapped exception will be re-raised.
Byte Strings and Text Strings¶
Several APIs which on Python 2 accepted or produced byte strings
(instances of str , sometimes just called bytes ) have
changed to accept or produce text strings (instances of str ,
sometimes just called text or unicode ) on Python 3.
From twisted.internet.address , the IPv4Address
and IPv6Address classes have had two attributes change from
byte strings to text strings: type and host .
twisted.python.log has shifted significantly towards text
strings from byte strings. Logging events, particular those produced by a
call like msg("foo") , must now be text strings. Consequently,
on Python 3, event dictionaries passed to log observes will contain text
strings where they previously contained byte strings.
twisted.python.runtime.platformType and the return value
from twisted.python.runtime.Platform.getType are now both text
strings.
twisted.python.filepath.FilePath has not changed.
It supports only byte strings. This will probably require applications to
update their usage of FilePath , at least to pass explicit byte
string literals rather than “native” string literals (which are text on
Python 3).
reactor.addSystemEventTrigger arguments that were
previously byte strings are now native strings.
twisted.names.dns deals with strings with a wide range of
meanings, often several for each DNS record type. Most of these strings
have remained as byte strings, which will probably require application
updates (for the reason given in the FilePath section above).
Some strings have changed to text strings, though. Any string representing
a human readable address (for
example, Record_A ‘s address parameter) is now a
text string. Additionally, time-to-live (ttl) values given as strings must
now be given as text strings.
twisted.web.resource.IResource continues to deal with URLs
and all URL-derived values as byte strings.
twisted.web.resource.ErrorPage has several string attributes
(template , brief , and detail ) which
were previously byte strings. On Python 3 only, these must now be text
strings.
Twisted Positioning¶
twisted.positioning: geolocation in Twisted
Introduction¶
twisted.positioning is a package for doing geospatial positioning (trying to find where you are on Earth) using Twisted.
High-level overview¶
In twisted.positioning, you write an IPositioningReceiver implementation that will get called whenever some information about your position is known (such as position, altitude, heading...).
The package provides a base class, BasePositioningReceiver you might want to use that implements all of the receiver methods as stubs.
Secondly, you will want a positioning source, which will call your IPositioningReceiver.
Currently, twisted.positioning provides an NMEA implementation, which is a standard protocol spoken by many positioning devices, usually over a serial port.
Examples¶
#!/usr/bin/env python
# Copyright (c) Twisted Matrix Laboratories.
# See LICENSE for details.
"""
Connects to an NMEA device, logs beacon information and position.
"""
import sys
from twisted.internet import reactor, serialport
from twisted.positioning import base, nmea
from twisted.python import log, usage
class PositioningReceiver(base.BasePositioningReceiver):
def positionReceived(self, latitude, longitude):
log.msg("I'm at {} lat, {} lon".format(latitude, longitude))
def beaconInformationReceived(self, beaconInformation):
template = "{0.seen} beacons seen, {0.used} beacons used"
log.msg(template.format(beaconInformation))
class Options(usage.Options):
optParameters = [
['baud-rate', 'b', 4800, "Baud rate (default: 4800)"],
['serial-port', 'p', '/dev/ttyS0', 'Serial Port device'],
]
def run():
log.startLogging(sys.stdout)
opts = Options()
try:
opts.parseOptions()
except usage.UsageError, message:
print "{}: {}".format(sys.argv[0], message)
return
positioningReceiver = PositioningReceiver()
nmeaReceiver = nmea.NMEAAdapter(positioningReceiver)
proto = nmea.NMEAProtocol(nmeaReceiver)
port, baudrate = opts["serial-port"], opts["baud-rate"]
serialport.SerialPort(proto, port, reactor, baudrate=baudrate)
reactor.run()
if __name__ == "__main__":
run()
- Connects to an NMEA device on a serial port, and reports whenever it receives a position.
Twisted Glossary¶
adaptee
An object that has been adapted, also called “original” . See Adapter .
An object whose sole purpose is to implement an Interface for another object. See Interfaces and Adapters .
A twisted.application.service.Application . There are HOWTOs on creating and manipulating them as a system-administrator, as well as using them in your code.
Avatar
(from Twisted Cred ) business logic for specific user. For example, in PB these are perspectives, in POP3 these are mailboxes, and so on.
The low-level data marshalling layer of Twisted Spread . See twisted.spread.banana .
A twisted.spread.pb.Broker , the object request broker for Twisted Spread .
cache
A way to store data in readily accessible place for later reuse. Caching data is often done because the data is expensive to produce or access. Caching data risks being stale, or out of sync with the original data.
component
A special kind of (persistent) Adapter that works with a twisted.python.components.Componentized . See also Interfaces and Adapters .
A Componentized object is a collection of information, separated into domain-specific or role-specific instances, that all stick together and refer to each other. Each object is an Adapter , which, in the context of Componentized, we call “components” . See also Interfaces and Adapters .
Twisted’s SSH implementation.
Connector
Object used to interface between client connections and protocols, usually used with a twisted.internet.protocol.ClientFactory to give you control over how a client connection reconnects. See twisted.internet.interfaces.IConnector and Writing Clients .
Consumer
An object that consumes data from a Producer . See twisted.internet.interfaces.IConsumer .
Cred
Twisted’s authentication API, twisted.cred . See Introduction to Twisted Cred and Twisted Cred usage .
credentials
A username/password, public key, or some other information used for authentication.
credential checker
Where authentication actually happens. See ICredentialsChecker .
CVSToys
A nifty set of tools for CVS, available at http://twistedmatrix.com/users/acapnotic/wares/code/CVSToys/ .
Daemon
A background process that does a job or handles client requests. Daemon is a Unix term; service is the Windows equivalent.
A instance of twisted.internet.defer.Deferred , an abstraction for handling chains of callbacks and error handlers (“errbacks” ). See the Deferring Execution HOWTO.
Enterprise
Twisted’s RDBMS support. It contains twisted.enterprise.adbapi for asynchronous access to any standard DB-API 2.0 module. See Introduction to Twisted Enterprise for more details.
errback
A callback attached to a Deferred with.addErrbackto handle errors.
In general, an object that constructs other objects. In Twisted, a Factory usually refers to a twisted.internet.protocol.Factory , which constructs Protocol instances for incoming or outgoing connections. See Writing Servers and Writing Clients .
Basically, an asynchronous exception that contains traceback information; these are used for passing errors through asynchronous callbacks.
im
Abbreviation of “(Twisted) Instance Messenger” .
Instance Messenger
Instance Messenger is a multi-protocol chat program that comes with Twisted. It can communicate via TOC with the AOL servers, via IRC, as well as via PB with Twisted Words . See twisted.words.im .
Interface
A class that defines and documents methods that a class conforming to that interface needs to have. A collection of core twisted.internet interfaces can be found in twisted.internet.interfaces . See also Interfaces and Adapters .
Jelly
The serialization layer for Twisted Spread , although it can be used separately from Twisted Spread as well. It is similar in purpose to Python’s standardpicklemodule, but is more network-friendly, and depends on a separate marshaller (Banana , in most cases). See twisted.spread.jelly .
Manhole
A debugging/administration interface to a Twisted application.
Microdom
A partial DOM implementation using SUX . It is simple and pythonic, rather than strictly standards-compliant. See twisted.web.microdom .
Names
Twisted’s DNS server, found in twisted.names .
Nevow
PB
Abbreviation of “Perspective Broker” .
Perspective Broker
The high-level object layer of Twisted Spread , implementing semantics for method calling and object copying, caching, and referencing. See twisted.spread.pb .
Portal
Glues credential checkers and realm s together.
Producer
An object that generates data a chunk at a time, usually to be processed by a Consumer . See twisted.internet.interfaces.IProducer .
In general each network connection has its own Protocol instance to manage connection-specific state. There is a collection of standard protocol implementations in twisted.protocols . See also Writing Servers and Writing Clients .
PSU
There is no PSU.
Reactor
The core event-loop of a Twisted application. See Reactor Basics .
Reality
See “Twisted Reality“
realm
A twisted.web.resource.Resource , which are served by Twisted Web. Resources can be as simple as a static file on disk, or they can have dynamically generated content.
Service
A twisted.application.service.Service . See Application howto for a description of how they relate to Applications .
Spread
Twisted Spread is Twisted’s remote-object suite. It consists of three layers: Perspective Broker , Jelly and Banana. See Writing Applications with Perspective Broker .
SUX
S mall U ncomplicated X ML, Twisted’s simple XML parser written in pure Python. See twisted.web.sux .
TAC
A T wisted A pplication C onfiguration is a Python source file, generally with the .tac extension, which defines configuration to make an application runnable usingtwistd.
TAP
T wisted A pplication P ickle (no longer supported), or simply just a*T* wisted AP plication. A serialised application that was created withmktap(no longer supported) and runnable bytwistd. See:doc:Using the Utilities <basics> .
Trial
twisted.trial , Twisted’s unit-testing framework, based on theunitteststandard library module. See also Writing tests for Twisted code .
Twisted Matrix Laboratories
The team behind Twisted. http://twistedmatrix.com/ .
Twisted Reality
In days of old, the Twisted Reality multiplayer text-based interactive-fiction system was the main focus of Twisted Matrix Labs; Twisted, the general networking framework, grew out of Reality’s need for better network functionality. Twisted Reality has been superseded by the Imaginary project.
The twisted.python.usage module, a replacement for the standardgetoptmodule for parsing command-lines which is much easier to work with. See Parsing command-lines .
Words
Twisted Words is a multi-protocol chat server that uses the Perspective Broker protocol as its native communication style. See twisted.words .
Woven
W eb O bject V isualization En vironment. A templating system previously, but no longer, included with Twisted. Woven has largely been superseded by Divmod Nevow .
Debugging Python(Twisted) with Emacs¶
Open up your project files. sometimes emacs can’t find them if you don’t have them open before-hand.
Make sure you have a program called
pdbsomewhere in your PATH, with the following contents:#!/bin/sh exec python -m pdb $1 $2 $3 $4 $5 $6 $7 $8 $9
Run
M-x pdbin emacs. If you usually run your program aspython foo.py, your command line should bepdb foo.py, fortwistdandtrialjust add -b to the command line, e.g.:twistd -b -y my.tacWhile pdb waits for your input, go to a place in your code and hit
C-x SPCto insert a break-point. pdb should say something happy. Do this in as many points as you wish.Go to your pdb buffer and hit
c; this runs as normal until a break-point is found.Once you get to a breakpoint, use
sto step,nto run the current line without stepping through the functions it calls,wto print out the current stack,uanddto go up and down a level in the stack,p footo print result of expressionfoo.Recommendations for effective debugging:
- use
p selfa lot; just knowing the class where the current code is isn’t enough most of the time. - use
wto get your bearings, it’ll re-display the current-line/arrow - after you use
w, useuanddand lots morep selfon the different stack-levels. - If you’ve got a big code-path that you need to grok, keep another buffer open and list the code-path there (e.g., I had a nasty-evil Deferred recursion, and this helped me tons)
- use
Footnotes
Introduction
-
Connecting your software - and having fun too!
-
Getting Started
-
Basic network servers with Twisted.
-
And basic clients.
Test-driven development with Twisted
Code without tests is broken by definition; Twisted makes it easy to test your network code.
Tutorial: Twisted From Scratch
- The Evolution of Finger: building a simple finger service
- The Evolution of Finger: adding features to the finger service
- The Evolution of Finger: cleaning up the finger code
- The Evolution of Finger: moving to a component based architecture
- The Evolution of Finger: pluggable backends
- The Evolution of Finger: a clean web frontend
- The Evolution of Finger: Twisted client support using Perspective Broker
- The Evolution of Finger: using a single factory for multiple protocols
- The Evolution of Finger: a Twisted finger client
- The Evolution of Finger: making a finger library
- The Evolution of Finger: configuration and packaging of the finger service
-
Networking and Other Event Sources
-
A brief overview of the
twisted.internetpackage. -
The event loop at the core of your program.
-
Add some security to your network transport.
-
How to use Twisted’s UDP implementation, including multicast and broadcast functionality.
-
Launching sub-processes, the correct way.
-
Like callback functions, only a lot better.
-
In-depth information on Deferreds.
-
More about Deferreds.
-
Timeouts, repeated events, and more: when you want things to happen later.
-
Running code in threads, and interacting with Twisted in a thread-safe manner.
Producers and Consumers: Efficient High-Volume Streaming
How to pause when buffers fill up.
Choosing a reactor and GUI toolkit integration
GTK+, Windows, epoll() and more: use your GUI of choice, or a faster event loop.
-
High-Level Infrastructure
Getting Connected with Endpoints
Create configurable applications that support multiple transports (e.g. TCP and SSL).
Interfaces and Adapters (Component Architecture)
When inheritance isn’t enough.
Cred: Pluggable Authentication
Implementing authentication and authorization that is configurable, pluggable and re-usable.
-
A generic plugin system for extendable programs.
Deploying Twisted Applications
Helper programs and scripts (twistd, ..)
twistdlets you daemonize and run your application.Using the Twisted Application Framework
Writing code that
twistdcan run.Writing Twisted Application Plugins for twistd
More powerful
twistddeployment method.Deploying Twisted with systemd
Use
systemdto launch and monitor Twisted applications.
Utilities
-
Keep a record of what your application is up to, and inspect that record to discover interesting information. (You may also be interested in the legacy logging system if you are maintaining code written to work with older versions of Twisted.)
-
enum-like constants.
Twisted RDBMS support with adbapi
Using SQL with your relational database via DB-API adapters.
Parsing command-line arguments
The command-line argument parsing used by
twistd.Using Dirdbm: Directory-based Storage
A simplistic way to store data on your filesystem.
Tips for writing tests for Twisted code using Trial
More information on writing tests.
Extremely Low-Level Socket Operations
Using wrappers for sendmsg(2) and recvmsg(2).
-
Asynchronous Messaging Protocol (AMP)
Asynchronous Messaging Protocol Overview
A two-way asynchronous message passing protocol, for when HTTP isn’t good enough.
Perspective Broker
-
A remote method invocation (RMI) protocol: call methods on remote objects.
-
Positioning
Appendix
Examples¶
Simple Echo server and client¶
simpleclient.py- simple TCP clientsimpleserv.py- simple TCP echo server
Chat¶
chatserver.py- shows how to communicate between clients
Echo server & client variants¶
echoserv.py- variant on a simple TCP echo serverechoclient.py- variant on a simple TCP clientechoserv_udp.py- simplest possible UDP serverechoclient_udp.py- simple UDP clientechoserv_ssl.py- simple SSL serverechoclient_ssl.py- simple SSL client
AMP server & client variants¶
ampserver.py- do math using AMPampclient.py- do math using AMP
Perspective Broker¶
pbsimple.py- simplest possible PB serverpbsimpleclient.py- simplest possible PB clientpbbenchclient.py- benchmarking clientpbbenchserver.py- benchmarking serverpbecho.py- echo server that uses loginpbechoclient.py- echo client using loginpb_exceptions.py- example of exceptions over PBpbgtk2.py- example of using GTK2 with PBpbinterop.py- shows off various types supported by PBbananabench.py- benchmark for banana
Cred¶
GUI¶
wxdemo.py- demo of wxPython integration with Twistedpbgtk2.py- example of using GTK2 with PBpyuidemo.py- PyUI
FTP examples¶
ftpclient.py- example of using the FTP clientftpserver.py- create an FTP server which serves files for anonymous users from the working directory and serves files for authenticated users from/home.
Logging¶
twistd-logging.tac- logging example using ILogObservertestlogging.py- use twisted.python.log to log errors to standard outrotatinglog.py- example of log file rotation
POSIX Specific Tricks¶
Miscellaneous¶
shaper.py- example of rate-limiting your web serverstdiodemo.py- example using stdio, Deferreds, LineReceiver and twisted.web.client.mouse.py- example using MouseMan protocol with the SerialPort transportptyserv.py- serve shells in pseudo-terminals over TCPcourier.py- example of interfacing to Courier’s mail filter interfacelongex.py- example of doing arbitrarily long calculations nicely in Twistedlongex2.py- using generators to do long calculationsstdin.py- reading a line at a time from standard input without blocking the reactorstreaming.py- example of a push producer/consumer systemfilewatch.py- write the content of a file to standard out one line at a timeshoutcast.py- example Shoutcast clientgpsfix.py- example using the SerialPort transport and GPS protocols to display fix data as it is received from the devicewxacceptance.py- acceptance tests for wxreactorpostfix.py- test application for PostfixTCPMapServerudpbroadcast.py- broadcasting using UDPtls_alpn_npn_client.py- example of TLS next-protocol negotiation on the client side using NPN and ALPN.tls_alpn_npn_server.py- example of TLS next-protocol negotiation on the server side using NPN and ALPN.
Specifications¶
Banana Protocol Specifications¶
Introduction¶
Banana is an efficient, extendable protocol for sending and receiving s-expressions. A s-expression in this context is a list composed of bytes, integers, large integers, floats and/or s-expressions. Unicode is not supported (but can be encoded to and decoded from bytes on the way into and out of Banana). Unsupported types must be converted into a supported type before sending them with Banana.
Banana Encodings¶
The banana protocol is a stream of data composed of elements. Each element has the
following general structure - first, the length of element encoded in base-128, least significant
bit first. For example length 4674 will be sent as 0x42 0x24 . For certain element
types the length will be omitted (e.g. float) or have a different meaning (it is the actual
value of integer elements).
Following the length is a delimiter byte, which tells us what kind of element this is. Depending on the element type, there will then follow the number of bytes specified in the length. The byte’s high-bit will always be set, so that we can differentiate between it and the length (since the length bytes use 128-base, their high bit will never be set).
Element Types¶
Given a series of bytes that gave us length N, these are the different delimiter bytes:
List – 0x80
The following bytes are a list of N elements. Lists may be nested, and a child list counts as only one element to its parent (regardless of how many elements the child list contains).
Integer – 0x81
The value of this element is the positive integer N. Following bytes are not part of this element. Integers can have values of 0 <= N <= 2147483647.
String – 0x82
The following N bytes are a string element.
Negative Integer – 0x83
The value of this element is the integer N * -1, i.e. -N. Following bytes are not part of this element. Negative integers can have values of 0 >= -N >= -2147483648.
Float - 0x84
The next 8 bytes are the float encoded in IEEE 754 floating-point “double format” bit layout. No length bytes should have been defined.
Large Integer – 0x85
The value of this element is the positive large integer N. Following bytes are not part of this element. Large integers have no size limitation.
Large Negative Integer – 0x86
The value of this element is the negative large integer -N. Following bytes are not part of this element. Large integers have no size limitation.
Large integers are intended for arbitrary length integers. Regular integers types (positive and negative) are limited to 32-bit values.
Examples¶
Here are some examples of elements and their encodings - the type bytes are marked in bold:
1
0x01 **0x81**
-1
0x01 **0x83**
1.5
**0x84** 0x3f 0xf8 0x00 0x00 0x00 0x00 0x00 0x00
"hello"
0x05 **0x82** 0x68 0x65 0x6c 0x6c 0x6f
[]
0x00 **0x80**
[1, 23]
0x02 **0x80** 0x01 **0x81** 0x17 **0x81**
123456789123456789
0x15 0x3e 0x41 0x66 0x3a 0x69 0x26 0x5b 0x01 **0x85**
[1, ["hello"]]
0x02 **0x80** 0x01 **0x81** 0x01 **0x80** 0x05 **0x82** 0x68 0x65 0x6c 0x6c 0x6f
Profiles¶
The Banana protocol is extendable. Therefore, it supports the concept of profiles. Profiles allow developers to extend the banana protocol, adding new element types, while still keeping backwards compatibility with implementations that don’t support the extensions. The profile used in each session is determined at the handshake stage (see below.)
A profile is specified by a unique string. This specification defines two profiles
- "none" and "pb" . The "none" profile is the standard
profile that should be supported by all Banana implementations.
Additional profiles may be added in the future.
Extensions defined by a profile may only be used if that profile has been selected by client and server.
The "none" Profile¶
The "none" profile is identical to the delimiter types listed above. It is highly recommended
that all Banana clients and servers support the "none" profile.
The "pb" Profile¶
The "pb" profile is intended for use with the Perspective Broker protocol, that runs on top
of Banana. Basically, it converts commonly used PB strings into shorter versions, thus
minimizing bandwidth usage. It starts with a single byte, which tells us to which string element
to convert it, and ends with the delimiter byte, 0x87 , which should not be prefixed
by a length.
- 0x01
- ‘None’
- 0x02
- ‘class’
- 0x03
- ‘dereference’
- 0x04
- ‘reference’
- 0x05
- ‘dictionary’
- 0x06
- ‘function’
- 0x07
- ‘instance’
- 0x08
- ‘list’
- 0x09
- ‘module’
- 0x0a
- ‘persistent’
- 0x0b
- ‘tuple’
- 0x0c
- ‘unpersistable’
- 0x0d
- ‘copy’
- 0x0e
- ‘cache’
- 0x0f
- ‘cached’
- 0x10
- ‘remote’
- 0x11
- ‘local’
- 0x12
- ‘lcache’
- 0x13
- ‘version’
- 0x14
- ‘login’
- 0x15
- ‘password’
- 0x16
- ‘challenge’
- 0x17
- ‘logged_in’
- 0x18
- ‘not_logged_in’
- 0x19
- ‘cachemessage’
- 0x1a
- ‘message’
- 0x1b
- ‘answer’
- 0x1c
- ‘error’
- 0x1d
- ‘decref’
- 0x1e
- ‘decache’
- 0x1f
- ‘uncache’
Protocol Handshake and Behaviour¶
The initiating side of the connection will be referred to as “client” , and the other side as “server” .
Upon connection, the server will send the client a list of string elements, signifying
the profiles it supports. It is recommended that "none" be included in this list. The client
then sends the server a string from this list, telling the server which profile it wants to
use. At this point the whole session will use this profile.
Once a profile has been established, the two sides may start exchanging elements. There is no limitation on order or dependencies of messages. Any such limitation (e.g. “server can only send an element to client in response to a request from client” ) is application specific.
Upon receiving illegal messages, failed handshakes, etc., a Banana client or server should close its connection.
Development of Twisted¶
Naming Conventions¶
While this may sound like a small detail, clear method naming is important to provide an API that developers familiar with event-based programming can pick up quickly.
Since the idea of a method call maps very neatly onto that of a received event, all event handlers are simply methods named after past-tense verbs. All class names are descriptive nouns, designed to mirror the is-a relationship of the abstractions they implement. All requests for notification or transmission are present-tense imperative verbs.
Here are some examples of this naming scheme:
- An event notification of data received from peer:
dataReceived(data) - A request to send data:
write(data) - A class that implements a protocol:
Protocol
The naming is platform neutral. This means that the names are equally appropriate in a wide variety of environments, as long as they can publish the required events.
It is self-consistent. Things that deal with TCP use the acronym TCP, and it is always capitalized. Dropping, losing, terminating, and closing the connection are all referred to as “losing” the connection. This symmetrical naming allows developers to easily locate other API calls if they have learned a few related to what they want to do.
It is semantically clear. The semantics of dataReceived are simple: there are some bytes available for processing. This remains true even if the lower-level machinery to get the data is highly complex.
Philosophy¶
Abstraction Levels¶
When implementing interfaces to the operating system or the network, provide two interfaces:
- One that doesn’t hide platform specific or library specific functionality. For example, you can use file descriptors on Unix, and Win32 events on Windows.
- One that provides a high level interface hiding platform specific details. E.g. process running uses same API on Unix and Windows, although the implementation is very different.
Restated in a more general way:
- Provide all low level functionality for your specific domain, without limiting the policies and decisions the user can make.
- Provide a high level abstraction on top of the low level implementation (or implementations) which implements the common use cases and functionality that is used in most cases.
Learning Curves¶
Require the minimal amount of work and learning on part of the user to get started. If this means they have less functionality, that’s OK, when they need it they can learn a bit more. This will also lead to a cleaner, easier to test design.
For example - using twistd is a great way to deploy applications. But to get started you don’t need to know about it. Later on you can start using twistd, but its usage is optional.
Security¶
We need to do a full audit of Twisted, module by module. This document list the sort of things you want to look for when doing this, or when writing your own code.
Bad input¶
Any place we receive untrusted data, we need to be careful.
In some cases we are not careful enough. For example, in HTTP
there are many places where strings need to be converted to
ints, so we use int() . The problem
is that this well accept negative numbers as well, whereas
the protocol should only be accepting positive numbers.
Resource Exhaustion and DoS¶
Make sure we never allow users to create arbitrarily large strings or files. Some of the protocols still have issues like this. Place a limit which allows reasonable use but will cut off huge requests, and allow changing of this limit.
Another operation to look out for are exceptions. They can fill up logs and take a lot of CPU time to render in web pages.
Twisted Development Policy¶
Twisted Coding Standard¶
Naming¶
Try to choose names which are both easy to remember and meaningful. Some silliness is OK at the module naming level (see twisted.spread ...) but when choosing class names, be as precise as possible.
Try to avoid overloaded terms. This rule is often broken, since it is incredibly difficult, as most normal words have already been taken by some other software. More importantly, try to avoid meaningless words. In particular, words like “handler” , “processor” , “engine” , “manager” and “component” don’t really indicate what something does, only that it does something .
Use American spelling in both names and docstrings. For compound technical terms such as ‘filesystem’, use a non-hyphenated spelling in both docstrings and code in order to avoid unnecessary capitalization.
Testing¶
Twisted development should always be test-driven . The complete test suite in the head of the SVN trunk is required to be passing on supported platforms at all times. Regressions in the test suite are addressed by reverting whatever revisions introduced them. For complete documentation about testing Twisted itself, refer to the Test Standard . What follows is intended to be a synopsis of the most important points.
The Twisted test suite is spread across many subpackages of the
twisted package. Many older tests are in
twisted.test . Others can be found at places such as
twisted.web.test (for twisted.web tests)
or twisted.internet.test (for twisted.internet
tests). The latter arrangement, twisted.somepackage.test ,
is preferred for new tests except when a test module already exists in
twisted.test .
Parts of the Twisted test suite may serve as good examples of how to
write tests for Twisted or for Twisted-based libraries (newer parts of
the test suite are generally better examples than older parts - check
when the code you are looking at was written before you use it as an
example of what you should write). The names of test modules should
begin with test_ so that they are automatically discoverable by
test runners such as Trial. Twisted’s unit tests are written using
twisted.trial , an xUnit library which has been
extensively customized for use in testing Twisted and Twisted-based
libraries.
Implementation (ie, non-test) source files should begin with a
test-case-name tag which gives the name of any test
modules or packages which exercise them. This lets tools discover a
subset of the entire test suite which they can run first to find tests
which might be broken by a particular change.
It is strongly suggested that developers learn to use Emacs, and use
the twisted-dev.el file included in
twisted-emacs
to bind the F9 key to “run unit tests” and bang on it
frequently. Support for other editors is unavailable at this time but
we would love to provide it.
To run the whole Twisted test without using emacs, use trial:
$ bin/trial twisted
To run an individual test module, such as
twisted/mail/test/test_pop3.py , specify the module
name:
$ bin/trial twisted.mail.test.test_pop3
To run the tests associated with a particular implementation file,
such as twisted/mail/pop3.py , use the
testmodule option:
$ bin/trial twisted/mail/pop3.py
All unit test methods should have docstrings specifying at a high level the intent of the test. That is, a description that users of the method would understand.
If you modify, or write a new, HOWTO, please read the Lore documentation to learn how to format the docs.
Copyright Header¶
Whenever a new file is added to the repository, add the following license header at the top of the file:
# Copyright (c) Twisted Matrix Laboratories.
# See LICENSE for details.
When you update existing files, if there is no copyright header, add one.
Whitespace¶
Indentation is 4 spaces per indent. Tabs are not allowed. It is preferred that every block appears on a new line, so that control structure indentation is always visible.
Lines are flowed at 79 columns. They must not have trailing whitespace. Long lines must be wrapped using implied line continuation inside parentheses; backslashes aren’t allowed. To handle long import lines, please repeat the import like this:
from very.long.package import foo, bar, baz
from very.long.package import qux, quux, quuux
Top-level classes and functions must be separated with 3 blank lines, and class-level functions with 2 blank lines. The control-L (i.e. ^L) form feed character must not be used.
Modules¶
Modules must be named in all lower-case, preferably short, single words. If a module name contains multiple words, they may be separated by underscores or not separated at all.
Modules must have a copyright message, a docstring and a reference to a test module that contains the bulk of its tests. Use this template:
# -*- test-case-name: <test module> -*-
# Copyright (c) Twisted Matrix Laboratories.
# See LICENSE for details.
"""
Docstring goes here.
"""
__all__ = []
In most cases, modules should contain more than one class, function, or method; if a module contains only one object, consider refactoring to include more related functionality in that module.
Depending on the situation, it is acceptable to have imports that look like this:
from twisted.internet.defer import Deferred
or like this:
from twisted.internet import defer
That is, modules should import modules or classes and functions , but not packages .
Wildcard import syntax may not be used by code in Twisted. These imports lead to code which is difficult to read and maintain by introducing complexity which strains human readers and automated tools alike. If you find yourself with many imports to make from a single module and wish to save typing, consider importing the module itself, rather than its attributes.
Relative imports (or sibling imports ) may not be used by code in Twisted. Relative imports allow certain circularities to be introduced which can ultimately lead to unimportable modules or duplicate instances of a single module. Relative imports also make the task of refactoring more difficult.
In case of local names conflicts due to import, use the as
syntax, for example:
from twisted.trial import util as trial_util
The encoding must always be ASCII, so no coding cookie is necessary.
Packages¶
Package names should follow the same conventions as module names. All modules must be encapsulated in some package. Nested packages may be used to further organize related modules.
__init__.py must never contain anything other than a
docstring and (optionally) an __all__ attribute. Packages are
not modules and should be treated differently. This rule may be
broken to preserve backwards compatibility if a module is made
into a nested package as part of a refactoring.
If you wish to promote code from a module to a package, for example, to break a large module out into several smaller files, the accepted way to do this is to promote from within the module. For example,
# parent/
# --- __init__.py ---
import child
# --- child.py ---
import parent
class Foo:
pass
parent.Foo = Foo
Every package should be added to the list in
setup.py .
Packages must not depend circularly upon each other. To simplify
maintaining this state, packages must also not import each other
circularly. While this applies to all packages within Twisted, one
twisted.python deserves particular attention, as it may
not depend on any other Twisted package.
String Formatting Operations¶
When using string formatting operations like formatString % values you should always
use a tuple if you’re using non-mapping values . This is to
avoid unexpected behavior when you think you’re passing in a single value,
but the value is unexpectedly a tuple, e.g.:
def foo(x):
return "Hi %s\n" % x
The example shows you can pass in foo("foo") or
foo(3) fine, but if you pass in foo((1,2)) ,
it raises a TypeError . You should use this instead:
def foo(x):
return "Hi %s\n" % (x,)
Docstrings¶
Docstrings should always be used to describe the purpose of methods, functions, classes, and modules. Moreover, all methods, functions, classes, and modules must have docstrings. In addition to documenting the purpose of the object, the docstring must document all of parameters or attributes of the object.
When documenting a class with one or more attributes which are
initialized directly from the value of a __init__ argument by
the same name (or differing only in that the attribute is private), it is
sufficient to document the __init__ parameter (in
the __init__ docstring). For example:
class Ninja(object):
"""
@ivar speed: See L{__init__}
@ivar _stealth: See C{stealth} parameter of L{__init__}
"""
def __init__(self, speed, stealth):
"""
@param speed: The maximum rate at which this ninja can travel (m/s)
@type speed: C{int} or C{float}
@param stealth: This ninja's ability to avoid being noticed in its
activities, as a percentage modifier.
@type: C{int}
"""
self.speed = speed
self._stealth = stealth
It is not necessary to have a second copy of the documentation for the
attribute in the class docstring, only a reference to the method
(typically __init__ which does document the
attribute’s meaning. Of course, if there is any interesting additional
behavior about the attribute that does not apply to
the __init__ argument, that behavior should be
documented in the class docstring.
Docstrings are never to be used to provide semantic information about an object; this rule may be violated if the code in question is to be used in a system where this is a requirement (such as Zope).
Docstrings should be indented to the level of the code they are documenting.
Docstrings should be triple-quoted. The opening and the closing of the docstrings should be on a line by themselves. For example:
class Ninja(object):
"""
A L{Ninja} is a warrior specializing in various unorthodox arts of war.
"""
def attack(self, someone):
"""
Attack C{someone} with this L{Ninja}'s shuriken.
"""
Docstrings should be written in epytext format; more documentation is available in the Epytext Markup Language documentation .
Additionally, to accommodate emacs users, single quotes of the type of the docstring’s triple-quote should be escaped. This will prevent font-lock from accidentally fontifying large portions of the file as a string.
For example,
def foo2bar(f):
"""
Convert L{foo}s to L{bar}s.
A function that should be used when you have a C{foo} but you want a
C{bar}; note that this is a non-destructive operation. If this method
can't convert the C{foo} to a C{bar} it will raise a L{FooException}.
@param f: C{foo}
@type f: str
For example::
import wombat
def sample(something):
f = something.getFoo()
f.doFooThing()
b = wombat.foo2bar(f)
b.doBarThing()
return b
"""
# Optionally, actual code can go here.
Comments¶
Start by reading the PEP8 Comments section. Ignore Documentation Strings section from PEP8 as Twisted uses a different docstring standard.
FIXME/TODO comments must have an associated ticket and contain a reference to it in the form of a full URL to the ticket. A brief amount of text should provide info about why the FIXME was added. It does not have to be the full ticket description, just enough to help readers decide if their next step should be reading the ticket or continue reading the code.
# FIXME: https://twistedmatrix.com/trac/ticket/1235
# Threads that have died before calling stop() are not joined.
for thread in threads:
thread.join()
Versioning¶
The API documentation should be marked up with version information.
When a new API is added the class should be marked with the epytext @since: field including the version number when the change was introduced.
This version number should be in the form x.y (for example, @since: 15.1).
Scripts¶
For each “script” , that is, a program you expect a Twisted user to run from the command-line, the following things must be done:
Write a module in twisted.scripts which contains a callable global named
run. This will be called by the command line part with no arguments (it will usually readsys.argv). Feel free to write more functions or classes in this module, if you feel they are useful to others.Create a file which contains a shebang line for Python. For Twisted Core, this file should be placed in the
bin/directory; for example,bin/twistd. For sub-projects, it should be placed inbin/<subproject>; for example, the key-generation tool for the Conch sub-project is inbin/conch/ckeygen.#!/usr/bin/env python
To make sure that the script is portable across different UNIX like
operating systems we use the /usr/bin/env command. The env
command allows you to run a program in a modified environment. That way
you don’t have to search for a program via the PATH environment
variable. This makes the script more portable but note that it is not a
foolproof method. Always make sure that /usr/bin/env exists or
use a softlink/symbolic link to point it to the correct path. Python’s
distutils will rewrite the shebang line upon installation so this policy
only covers the source files in version control.
For core scripts, add this Twisted running-from-SVN header:
import sys try: import _preamble except ImportError: sys.clear_exc()
Or for sub-project scripts, add a modified version which also adjusts
sys.path:import sys, os extra = os.path.dirname(os.path.dirname(sys.argv[0])) sys.path.insert(0, extra) try: import _preamble except ImportError: sys.clear_exc() sys.path.remove(extra)
And end with:
from twisted.scripts.yourmodule import run run()
Write a manpage and add it to the
manfolder of a subproject’sdocfolder. On Debian systems you can find a skeleton example of a manpage in/usr/share/doc/man-db/examples/manpage.example.
This will insure your program will work correctly for users of SVN, Windows releases and Debian packages.
Examples¶
For example scripts you expect a Twisted user to run from the command-line, add this Python shebang line at the top of the file:
#!/usr/bin/env python
Standard Library Extension Modules¶
When using the extension version of a module for which there is also a Python version, place the import statement inside a try/except block, and import the Python version if the import fails. This allows code to work on platforms where the extension version is not available. For example:
try:
import cPickle as pickle
except ImportError:
import pickle
Use the “as” syntax of the import statement as well, to set the name of the extension module to the name of the Python module.
Some modules don’t exist across all supported Python versions. For
example, Python 2.3’s sets module was deprecated in Python 2.6
in favor of the set and frozenset builtins.
twisted.python.compat would be the place to add
set and frozenset implementations that work across
Python versions.
Classes¶
Classes are to be named in mixed case, with the first letter capitalized; each word separated by having its first letter capitalized. Acronyms should be capitalized in their entirety. Class names should not be prefixed with the name of the module they are in. Examples of classes meeting this criteria:
- twisted.spread.pb.ViewPoint
- twisted.parser.patterns.Pattern
Examples of classes not meeting this criteria:
- event.EventHandler
- main.MainGadget
An effort should be made to prevent class names from clashing with each other between modules, to reduce the need for qualification when importing. For example, a Service subclass for Forums might be named twisted.forum.service.ForumService, and a Service subclass for Words might be twisted.words.service.WordsService. Since neither of these modules are volatile (see above) the classes may be imported directly into the user’s namespace and not cause confusion.
Classes and instances in Python come in two flavors: old-style or
classic, and new-style. Up to Python 2.1, old-style classes were the
only flavour available to the user, new-style classes were introduced
in Python 2.2 to unify classes and types. All classes added to Twisted
should be written as new-style classes. If x
is an instance of a new-style class, then type(x)
is the same as x.__class__ .
Methods¶
Methods should be in mixed case, with the first letter lower
case, each word separated by having its first letter
capitalized. For example, someMethodName ,
method .
Sometimes, a class will dispatch to a specialized sort of
method using its name; for example, twisted.reflect.Accessor.
In those cases, the type of method should be a prefix in all
lower-case with a trailing underscore, so method names will
have an underscore in them. For example, get_someAttribute .
Underscores in method names in twisted code are therefore
expected to have some semantic associated with them.
Some methods, in particular addCallback and its
cousins return self to allow for chaining calls. In this case,
wrap the chain in parenthesis, and start each chained call on
a separate line, for example:
return (foo()
.addCallback(bar)
.addCallback(thud)
.addCallback(wozers))
Callback Arguments¶
There are several methods whose purpose is to help the user set up
callback functions, for example Deferred.addCallback or the
reactor’s callLater method. To make
access to the callback as transparent as possible, most of these methods
use **kwargs to capture arbitrary arguments
that are destined for the user’s callback. This allows the call to the
setup function to look very much like the eventual call to the target
callback function.
In these methods, take care to not have other argument names that will “steal” the user’s callback’s arguments. When sensible, prefix these “internal” argument names with an underscore. For example, RemoteReference.callRemote is meant to be called like this:
myref.callRemote("addUser", "bob", "555-1212")
# on the remote end, the following method is invoked:
def addUser(name, phone):
...
where “addUser” is the remote method name. The user might also choose to call it with named parameters like this:
myref.callRemote("addUser", name="bob", phone="555-1212")
In this case, callRemote (and any code that uses the
**kwargs syntax) must be careful to not use
“name” , “phone” , or any other name that might overlap with
a user-provided named parameter. Therefore, callRemote is
implemented with the following signature:
class SomeClass(object):
def callRemote(self, _name, *args, **kw):
...
Do whatever you can to reduce user confusion. It may also be
appropriate to assert that the kwargs
dictionary does not contain parameters with names that will eventually
cause problems.
Special Methods¶
The augmented assignment protocol, defined by __iadd__ and other
similarly named methods, can be used to allow objects to be modified in
place or to rebind names if an object is immutable – both through use
of the same operator. This can lead to confusing code, which in turn
leads to buggy code. For this reason, methods of the augmented
assignment protocol should not be used in Twisted.
Functions¶
Functions should be named similarly to methods.
Functions or methods which are responding to events to
complete a callback or errback should be named _cbMethodName or
_ebMethodName , in order to distinguish them from normal
methods.
Attributes¶
Attributes should be named similarly to functions and
methods. Attributes should be named descriptively; attribute
names like mode , type , and
buf are generally discouraged. Instead, use
displayMode , playerType , or
inputBuffer .
Do not use Python’s “private” attribute syntax; prefix non-public attributes with a single leading underscore. Since several classes have the same name in Twisted, and they are distinguished by which package they come from, Python’s double-underscore name mangling will not work reliably in some cases. Also, name-mangled private variables are more difficult to address when unit testing or persisting a class.
An attribute (or function, method or class) should be considered private when one or more of the following conditions are true:
- The attribute represents intermediate state which is not always kept up-to-date.
- Referring to the contents of the attribute or otherwise maintaining a reference to it may cause resources to leak.
- Assigning to the attribute will break internal assumptions.
- The attribute is part of a known-to-be-sub-optimal interface and will certainly be removed in a future release.
Python 3¶
Twisted is being ported to Python 3. Be sure to follow the guidelines for Python 3 compatibility. Check on them often as they may change as lessons are learned during the porting effort.
Database¶
Database tables will be named with plural nouns.
Database columns will be named with underscores between words, all lower case, since most databases do not distinguish between case.
Any attribute, method argument, or method name that corresponds directly to a column in the database will be named exactly the same as that column, regardless of other coding conventions surrounding that circumstance.
All SQL keywords should be in upper case.
C Code¶
Wherever possible, C code should be optional, and the default python implementation should be maintained in tandem with it. C code should be strict ANSI C, and must build using GCC as well as Visual Studio for Windows, and really shouldn’t have any problems with other compilers either. Don’t do anything tricky.
C code should only be used for efficiency, not for binding to external libraries. If your particular code is not frequently run, write it in Python. If you require the use of an external library, develop a separate, external bindings package and make your twisted code depend on it.
Commit Messages¶
The commit messages are being distributed in a myriad of ways. Because of that, you need to observe a few simple rules when writing a commit message.
The first line of the message is being used as both the subject of the commit email and the announcement on #twisted. Therefore, it should be short (aim for < 80 characters) and descriptive – and must be able to stand alone (it is best if it is a complete sentence). The rest of the e-mail should be separated with hard line breaks into short lines (< 70 characters). This is free-format, so you can do whatever you like here.
Commit messages should be about what , not how : we can get how from SVN diff. Explain reasons for commits, and what they affect.
Each commit should be a single logical change, which is internally consistent. If you can’t summarize your changes in one short line, this is probably a sign that they should be broken into multiple checkins.
Source Control¶
Twisted currently uses Subversion for source control. All development should occur using branches; when a task is considered complete another Twisted developer may review it and if no problems are found, it may be merged into trunk. The Twisted wiki has a start . Branches must be used for major development. Branches should be managed using Combinator (but if you can manage them in some other way without anyone noticing, knock yourself out).
Certain features of Subversion should be avoided.
Do not set the
svn:ignoreproperty on any file or directory. What you wish to ignore, others may wish to examine. What others may wish you ignore, you may wish you examine.svn:ignorewill affect everyone who uses the repository, and so it is not the right mechanism to express personal preferences.If you wish to ignore certain files use the
global-ignoresfeature of~/.subversion/config, for example:[miscellany] global-ignores = dropin.cache *.pyc *.pyo *.o *.lo *.la #*# .*.rej *.rej .*~
Fallback¶
In case of conventions not enforced in this document, the reference documents to use in fallback is PEP 8 for Python code and PEP 7 for C code. For example, the paragraph Whitespace in Expressions and Statements in PEP 8 describes what should be done in Twisted code.
Recommendations¶
These things aren’t necessarily standardizeable (in that code can’t be easily checked for compliance) but are a good idea to keep in mind while working on Twisted.
If you’re going to work on a fragment of the Twisted codebase, please consider finding a way that you would use such a fragment in daily life. Using a Twisted Web server on your website encourages you to actively maintain and improve your code, as the little everyday issues with using it become apparent.
Twisted is a big codebase! If you’re refactoring something, please make sure to recursively grep for the names of functions you’re changing. You may be surprised to learn where something is called. Especially if you are moving or renaming a function, class, method, or module, make sure that it won’t instantly break other code.
Twisted Writing Standard¶
The Twisted writing standard describes the documentation writing styles we prefer in our documentation. This standard applies particularly to howtos and other descriptive documentation.
This document is meant to help Twisted documentation authors produce documentation that does not have the following problems:
- misleads users about what is good Twisted style;
- misleads users into thinking that an advanced howto is an introduction to writing their first Twisted server; and
- misleads users about whether they fit the document’s target audience: for example, that they are able to use enterprise without knowing how to write SQL queries.
General style¶
Documents should aim to be clear and concise, allowing the API documentation and the example code to tell as much of the story as they can. Demonstrations and where necessary supported arguments should always preferred to simple statements (“here is how you would simplify this code with Deferreds” rather than “Deferreds make code simpler”).
Documents should be clearly delineated into sections and subsections. Each of these sections, like the overall document, should have a single clear purpose. This is most easily tested by trying to have meaningful headings: a section which is headed by “More details” or “Advanced stuff” is not purposeful enough. There should be fairly obvious ways to split a document. The two most common are task based sectioning and sectioning which follows module and class separations.
Documentation must use American English spelling, and where possible avoid any local variants of either vocabulary or grammar. Grammatically complex sentences should ideally be avoided: these make reading unnecessarily difficult, particularly for non-native speakers.
When referring to a hypothetical person, (such as “a user of a website written with twisted.web”), gender neutral pronouns (they/their/them) should be used.
For reStructuredText documents which are handled by the Sphinx documentation generator make lines short, and break lines at natural places, such as after commas and semicolons, rather than after the 79th column. We call this semantic newlines. This rule does not apply to docstrings.
1 2 3 4 5 | Sometimes when editing a narrative documentation file, I wrap the lines semantically.
Instead of inserting a newline at 79 columns (or whatever),
or making paragraphs one long line,
I put in newlines at a point that seems logical to me.
Modern code-oriented text editors are very good at wrapping and arranging long lines.
|
Evangelism and usage documents¶
The Twisted documentation should maintain a reasonable distinction between “evangelism” documentation, which compares the Twisted design or Twisted best practice with other approaches and argues for the Twisted approach, and “usage” documentation, which describes the Twisted approach in detail without comparison to other possible approaches.
While both kinds of documentation are useful, they have different audiences. The first kind of document, evangelical documents, is useful to a reader who is researching and comparing approaches and seeking to understand the Twisted approach or Twisted functionality in order to decide whether it is useful to them. The second kind of document, usage documents, are useful to a reader who has decided to use Twisted and simply wants further information about available functions and architectures they can use to accomplish their goal.
Since they have distinct audiences, evangelism and detailed usage documentation belongs in separate files. There should be links between them in ‘Further reading’ or similar sections.
Descriptions of features¶
Descriptions of any feature added since release 2.0 of Twisted core must have a note describing which release of which Twisted project they were added in at the first mention in each document. If they are not yet released, give them the number of the next minor release.
For example, a substantial change might have a version number added in the introduction:
This document describes the Application infrastructure for deploying Twisted applications (added in Twisted 1.3) .
The version does not need to be mentioned elsewhere in the document except for specific features which were added in subsequent releases, which might should be mentioned separately.
The simplest way to create a.tacfile, SuperTac (added in Twisted Core 99.7) ...
In the case where the usage of a feature has substantially changed, the number should be that of the release in which the current usage became available. For example:
This document describes the Application infrastructure for deploying Twisted applications (updated[/substantially updated] in Twisted 2.7) .
Linking¶
The first occurrence of the name of any module, class or function should always link to the API documents. Subsequent mentions may or may not link at the author’s discretion: discussions which are very closely bound to a particular API should probably link in the first mention in the given section.
Links between howtos are encouraged. Overview documents and tutorials should always link to reference documents and in depth documents. These documents should link among themselves wherever it’s needed: if you’re tempted to re-describe the functionality of another module, you should certainly link instead.
Linking to standard library documentation is also encouraged when referencing
standard library objects. Intersphinx
is supported in Twisted documentation, with prefixes for linking to either
the Python 2 standard library documentation (via py2) or Python 3 (via
py3) as needed.
Introductions¶
The introductory section of a Twisted howto should immediately follow the top-level heading and precede any subheadings.
The following items should be present in the introduction to Twisted howtos: the introductory paragraph and the description of the target audience.
The introductory paragraph of a document should summarize what the document is designed to present. It should use the both proper names for the Twisted technologies and simple non-Twisted descriptions of the technologies. For example, in this paragraph both the name of the technology (“Conch”) and a description (“SSH server”) are used:
This document describes setting up a SSH server to serve data from the file system using Conch, the Twisted SSH implementation.
The introductory paragraph should be relatively short, but should, like the above, somewhere define the document’s objective: what the reader should be able to do using instructions in the document.
Subsequent paragraphs in the introduction should describe the target audience of the document: who would want to read it, and what they should know before they can expect to use your document. For example:
The target audience of this document is a Twisted user who has a set of filesystem like data objects that they would like to make available to authenticated users over SFTP.
Following the directions in this document will require that you are familiar with managing authentication via the Twisted Cred system.
Use your discretion about the extent to which you list assumed knowledge. Very introductory documents that are going to be among a reader’s first exposure to Twisted will even need to specify that they rely on knowledge of Python and of certain networking concepts (ports, servers, clients, connections) but documents that are going to be sought out by existing Twisted users for particular purposes only need to specify other Twisted knowledge that is assumed.
Any knowledge of technologies that wouldn’t be considered “core Python” and/or “simple networking” need to be explicitly specified, no matter how obvious they seem to someone familiar with the technology. For example, it needs to be stated that someone using enterprise should know SQL and should know how to set up and populate databases for testing purposes.
Where possible, link to other documents that will fill in missing knowledge for the reader. Linking to documents in the Twisted repository is preferred but not essential.
The introduction should finish with a list of tasks that the user can expect to see the document accomplish. These tasks should be concrete rather than abstract, so rather than telling the user that they will “understand Twisted Conch”, you would list the specific tasks that they will see the document do. For example:
This document will demonstrate the following tasks using Twisted Conch:
- creating an anonymous access read-only SFTP server using a filesystem backend;
- creating an anonymous access read-only SFTP server using a proxy backend connecting to an HTTP server; and
- creating a anonymous access read and write SFTP server using a filesystem backend.
In many cases this will essentially be a list of your code examples, but it need not be. If large sections of your code are devoted to design discussions, your goals might resemble the following:
This document will discuss the following design aspects of writing Conch servers:
- authentication of users; and
- choice of data backends.
Example code¶
Wherever possible, example code should be provided to illustrate a certain technique or piece of functionality.
Example code should try and meet as many of the following requirements as possible:
- example code should be a complete working example suitable for copying and pasting and running by the reader (where possible, provide a link to a file to download);
- example code should be short;
- example code should be commented very extensively, with the assumption that this code may be read by a Twisted newcomer;
- example code should conform to the coding standard ; and
- example code should exhibit ‘best practice’, not only for dealing with the target functionality, but also for use of the application framework and so on.
The requirement to have a complete working example will occasionally impose upon authors the need to have a few dummy functions: in Twisted documentation the most common example is where a function is needed to generate a Deferred and fire it after some time has passed. An example might be this, where deferLater is used to fire a callback after a period of time:
from twisted.internet import task, reactor
def getDummyDeferred():
"""
Dummy method which returns a deferred that will fire in 5 seconds with
a result
"""
return task.deferLater(reactor, 5, lambda x: "RESULT")
As in the above example, it is imperative to clearly mark that the
function is a dummy in as many ways as you can: using Dummy in
the function name, explaining that it is a dummy in the docstring, and
marking particular lines as being required to create an effect for the
purposes of demonstration. In most cases, this will save the reader from
mistaking this dummy method for an idiom they should use in their Twisted
code.
Conclusions¶
The conclusion of a howto should follow the very last section heading in a file. This heading would usually be called “Conclusion”.
The conclusion of a howto should remind the reader of the tasks that they have done while reading the document. For example:
In this document, you have seen how to:
- set up an anonymous read-only SFTP server;
- set up a SFTP server where users authenticate;
- set up a SFTP server where users are restricted to some parts of the filesystem based on authentication; and
- set up a SFTP server where users have write access to some parts of the filesystem based on authentication.
If appropriate, the howto could follow this description with links to other documents that might be of interest to the reader with their newfound knowledge. However, these links should be limited to fairly obvious extensions of at least one of the listed tasks.
Unit Tests in Twisted¶
Each unit test tests one bit of functionality in the software. Unit tests are entirely automated and complete quickly. Unit tests for the entire system are gathered into one test suite, and may all be run in a single batch. The result of a unit test is simple: either it passes, or it doesn’t. All this means you can test the entire system at any time without inconvenience, and quickly see what passes and what fails.
Unit Tests in the Twisted Philosophy¶
The Twisted development team adheres to the practice of Extreme Programming (XP), and the usage of unit tests is a cornerstone XP practice. Unit tests are a tool to give you increased confidence. You changed an algorithm – did you break something? Run the unit tests. If a test fails, you know where to look, because each test covers only a small amount of code, and you know it has something to do with the changes you just made. If all the tests pass, you’re good to go, and you don’t need to second-guess yourself or worry that you just accidentally broke someone else’s program.
What to Test, What Not to Test¶
You don’t have to write a test for every single method you write, only production methods that could possibly break.
– Kent Beck, Extreme Programming Explained
Running the Tests¶
From the root of the Twisted source tree, run Trial :
$ bin/trial twisted
You’ll find that having something like this in your emacs init files is quite handy:
(defun runtests () (interactive)
(compile "python /somepath/Twisted/bin/trial /somepath/Twisted"))
(global-set-key [(alt t)] 'runtests)
Always, always, always be sure all the tests pass before committing any code. If someone else checks out code at the start of a development session and finds failing tests, they will not be happy and may decide to hunt you down .
Since this is a geographically dispersed team, the person who can help you get your code working probably isn’t in the room with you. You may want to share your work in progress over the network, but you want to leave the main Subversion tree in good working order. So use a branch , and merge your changes back in only after your problem is solved and all the unit tests pass again.
Adding a Test¶
Please don’t add new modules to Twisted without adding tests for them too. Otherwise we could change something which breaks your module and not find out until later, making it hard to know exactly what the change that broke it was, or until after a release, and nobody wants broken code in a release.
Tests go into dedicated test packages such as
twisted/test/ or twisted/conch/test/ ,
and are named test_foo.py , where foo is the name
of the module or package being tested. Extensive documentation on using
the PyUnit framework for writing unit tests can be found in the
links section below.
One deviation from the standard PyUnit documentation: To ensure
that any variations in test results are due to variations in the
code or environment and not the test process itself, Twisted ships
with its own, compatible, testing framework. That just
means that when you import the unittest module, you will from twisted.trial import unittest instead of the
standard import unittest .
As long as you have followed the module naming and placement
conventions, trial will be smart
enough to pick up any new tests you write.
PyUnit provides a large number of assertion methods to be used when
writing tests. Many of these are redundant. For consistency, Twisted
unit tests should use the assert forms rather than the
fail forms. Also, use assertEqual ,
assertNotEqual , and assertAlmostEqual rather
than assertEquals , assertNotEquals , and
assertAlmostEquals . assertTrue is also
preferred over assert_ . You may notice this convention is
not followed everywhere in the Twisted codebase. If you are changing
some test code and notice the wrong method being used in nearby code,
feel free to adjust it.
When you add a unit test, make sure all methods have docstrings specifying at a high level the intent of the test. That is, a description that users of the method would understand.
Test Implementation Guidelines¶
Here are some guidelines to follow when writing tests for the Twisted test suite. Many tests predate these guidelines and so do not follow them. When in doubt, follow the guidelines given here, not the example of old unit tests.
When writing tests for the Twisted test suite, test classes are named
FooTests, where Foo is the name of the component being tested.
Here is an example:
class SSHClientTests(unittest.TestCase):
def test_sshClient(self):
foo() # the actual test
Most unit tests should avoid performing real, platform-implemented I/O operations. Real I/O is slow, unreliable, and unwieldy.
When implementing a protocol, twisted.test.proto_helpers.StringTransport can be used instead of a real TCP transport.
StringTransport is fast, deterministic, and can easily be used to exercise all possible network behaviors.
If you need pair a client to a server and have them talk to each other, use twisted.test.iosim.connect with twisted.test.iosim.FakeTransport transports.
Most unit tests should also avoid waiting for real time to pass. Unit tests which construct and advance a twisted.internet.task.Clock are fast and deterministic.
When designing your code allow for the reactor to be injected during tests.
from twisted.internet.task import Clock
def test_timeBasedFeature(self):
"""
In this test a Clock scheduler is used.
"""
clock = Clock()
yourThing = SomeThing()
yourThing._reactor = clock
state = yourThing.getState()
clock.advance(10)
# Get state after 10 seconds.
state = yourThing.getState()
Keeping test data in the source tree should be avoided where possible.
In some cases it can not be avoided, but where it’s obvious how to do so, do it. Test data can be generated at run time or stored inside the test modules as constants.
When file system access is required, dumping data into a temporary path during the test run opens up more testing opportunities. Inside the temporary path you can control various path properties or permissions.
You should design your code so that data can be read from arbitrary input streams.
Tests should be able to run even if they are run inside an installed copy of Twisted.
publicRSA_openssh = ("ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAGEArzJx8OYOnJmzf4tf"
"vLi8DVPrJ3/c9k2I/Az64fxjHf9imyRJbixtQhlH9lfNjUIx+4LmrJH5QNRsFporcHDKOTwTT"
"h5KmRpslkYHRivcJSkbh/C+BR3utDS555mV comment")
def test_loadOpenSSHRSAPublic(self):
"""
L{keys.Key.fromStrea} can load RSA keys serialized in OpenSSH format.
"""
keys.Key.fromStream(StringIO(publicRSA_openssh))
Since unit tests are avoiding real I/O and real time, they can usually avoid using a real reactor. The only exceptions to this are unit tests for a real reactor implementation. Unit tests for protocol implementations or other application code should not use a reactor. Unit tests for real reactor implementations should not use the global reactor, but should instead use twisted.internet.test.reactormixins.ReactorBuilder so they can be applied to all of the reactor implementations automatically. In no case should new unit tests use the global reactor.
Skipping Tests¶
Trial, the Twisted unit test framework, has some extensions which are designed to encourage developers to add new tests. One common situation is that a test exercises some optional functionality: maybe it depends upon certain external libraries being available, maybe it only works on certain operating systems. The important common factor is that nobody considers these limitations to be a bug.
To make it easy to test as much as possible, some tests may be skipped in
certain situations. Individual test cases can raise the SkipTest exception to indicate that they should be skipped, and
the remainder of the test is not run. In the summary (the very last thing
printed, at the bottom of the test output) the test is counted as a”skip” instead of a “success” or “fail” . This should be used
inside a conditional which looks for the necessary prerequisites:
class SSHClientTests(unittest.TestCase):
def test_sshClient(self):
if not ssh_path:
raise unittest.SkipTest("cannot find ssh, nothing to test")
foo() # do actual test after the SkipTest
You can also set the .skip attribute on the method, with a
string to indicate why the test is being skipped. This is convenient for
temporarily turning off a test case, but it can also be set conditionally (by
manipulating the class attributes after they’ve been defined):
class SomeThingTests(unittest.TestCase):
def test_thing(self):
dotest()
test_thing.skip = "disabled locally"
class MyTests(unittest.TestCase):
def test_one(self):
...
def test_thing(self):
dotest()
if not haveThing:
MyTests.test_thing.im_func.skip = "cannot test without Thing"
# but test_one() will still run
Finally, you can turn off an entire TestCase at once by setting the .skip attribute on the class. If you organize your tests by the functionality they depend upon, this is a convenient way to disable just the tests which cannot be run.
class TCPTests(unittest.TestCase):
...
class SSLTests(unittest.TestCase):
if not haveSSL:
skip = "cannot test without SSL support"
# but TCPTests will still run
...
Testing New Functionality¶
Two good practices which arise from the “XP” development process are sometimes at odds with each other:
- Unit tests are a good thing. Good developers recoil in horror when they see a failing unit test. They should drop everything until the test has been fixed.
- Good developers write the unit tests first. Once tests are done, they write implementation code until the unit tests pass. Then they stop.
These two goals will sometimes conflict. The unit tests that are written first, before any implementation has been done, are certain to fail. We want developers to commit their code frequently, for reliability and to improve coordination between multiple people working on the same problem together. While the code is being written, other developers (those not involved in the new feature) should not have to pay attention to failures in the new code. We should not dilute our well-indoctrinated Failing Test Horror Syndrome by crying wolf when an incomplete module has not yet started passing its unit tests. To do so would either teach the module author to put off writing or committing their unit tests until after all the functionality is working, or it would teach the other developers to ignore failing test cases. Both are bad things.
”.todo” is intended to solve this problem. When a developer first
starts writing the unit tests for functionality that has not yet been
implemented, they can set the .todo attribute on the test
methods that are expected to fail. These methods will still be run, but
their failure will not be counted the same as normal failures: they will go
into an “expected failures” category. Developers should learn to treat
this category as a second-priority queue, behind actual test failures.
As the developer implements the feature, the tests will eventually start passing. This is surprising: after all those tests are marked as being expected to fail. The .todo tests which nevertheless pass are put into a”unexpected success” category. The developer should remove the .todo tag from these tests. At that point, they become normal tests, and their failure is once again cause for immediate action by the entire development team.
The life cycle of a test is thus:
- Test is created, marked
.todo. Test fails: “expected failure” . - Code is written, test starts to pass. “unexpected success” .
.todotag is removed. Test passes. “success” .- Code is broken, test starts to fail. “failure” . Developers spring into action.
- Code is fixed, test passes once more. “success” .
.todo may be of use while you are developing a feature, but by the time you are ready to commit anything all the tests you have written should be passing.
In other words never commit to trunk tests marked as .todo.
For unfinished tests you should create a follow up ticket and add the tests to the ticket’s description.
You can also ignore the .todo marker and just make sure you write test first to see them failing before starting to work on the fix.
Line Coverage Information¶
Trial provides line coverage information, which is very useful to ensure
old code has decent coverage. Passing the --coverage option to Trial will generate the coverage information in a file called coverage which can be found in the _trial_temp
folder.
Associating Test Cases With Source Files¶
Please add a test-case-name tag to the source file that is
covered by your new test. This is a comment at the beginning of the file
which looks like one of the following:
# -*- test-case-name: twisted.test.test_defer -*-
or
#!/usr/bin/env python
# -*- test-case-name: twisted.test.test_defer -*-
This format is understood by emacs to mark “File Variables” . The
intention is to accept test-case-name anywhere emacs would on
the first or second line of the file (but not in the File Variables: block that emacs accepts at the end of the file). If you
need to define other emacs file variables, you can either put them in the File Variables: block or use a semicolon-separated list of
variable definitions:
# -*- test-case-name: twisted.test.test_defer; fill-column: 75; -*-
If the code is exercised by multiple test cases, those may be marked by
using a comma-separated list of tests, as follows: (NOTE: not all tools can
handle this yet.. trial --testmodule does, though)
# -*- test-case-name: twisted.test.test_defer,twisted.test.test_tcp -*-
The test-case-name tag will allow trial --testmodule twisted/dir/myfile.py to determine which test cases need
to be run to exercise the code in myfile.py . Several tools (as
well as http://launchpad.net/twisted-emacs’s twisted-dev.el ‘s F9 command) use this to automatically
run the right tests.
Links¶
- A chapter on Unit Testing in Mark Pilgrim’s Dive Into Python .
- unittest module documentation, in the Python Library Reference .
- UnitTest on the PortlandPatternRepository Wiki , where all the cool ExtremeProgramming kids hang out.
- Unit Tests in Extreme Programming: A Gentle Introduction .
- Ron Jeffries expounds on the importance of Unit Tests at 100% .
- Ron Jeffries writes about the Unit Test in the Extreme Programming practices of C3 .
- PyUnit’s homepage .
- The top-level tests directory, twisted/test , in Subversion.
See also Tips for writing tests for Twisted code .
Working from Twisted’s Subversion repository¶
If you’re going to be doing development on Twisted itself, or if you want to take advantage of bleeding-edge features (or bug fixes) that are not yet available in a numbered release, you’ll probably want to check out a tree from the Twisted Subversion repository. The Trunk is where all current development takes place.
This document lists some useful tips for working on this cutting edge.
Checkout¶
Subversion tutorials can be found elsewhere, see in particular the Subversion homepage . The relevant data you need to check out a copy of the Twisted tree is available on the development page , and is as follows:
$ svn co svn://svn.twistedmatrix.com/svn/Twisted/trunk Twisted
Alternate tree names¶
By using svn co svn://svn.twistedmatrix.com/svn/Twisted/trunk otherdir , you can put the workspace tree in a directory other than “Twisted” . I do this (with a name like “Twisted-Subversion” ) to
remind myself that this tree comes from Subversion and not from a released
version (like “Twisted-1.0.5” ). This practice can cause a few problems,
because there are a few places in the Twisted tree that need to know where
the tree starts, so they can add it to sys.path without
requiring the user manually set their PYTHONPATH. These functions walk the
current directory up to the root, looking for a directory named “Twisted” (sometimes exactly that, sometimes with a .startswith test). Generally these are test scripts or other
administrative tools which expect to be launched from somewhere inside the
tree (but not necessarily from the top).
If you rename the tree to something other than Twisted , these
tools may wind up trying to use Twisted source files from /usr/lib/python2.5
or elsewhere on the default sys.path . Normally this won’t
matter, but it is good to be aware of the issue in case you run into
problems.
twisted/test/process_twisted.py is one of these programs.
Combinator¶
In order to simplify the use of Subversion, we typically use Divmod Combinator . You may find it to be useful, too. In particular, because Twisted uses branches for almost all feature development, if you plan to contribute to Twisted you will probably find Combinator very useful. For more details, see the Combinator website, as well as the UQDS page.
Compiling C extensions¶
There are currently several C extension modules in Twisted: twisted.internet.cfsupport , twisted.internet.iocpreactor._iocp ,
and twisted.python._epoll . These modules
are optional, but you’ll have to compile them if you want to experience their
features, performance improvements, or bugs. There are two approaches.
The first is to do a regular distutils ./setup.py build , which
will create a directory under build/ to hold both the generated .so files as well as a copy of the 600-odd .py files
that make up Twisted. If you do this, you will need to set your PYTHONPATH to
something like MyDir/Twisted/build/lib.linux-i686-2.5 in order to
run code against the Subversion twisted (as opposed to whatever’s installed in /usr/lib/python2.5 or wherever python usually looks). In
addition, you will need to re-run the build command every time you change a .py file. The build/lib.foo
directory is a copy of the main tree, and that copy is only updated when you
re-run setup.py build . It is easy to forget this and then wonder
why your code changes aren’t being expressed.
The second technique is to build the C modules in place, and point your
PYTHONPATH at the top of the tree, like MyDir/Twisted . This way
you’re using the .py files in place too, removing the confusion a forgotten
rebuild could cause with the separate build/ directory above. To build the C
modules in place, do ./setup.py build_ext -i . You only need to
re-run this command when you change the C files. Note that setup.py is not Make, it does not always get the dependencies
right (.h files in particular), so if you are hacking on the
cReactor you may need to manually delete the .o files before
doing a rebuild. Also note that doing a setup.py clean will
remove the .o files but not the final .so files,
they must be deleted by hand.
Running tests¶
To run the full unit-test suite, do:
./bin/trial twisted
To run a single test file (like twisted/test/test_defer.py ),
do one of:
./bin/trial twisted.test.test_defer
or
./bin/trial twisted/test/test_defer.py
To run any tests that are related to a code file, like twisted/protocols/imap4.py , do:
./bin/trial --testmodule twisted/mail/imap4.py
This depends upon the .py file having an appropriate “test-case-name” tag that indicates which test cases provide coverage.
See the Test Standards document for
details about using “test-case-name” . In this example, the twisted.mail.test.test_imap test will be run.
Many tests create temporary files in /tmp or ./_trial_temp, but
everything in /tmp should be deleted when the test finishes. Sometimes these
cleanup calls are commented out by mistake, so if you see a stray /tmp/@12345.1 directory, it is probably from test_dirdbm or test_popsicle .
Look for an rmtree that has been commented out and complain to
the last developer who touched that file.
Building docs¶
Twisted documentation (not including the automatically-generated API docs) is generated by Sphinx .
The docs are written in Restructured Text (.rst) and translated into .html files by the bin/admin/build-docs script.
To build the HTML form of the docs into the doc/ directory, do the following:
./bin/admin/build-docs .
Committing and Post-commit Hooks¶
Twisted uses a customized trac-post-commit-hook to enable ticket updates based on svn commit
logs. When making a branch for a ticket, the branch name should end
in -<ticket number> , for
example my-branch-9999 . This will add a ticket comment containing a
changeset link and branch name. To make your commit message show up as a comment
on a Trac ticket, add a refs #<ticket number> line at the
bottom of your commit message. To automatically close a ticket on Trac
as Fixed and add a comment with the closing commit message, add
a Fixes: #<ticket number> line to your commit message. In
general, a commit message closing a ticket looks like this:
Merge my-branch-9999: A single-line summary.
Author: jesstess
Reviewers: exarkun, glyph
Fixes: #9999
My longer description of the changes made.
The Twisted Coding Standard elaborates on commit messages and source control.
Emacs¶
A minor mode for development with Twisted using Emacs is available. See twisted-dev.el , provided by twisted-emacs ,
for several utility functions which make it easier to grep for methods, run test cases, etc.
This series of documents is designed for people who wish to contribute to the Twisted codebase.
This documentation is for people who work on the Twisted codebase itself, rather than for people who want to use Twisted in their own projects.
- Developer guides : documentation on using Twisted Core to develop your own applications
- Examples : short code examples using Twisted Core
- Specifications : specification documents for elements of Twisted Core
- Development of Twisted : for people who want to work on Twisted itself
An API reference is available on the twistedmatrix web site.
Twisted Conch¶
Developer Guides¶
Writing a client with Twisted Conch¶
Introduction¶
In the original days of computing, rsh/rlogin were used to connect to remote computers and execute commands. These commands had the problem that the passwords and commands were sent in the clear. To solve this problem, the SSH protocol was created. Twisted Conch implements the second version of this protocol.
Using an SSH Command Endpoint¶
If your objective is to execute a command on a remote host over an SSH connection, then the easiest approach may be to use twisted.conch.endpoints.SSHCommandClientEndpoint . If you haven’t used endpoints before, first take a look at the endpoint howto to get an idea of how endpoints work in general.
Conch provides an endpoint implementation which establishes an SSH connection, performs necessary authentication, opens a channel, and launches a command in that channel. It then associates the output of that command with the input of a protocol you supply, and associates output from that protocol with the input of that command. Effectively, this lets you ignore most of the complexity of SSH and just interact with a remote process as though it were any other stream-oriented connection - such as TCP or SSL.
Conch also provides an endpoint that is initialized with an already established SSH connection. This endpoint just opens a new channel on the existing connection and launches a command in that.
Using the SSHCommandClientEndpoint is about as simple as using any
other stream-oriented client endpoint. Just create the endpoint defining
where the SSH server to connect to is and a factory defining what kind of
protocol to use to interact with the command and let them get to work
using the endpoint’s connect method.
#!/usr/bin/env python
# Copyright (c) Twisted Matrix Laboratories.
# See LICENSE for details.
if __name__ == '__main__':
import sys
import echoclient_ssh
from twisted.internet.task import react
react(echoclient_ssh.main, sys.argv[1:])
import os, getpass
from twisted.python.filepath import FilePath
from twisted.python.usage import Options
from twisted.internet.defer import Deferred
from twisted.internet.protocol import Factory, Protocol
from twisted.internet.endpoints import UNIXClientEndpoint
from twisted.conch.ssh.keys import EncryptedKeyError, Key
from twisted.conch.client.knownhosts import KnownHostsFile
from twisted.conch.endpoints import SSHCommandClientEndpoint
class EchoOptions(Options):
optParameters = [
("host", "h", "localhost",
"hostname of the SSH server to which to connect"),
("port", "p", 22,
"port number of SSH server to which to connect", int),
("username", "u", getpass.getuser(),
"username with which to authenticate with the SSH server"),
("identity", "i", None,
"file from which to read a private key to use for authentication"),
("password", None, None,
"password to use for authentication"),
("knownhosts", "k", "~/.ssh/known_hosts",
"file containing known ssh server public key data"),
]
optFlags = [
["no-agent", None, "Disable use of key agent"],
]
class NoiseProtocol(Protocol):
def connectionMade(self):
self.finished = Deferred()
self.strings = ["bif", "pow", "zot"]
self.sendNoise()
def sendNoise(self):
if self.strings:
self.transport.write(self.strings.pop(0) + "\n")
else:
self.transport.loseConnection()
def dataReceived(self, data):
print "Server says:", data
self.sendNoise()
def connectionLost(self, reason):
self.finished.callback(None)
def readKey(path):
try:
return Key.fromFile(path)
except EncryptedKeyError:
passphrase = getpass.getpass("%r keyphrase: " % (path,))
return Key.fromFile(path, passphrase=passphrase)
class ConnectionParameters(object):
def __init__(self, reactor, host, port, username, password, keys,
knownHosts, agent):
self.reactor = reactor
self.host = host
self.port = port
self.username = username
self.password = password
self.keys = keys
self.knownHosts = knownHosts
self.agent = agent
@classmethod
def fromCommandLine(cls, reactor, argv):
config = EchoOptions()
config.parseOptions(argv)
keys = []
if config["identity"]:
keyPath = os.path.expanduser(config["identity"])
if os.path.exists(keyPath):
keys.append(readKey(keyPath))
knownHostsPath = FilePath(os.path.expanduser(config["knownhosts"]))
if knownHostsPath.exists():
knownHosts = KnownHostsFile.fromPath(knownHostsPath)
else:
knownHosts = None
if config["no-agent"] or "SSH_AUTH_SOCK" not in os.environ:
agentEndpoint = None
else:
agentEndpoint = UNIXClientEndpoint(
reactor, os.environ["SSH_AUTH_SOCK"])
return cls(
reactor, config["host"], config["port"],
config["username"], config["password"], keys,
knownHosts, agentEndpoint)
def endpointForCommand(self, command):
return SSHCommandClientEndpoint.newConnection(
self.reactor, command, self.username, self.host,
port=self.port, keys=self.keys, password=self.password,
agentEndpoint=self.agent, knownHosts=self.knownHosts)
def main(reactor, *argv):
parameters = ConnectionParameters.fromCommandLine(reactor, argv)
endpoint = parameters.endpointForCommand(b"/bin/cat")
factory = Factory()
factory.protocol = NoiseProtocol
d = endpoint.connect(factory)
d.addCallback(lambda proto: proto.finished)
return d
For completeness, this example includes a lot of code to support different
styles of authentication, reading (and possibly updating) existing
known_hosts files, and parsing command line options. Focus on
the latter half of the main function to see the code that is
most directly responsible for actually doing the necessary SSH connection
setup. SSHCommandClientEndpoint accepts quite a few options, since
there is a lot of flexibility in SSH and many possible different server
configurations, but once the endpoint object itself is created, its use is
no more complicated than the use of any other endpoint: pass a factory to
its connect method and attach a callback to the
resulting Deferred to do something with the protocol
instance. If you use an endpoint that creates new connections, the connection
attempt can be cancelled by calling cancel() on this
Deferred .
In this case, the connected protocol instance is only used to make the
example wait until the client has finished talking to the server, which
happens after the small amount of example data has been sent to the server
and bounced back by the /bin/cat process the
protocol is interacting with.
Several of the options accepted by SSHCommandClientEndpoint.newConnection should be easy to understand.
The endpoint takes a reactor which it uses to do any and all I/O it needs to do.
It also takes a command which it executes on the remote server once the SSH connection is established and authenticated; this command is a single string, perhaps including spaces or other special shell symbols, and is interpreted by a shell on the server.
It takes a username with which it identifies itself to the server for authentication purposes.
It takes an optional password argument which will also be used for authentication - if the server supports password authentication (prefer keys instead where possible, see below).
It takes a host (either a name or an IP address) and a port number, defining where to connect.
Some of the other options may bear further explanation.
The keys argument gives any SSH Key objects which may be useful for authentication.
These keys are available to the endpoint for authentication, but only keys that the server indicates are useful will actually be used.
This argument is optional.
If key authentication against the server is either unnecessary or undesired, it may be omitted entirely.
The agentEndpoint argument gives the SSHCommandClientEndpoint an opportunity to connect to an SSH authentication agent.
The agent may already be loaded with keys, or may have some other way to authenticate a connection.
Using the agent can mean the process actually establishing the SSH connection doesn’t need to load any authentication material (passwords or keys) itself (often convenient in case keys are encrypted and potentially more secure, since only the agent process ever actually holds the secrets).
The value for this argument is another IStreamClientEndpoint .
Often in a typical NIX desktop environment, the *SSH_AUTH_SOCK environment variable will give the location of a AF_UNIX socket.
This explains the value echoclient_ssh.py assigns this parameter when –no-agent is not given.
The knownHosts argument accepts a KnownHostsFile instance and controls how server keys are checked and stored.
This object has the opportunity to reject server keys if they differ from expectations.
It can also save server keys when they are first observed.
Finally, there is one option that is not demonstrated in the example - the ui argument.
This argument is closely related to the knownHosts argument described above.
KnownHostsFile may require user-input under certain circumstances - for example, to ask if it should accept a server key the first time it is observed.
The ui object is how this user-input is obtained.
By default, a ConsoleUI instance associated with /dev/tty will be used.
This gives about the same behavior as is seen in a standard command-line ssh client.
See SSHCommandClientEndpoint.newConnection for details about how edge cases are handled for this default value.
For use of SSHCommandClientEndpoint that is intended to be completely autonomous, applications will probably want to specify a custom ui object which can make the necessary decisions without user-input.
It is also possible to run commands (one or more) over an
already-established connection. This is done using the alternate
constructor SSHCommandClientEndpoint.existingConnection .
#!/usr/bin/env python
# Copyright (c) Twisted Matrix Laboratories.
# See LICENSE for details.
if __name__ == '__main__':
import sys
import echoclient_shared_ssh
from twisted.internet.task import react
react(echoclient_shared_ssh.main, sys.argv[1:])
from twisted.internet.task import cooperate
from twisted.internet.defer import Deferred, gatherResults
from twisted.internet.protocol import Factory, Protocol
from twisted.conch.endpoints import SSHCommandClientEndpoint
from echoclient_ssh import ConnectionParameters
class PrinterProtocol(Protocol):
def dataReceived(self, data):
print "Got some data:", data,
def connectionLost(self, reason):
print "Lost my connection"
self.factory.done.callback(None)
def main(reactor, *argv):
parameters = ConnectionParameters.fromCommandLine(reactor, argv)
endpoint = parameters.endpointForCommand(b"/bin/cat")
done = []
factory = Factory()
factory.protocol = Protocol
d = endpoint.connect(factory)
def gotConnection(proto):
conn = proto.transport.conn
for i in range(50):
factory = Factory()
factory.protocol = PrinterProtocol
factory.done = Deferred()
done.append(factory.done)
e = SSHCommandClientEndpoint.existingConnection(
conn, b"/bin/echo %d" % (i,))
yield e.connect(factory)
d.addCallback(gotConnection)
d.addCallback(lambda work: cooperate(work).whenDone())
d.addCallback(lambda ignored: gatherResults(done))
return d
Writing a client¶
In case the endpoint is missing some necessary functionality, or in case you want to interact with a different part of an SSH server - such as one of its subsystems (for example, SFTP), you may need to use the lower-level Conch client interface. This is described below.
Writing a client with Conch involves sub-classing 4 classes: twisted.conch.ssh.transport.SSHClientTransport , twisted.conch.ssh.userauth.SSHUserAuthClient , twisted.conch.ssh.connection.SSHConnection , and twisted.conch.ssh.channel.SSHChannel . We’ll start out
with SSHClientTransport because it’s the base
of the client.
The Transport¶
from twisted.conch import error
from twisted.conch.ssh import transport
from twisted.internet import defer
class ClientTransport(transport.SSHClientTransport):
def verifyHostKey(self, pubKey, fingerprint):
if fingerprint != 'b1:94:6a:c9:24:92:d2:34:7c:62:35:b4:d2:61:11:84':
return defer.fail(error.ConchError('bad key'))
else:
return defer.succeed(1)
def connectionSecure(self):
self.requestService(ClientUserAuth('user', ClientConnection()))
See how easy it is? SSHClientTransport
handles the negotiation of encryption and the verification of keys
for you. The one security element that you as a client writer need to
implement is verifyHostKey() . This method
is called with two strings: the public key sent by the server and its
fingerprint. You should verify the host key the server sends, either
by checking against a hard-coded value as in the example, or by asking
the user. verifyHostKey returns a twisted.internet.defer.Deferred which gets a callback
if the host key is valid, or an errback if it is not. Note that in the
above, replace ‘user’ with the username you’re attempting to ssh with,
for instance a call to os.getlogin() for the
current user.
The second method you need to implement is connectionSecure() . It is called when the
encryption is set up and other services can be run. The example requests
that the ClientUserAuth service be started.
This service will be discussed next.
The Authorization Client¶
from twisted.conch.ssh import keys, userauth
# these are the public/private keys from test_conch
publicKey = 'ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAGEArzJx8OYOnJmzf4tfBEvLi8DVPrJ3\
/c9k2I/Az64fxjHf9imyRJbixtQhlH9lfNjUIx+4LmrJH5QNRsFporcHDKOTwTTYLh5KmRpslkYHR\
ivcJSkbh/C+BR3utDS555mV'
privateKey = """-----BEGIN RSA PRIVATE KEY-----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-----END RSA PRIVATE KEY-----"""
class ClientUserAuth(userauth.SSHUserAuthClient):
def getPassword(self, prompt = None):
return
# this says we won't do password authentication
def getPublicKey(self):
return keys.Key.fromString(data = publicKey).blob()
def getPrivateKey(self):
return defer.succeed(keys.Key.fromString(data = privateKey).keyObject)
Again, fairly simple. The SSHUserAuthClient takes care of most
of the work, but the actual authentication data needs to be
supplied. getPassword() asks for a
password, getPublicKey() and getPrivateKey() get public and private keys,
respectively. getPassword() returns
a Deferred that is called back with
the password to use.
getPublicKey() returns the SSH key data for the public key to use.
twisted.conch.ssh.keys.Key.fromString() will take a key in OpenSSH, LSH or any supported format, as a string, and generate a new twisted.conch.ssh.keys.Key.
Alternatively, keys.Key.fromFile() can be used instead, which
will take the filename of a key in the supported format, and and generate a new twisted.conch.ssh.keys.Key.
getPrivateKey() returns a Deferred which is called back with the private twisted.conch.ssh.keys.Key.
getPassword() and getPrivateKey() return Deferreds because they may need to ask the user for input.
Once the authentication is complete, SSHUserAuthClient takes care of starting the code SSHConnection object given to it. Next, we’ll
look at how to use the SSHConnection
The Connection¶
from twisted.conch.ssh import connection
class ClientConnection(connection.SSHConnection):
def serviceStarted(self):
self.openChannel(CatChannel(conn = self))
SSHConnection is the easiest,
as it’s only responsible for starting the channels. It has
other methods, those will be examined when we look at SSHChannel .
The Channel¶
from twisted.conch.ssh import channel, common
class CatChannel(channel.SSHChannel):
name = 'session'
def channelOpen(self, data):
d = self.conn.sendRequest(self, 'exec', common.NS('cat'),
wantReply = 1)
d.addCallback(self._cbSendRequest)
self.catData = ''
def _cbSendRequest(self, ignored):
self.write('This data will be echoed back to us by "cat."\r\n')
self.conn.sendEOF(self)
self.loseConnection()
def dataReceived(self, data):
self.catData += data
def closed(self):
print 'We got this from "cat":', self.catData
Now that we’ve spent all this time getting the server and
client connected, here is where that work pays off. SSHChannel is the interface between you and the
other side. This particular channel opens a session and plays with the
‘cat’ program, but your channel can implement anything, so long as the
server supports it.
The channelOpen() method is
where everything gets started. It gets passed a chunk of data;
however, this chunk is usually nothing and can be ignored.
Our channelOpen() initializes our
channel, and sends a request to the other side, using the sendRequest() method of the SSHConnection object. Requests are used to send
events to the other side. We pass the method self so that it knows to
send the request for this channel. The 2nd argument of ‘exec’ tells the
server that we want to execute a command. The third argument is the data
that accompanies the request. common.NS encodes
the data as a length-prefixed string, which is how the server expects
the data. We also say that we want a reply saying that the process has a
been started. sendRequest() then returns a Deferred which we add a callback for.
Once the callback fires, we send the data. SSHChannel supports the twisted.internet.interfaces.ITransport
interface, so
it can be given to Protocols to run them over the secure
connection. In our case, we just write the data directly. sendEOF() does not follow the interface,
but Conch uses it to tell the other side that we will write no
more data. loseConnection() shuts
down our side of the connection, but we will still receive data
through dataReceived() . The closed() method is called when both sides of the
connection are closed, and we use it to display the data we received
(which should be the same as the data we sent.)
Finally, let’s actually invoke the code we’ve set up.
The main() function¶
from twisted.internet import protocol, reactor
def main():
factory = protocol.ClientFactory()
factory.protocol = ClientTransport
reactor.connectTCP('localhost', 22, factory)
reactor.run()
if __name__ == "__main__":
main()
We call connectTCP() to connect to
localhost, port 22 (the standard port for ssh), and pass it an instance
of twisted.internet.protocol.ClientFactory .
This instance has the attribute protocol
set to our earlier ClientTransport
class. Note that the protocol attribute is set to the class ClientTransport , not an instance of ClientTransport ! When the connectTCP call completes, the protocol will be
called to create a ClientTransport() object
- this then invokes all our previous work.
It’s worth noting that in the example main()
routine, the reactor.run() call never returns.
If you want to make the program exit, call reactor.stop() in the earlier closed() method.
If you wish to observe the interactions in more detail, adding a call
to log.startLogging(sys.stdout, setStdout=0)
before the reactor.run() call will send all
logging to stdout.
- Tutorial
Examples¶
Simple SSH server and client¶
sshsimpleclient.py- simple SSH clientsshsimpleserver.py- simple SSH server
Simple telnet server¶
telnet_echo.tac- A telnet server which echoes data and events back to the client
twisted.conch.insults examples¶
demo.tac- Nearly pointless demonstration of the manhole interactive interpreterdemo_draw.tac- A trivial drawing applicationdemo_insults.tac- Various simple terminal manipulations using the insults moduledemo_recvline.tac- Demonstrates line-at-a-time handling with basic line-editing supportdemo_scroll.tac- Simple echo-ish server that uses the scroll-regiondemo_manhole.tac- An interactive Python interpreter with syntax coloringwindow.tac- An example of various widgets
- Developer guides: documentation on using Twisted Conch to develop your own applications
- Examples: short code examples using Twisted Conch
Twisted Mail¶
Examples¶
SMTP servers¶
emailserver.tac- a toy email server.
SMTP clients¶
sendmail_smtp.py- sending email over plain SMTP with the high-level sendmail client.sendmail_gmail.py- sending email encrypted ESMTP to GMail with the high-level sendmail client.sendmail_message.py- sending a complex message with the high-level sendmail client.smtpclient_simple.py- sending email using SMTP.smtpclient_tls.py- send email using authentication and transport layer security.
IMAP clients¶
imap4client.py- Simple IMAP4 client which displays the subjects of all messages in a particular mailbox.
Developer Guides¶
Sending Mail¶
Twisted contains many ways of sending email, but the simplest is sendmail.
Intended as a near drop-in replacement of smtplib.SMTP‘s sendmail method, it provides the ability to send email over SMTP/ESMTP with minimal fuss or configuration.
Knowledge of Twisted’s Deferreds is required for making full use of this document.
Sending an Email over SMTP¶
Although insecure, some email systems still use plain SMTP for sending email. Plain SMTP has no authentication, no transport security (emails are transmitted in plain text), and should not be done over untrusted networks.
sendmail‘s positional arguments are, in order:
- The SMTP/ESMTP server you are sending the message to
- The email address you are sending from
- A
listof email addresses you are sending to - The message.
The following example shows these in action.
from __future__ import print_function
from twisted.mail.smtp import sendmail
from twisted.internet.task import react
def main(reactor):
d = sendmail("myinsecuremailserver.example.com",
"alice@example.com",
["bob@gmail.com", "charlie@gmail.com"],
"This is my super awesome email, sent with Twisted!")
d.addBoth(print)
return d
react(main)
Assuming that the values in it were replaced with real emails and a real SMTP server, it would send an email to the two addresses specified and print the return status.
Sending an Email over ESMTP¶
Extended SMTP (ESMTP) is an improved version of SMTP, and is used by most modern mail servers. Unlike SMTP, ESMTP supports authentication and transport security (emails are encrypted in transit). If you wish to send mail through services like GMail/Google Apps or Outlook.com/Office 365, you will have to use ESMTP.
Using ESMTP requires more options – usually the default port of 25 is not open, so you must find out your email provider’s TLS-enabled ESMTP port. It also allows the use of authentication via a username and password.
The following example shows part of the ESMTP functionality of sendmail.
from __future__ import print_function
from twisted.mail.smtp import sendmail
from twisted.internet.task import react
def main(reactor):
d = sendmail("smtp.gmail.com",
"alice@gmail.com",
["bob@gmail.com", "charlie@gmail.com"],
"This is my super awesome email, sent with Twisted!",
port=587, username="alice@gmail.com", password="*********")
d.addBoth(print)
return d
react(main)
Assuming you own the account alice@gmail.com, this would send an email to both bob@gmail.com and charlie@gmail.com, and print out something like the following (formatted for clarity):
(2, [('bob@gmail.com', 250, '2.1.5 OK hz13sm11691456pac.6 - gsmtp'),
('charlie@gmail.com', 250, '2.1.5 OK hz13sm11691456pac.6 - gsmtp')])
sendmail returns a 2-tuple, containing the number of emails sent successfully (note that this is from you to the server you specified, not to the recepient – emails may still be lost between that server and the recepient) and a list of statuses of the sent mail.
Each status is a 3-tuple containing the address it was sent to, the SMTP status code, and the server response.
Sending Complex Emails¶
Sometimes you want to send more complicated emails – ones with headers, or with attachments.
sendmail supports using Python’s email.Message, which lets you make complex emails:
from __future__ import print_function
from twisted.mail.smtp import sendmail
from twisted.internet.task import react
from email.mime.text import MIMEText
def main(reactor):
me = "alice@gmail.com"
to = ["bob@gmail.com", "charlie@gmail.com"]
message = MIMEText("This is my super awesome email, sent with Twisted!")
message["Subject"] = "Twisted is great!"
message["From"] = me
message["To"] = ", ".join(to)
d = sendmail("smtp.gmail.com", me, to, message,
port=587, username=me, password="*********",
requireAuthentication=True,
requireTransportSecurity=True)
d.addBoth(print)
return d
react(main)
For more information on how to use Message, please see the module’s Python docs.
Enforcing Transport Security¶
To prevent downgrade attacks, you can pass requireTransportSecurity=True to sendmail.
This means that your emails will not be transmitted in plain text.
For example:
sendmail("smtp.gmail.com", me, to, message,
port=587, username=me, password="*********",
requireTransportSecurity=True)
Conclusion¶
In this document, you have seen how to:
- Sending Mail: Sending mail with Twisted
Twisted Mail Tutorial: Building an SMTP Client from Scratch¶
Introduction¶
This tutorial will walk you through the creation of an extremely simple SMTP client application. By the time the tutorial is complete, you will understand how to create and start a TCP client speaking the SMTP protocol, have it connect to an appropriate mail exchange server, and transmit a message for delivery.
For the majority of this tutorial, twistd will be used
to launch the application. Near the end we will explore other
possibilities for starting a Twisted application. Until then, make
sure that you have twistd installed and conveniently
accessible for use in running each of the example .tac
files.
SMTP Client 1¶
The first step is to create smtpclient-1.tac possible for use by twistd .
from twisted.application import service
The first line of the .tac file
imports twisted.application.service , a module which
contains many of the basic service classes and helper
functions available in Twisted. In particular, we will be using
the Application function to create a new application service . An application service simply acts as a
central object on which to store certain kinds of deployment
configuration.
application = service.Application("SMTP Client Tutorial")
The second line of the .tac file creates a
new application service and binds it to the local
name application . twistd requires this
local name in each .tac file it runs. It uses various
pieces of configuration on the object to determine its behavior. For
example, "SMTP Client Tutorial" will be used as the name
of the .tap file into which to serialize application
state, should it be necessary to do so.
That does it for the first example. We now have enough of
a .tac file to pass to twistd . If we
run smtpclient-1.tac using
the twistd command line:
twistd -ny smtpclient-1.tac
we are rewarded with the following output:
exarkun@boson:~/mail/tutorial/smtpclient$ twistd -ny smtpclient-1.tac
18:31 EST [-] Log opened.
18:31 EST [-] twistd 2.0.0 (/usr/bin/python2.4 2.4.1) starting up
18:31 EST [-] reactor class: twisted.internet.selectreactor.SelectReactor
18:31 EST [-] Loading smtpclient-1.tac...
18:31 EST [-] Loaded.
As we expected, not much is going on. We can shutdown this server
by issuing ^C :
18:34 EST [-] Received SIGINT, shutting down.
18:34 EST [-] Main loop terminated.
18:34 EST [-] Server Shut Down.
exarkun@boson:~/mail/tutorial/smtpclient$
SMTP Client 2¶
The first version of our SMTP client wasn’t very interesting. It
didn’t even establish any TCP connections! The smtpclient-2.tac will come a little bit
closer to that level of complexity. First, we need to import a few
more things:
from twisted.application import internet
from twisted.internet import protocol
twisted.application.internet is
another application service module. It provides services for
establishing outgoing connections (as well as creating network
servers, though we are not interested in those parts for the
moment). twisted.internet.protocol provides base
implementations of many of the core Twisted concepts, such
as factories and protocols .
The next line of smtpclient-2.tac
instantiates a new client factory .
smtpClientFactory = protocol.ClientFactory()
Client factories are responsible for constructing protocol instances whenever connections are established. They may be required to create just one instance, or many instances if many different connections are established, or they may never be required to create one at all, if no connection ever manages to be established.
Now that we have a client factory, we’ll need to hook it up to the
network somehow. The next line of smtpclient-2.tac does
just that:
smtpClientService = internet.TCPClient(None, None, smtpClientFactory)
We’ll ignore the first two arguments
to internet.TCPClient for the moment and instead focus on
the third. TCPClient is one of those application service classes. It creates TCP connections to a specified
address and then uses its third argument, a client factory ,
to get a protocol instance . It then associates the TCP
connection with the protocol instance and gets out of the way.
We can try to run smtpclient-2.tac the same way we
ran smtpclient-1.tac , but the results might be a little
disappointing:
exarkun@boson:~/mail/tutorial/smtpclient$ twistd -ny smtpclient-2.tac
18:55 EST [-] Log opened.
18:55 EST [-] twistd SVN-Trunk (/usr/bin/python2.4 2.4.1) starting up
18:55 EST [-] reactor class: twisted.internet.selectreactor.SelectReactor
18:55 EST [-] Loading smtpclient-2.tac...
18:55 EST [-] Loaded.
18:55 EST [-] Starting factory <twisted.internet.protocol.ClientFactory
instance at 0xb791e46c>
18:55 EST [-] Traceback (most recent call last):
File "twisted/scripts/twistd.py", line 187, in runApp
app.runReactorWithLogging(config, oldstdout, oldstderr)
File "twisted/application/app.py", line 128, in runReactorWithLogging
reactor.run()
File "twisted/internet/posixbase.py", line 200, in run
self.mainLoop()
File "twisted/internet/posixbase.py", line 208, in mainLoop
self.runUntilCurrent()
--- <exception caught here> ---
File "twisted/internet/base.py", line 533, in runUntilCurrent
call.func(*call.args, **call.kw)
File "twisted/internet/tcp.py", line 489, in resolveAddress
if abstract.isIPAddress(self.addr[0]):
File "twisted/internet/abstract.py", line 315, in isIPAddress
parts = string.split(addr, '.')
File "/usr/lib/python2.4/string.py", line 292, in split
return s.split(sep, maxsplit)
exceptions.AttributeError: 'NoneType' object has no attribute 'split'
18:55 EST [-] Received SIGINT, shutting down.
18:55 EST [-] Main loop terminated.
18:55 EST [-] Server Shut Down.
exarkun@boson:~/mail/tutorial/smtpclient$
What happened? Those first two arguments to TCPClient
turned out to be important after all. We’ll get to them in the next
example.
SMTP Client 3¶
Version three of our SMTP client only changes one thing. The line from version two:
smtpClientService = internet.TCPClient(None, None, smtpClientFactory)
has its first two arguments changed from None to
something with a bit more meaning:
smtpClientService = internet.TCPClient('localhost', 25, smtpClientFactory)
This directs the client to connect to localhost on
port 25 . This isn’t the address we want ultimately, but it’s
a good place-holder for the time being. We can
run smtpclient-3.tac and see what this
change gets us:
exarkun@boson:~/mail/tutorial/smtpclient$ twistd -ny smtpclient-3.tac
19:10 EST [-] Log opened.
19:10 EST [-] twistd SVN-Trunk (/usr/bin/python2.4 2.4.1) starting up
19:10 EST [-] reactor class: twisted.internet.selectreactor.SelectReactor
19:10 EST [-] Loading smtpclient-3.tac...
19:10 EST [-] Loaded.
19:10 EST [-] Starting factory <twisted.internet.protocol.ClientFactory
instance at 0xb791e48c>
19:10 EST [-] Enabling Multithreading.
19:10 EST [Uninitialized] Traceback (most recent call last):
File "twisted/python/log.py", line 56, in callWithLogger
return callWithContext({"system": lp}, func, *args, **kw)
File "twisted/python/log.py", line 41, in callWithContext
return context.call({ILogContext: newCtx}, func, *args, **kw)
File "twisted/python/context.py", line 52, in callWithContext
return self.currentContext().callWithContext(ctx, func, *args, **kw)
File "twisted/python/context.py", line 31, in callWithContext
return func(*args,**kw)
--- <exception caught here> ---
File "twisted/internet/selectreactor.py", line 139, in _doReadOrWrite
why = getattr(selectable, method)()
File "twisted/internet/tcp.py", line 543, in doConnect
self._connectDone()
File "twisted/internet/tcp.py", line 546, in _connectDone
self.protocol = self.connector.buildProtocol(self.getPeer())
File "twisted/internet/base.py", line 641, in buildProtocol
return self.factory.buildProtocol(addr)
File "twisted/internet/protocol.py", line 99, in buildProtocol
p = self.protocol()
exceptions.TypeError: 'NoneType' object is not callable
19:10 EST [Uninitialized] Stopping factory
<twisted.internet.protocol.ClientFactory instance at
0xb791e48c>
19:10 EST [-] Received SIGINT, shutting down.
19:10 EST [-] Main loop terminated.
19:10 EST [-] Server Shut Down.
exarkun@boson:~/mail/tutorial/smtpclient$
A meagre amount of progress, but the service still raises an exception. This time, it’s because we haven’t specified a protocol class for the factory to use. We’ll do that in the next example.
SMTP Client 4¶
In the previous example, we ran into a problem because we hadn’t
set up our client factory’s protocol attribute
correctly (or at all). ClientFactory.buildProtocol is
the method responsible for creating a protocol instance . The
default implementation calls the factory’s protocol attribute,
adds itself as an attribute named factory to the
resulting instance, and returns it. In smtpclient-4.tac , we’ll correct the
oversight that caused the traceback in smtpclient-3.tac:
smtpClientFactory.protocol = protocol.Protocol
Running this version of the client, we can see the output is once again traceback free:
exarkun@boson:~/doc/mail/tutorial/smtpclient$ twistd -ny smtpclient-4.tac
19:29 EST [-] Log opened.
19:29 EST [-] twistd SVN-Trunk (/usr/bin/python2.4 2.4.1) starting up
19:29 EST [-] reactor class: twisted.internet.selectreactor.SelectReactor
19:29 EST [-] Loading smtpclient-4.tac...
19:29 EST [-] Loaded.
19:29 EST [-] Starting factory <twisted.internet.protocol.ClientFactory
instance at 0xb791e4ac>
19:29 EST [-] Enabling Multithreading.
19:29 EST [-] Received SIGINT, shutting down.
19:29 EST [Protocol,client] Stopping factory
<twisted.internet.protocol.ClientFactory instance at
0xb791e4ac>
19:29 EST [-] Main loop terminated.
19:29 EST [-] Server Shut Down.
exarkun@boson:~/doc/mail/tutorial/smtpclient$
But what does this
mean? twisted.internet.protocol.Protocol is the
base protocol implementation. For those familiar with the
classic UNIX network services, it is equivalent to
the discard service. It never produces any output and it
discards all its input. Not terribly useful, and certainly nothing
like an SMTP client. Let’s see how we can improve this in the next
example.
SMTP Client 5¶
In smtpclient-5.tac , we will begin
to use Twisted’s SMTP protocol implementation for the first time.
We’ll make the obvious change, simply swapping
out twisted.internet.protocol.Protocol in favor
of twisted.mail.smtp.ESMTPClient . Don’t worry about
the E in ESMTP . It indicates we’re actually using a
newer version of the SMTP protocol. There is
an SMTPClient in Twisted, but there’s essentially no
reason to ever use it.
smtpclient-5.tac adds a new import:
from twisted.mail import smtp
All of the mail related code in Twisted exists beneath
the twisted.mail package. More specifically, everything
having to do with the SMTP protocol implementation is defined in
the twisted.mail.smtp module.
Next we remove a line we added in smtpclient-4.tac:
smtpClientFactory.protocol = protocol.Protocol
And add a similar one in its place:
smtpClientFactory.protocol = smtp.ESMTPClient
Our client factory is now using a protocol implementation which behaves as an SMTP client. What happens when we try to run this version?
exarkun@boson:~/doc/mail/tutorial/smtpclient$ twistd -ny smtpclient-5.tac
19:42 EST [-] Log opened.
19:42 EST [-] twistd SVN-Trunk (/usr/bin/python2.4 2.4.1) starting up
19:42 EST [-] reactor class: twisted.internet.selectreactor.SelectReactor
19:42 EST [-] Loading smtpclient-5.tac...
19:42 EST [-] Loaded.
19:42 EST [-] Starting factory <twisted.internet.protocol.ClientFactory
instance at 0xb791e54c>
19:42 EST [-] Enabling Multithreading.
19:42 EST [Uninitialized] Traceback (most recent call last):
File "twisted/python/log.py", line 56, in callWithLogger
return callWithContext({"system": lp}, func, *args, **kw)
File "twisted/python/log.py", line 41, in callWithContext
return context.call({ILogContext: newCtx}, func, *args, **kw)
File "twisted/python/context.py", line 52, in callWithContext
return self.currentContext().callWithContext(ctx, func, *args, **kw)
File "twisted/python/context.py", line 31, in callWithContext
return func(*args,**kw)
--- <exception caught here> ---
File "twisted/internet/selectreactor.py", line 139, in _doReadOrWrite
why = getattr(selectable, method)()
File "twisted/internet/tcp.py", line 543, in doConnect
self._connectDone()
File "twisted/internet/tcp.py", line 546, in _connectDone
self.protocol = self.connector.buildProtocol(self.getPeer())
File "twisted/internet/base.py", line 641, in buildProtocol
return self.factory.buildProtocol(addr)
File "twisted/internet/protocol.py", line 99, in buildProtocol
p = self.protocol()
exceptions.TypeError: __init__() takes at least 2 arguments (1 given)
19:42 EST [Uninitialized] Stopping factory
<twisted.internet.protocol.ClientFactory instance at
0xb791e54c>
19:43 EST [-] Received SIGINT, shutting down.
19:43 EST [-] Main loop terminated.
19:43 EST [-] Server Shut Down.
exarkun@boson:~/doc/mail/tutorial/smtpclient$
Oops, back to getting a traceback. This time, the default
implementation of buildProtocol seems no longer to be
sufficient. It instantiates the protocol with no arguments,
but ESMTPClient wants at least one argument. In the next
version of the client, we’ll override buildProtocol to
fix this problem.
SMTP Client 6¶
smtpclient-6.tac introduces
a twisted.internet.protocol.ClientFactory subclass with
an overridden buildProtocol method to overcome the
problem encountered in the previous example.
class SMTPClientFactory(protocol.ClientFactory):
protocol = smtp.ESMTPClient
def buildProtocol(self, addr):
return self.protocol(secret=None, identity='example.com')
The overridden method does almost the same thing as the base
implementation: the only change is that it passes values for two
arguments to twisted.mail.smtp.ESMTPClient ‘s initializer.
The secret argument is used for SMTP authentication
(which we will not attempt yet). The identity argument
is used as a to identify ourselves Another minor change to note is
that the protocol attribute is now defined in the class
definition, rather than tacked onto an instance after one is created.
This means it is a class attribute, rather than an instance attribute,
now, which makes no difference as far as this example is concerned.
There are circumstances in which the difference is important: be sure
you understand the implications of each approach when creating your
own factories.
One other change is required: instead of
instantiating twisted.internet.protocol.ClientFactory , we
will now instantiate SMTPClientFactory :
smtpClientFactory = SMTPClientFactory()
Running this version of the code, we observe that the code still isn’t quite traceback-free.
exarkun@boson:~/doc/mail/tutorial/smtpclient$ twistd -ny smtpclient-6.tac
21:17 EST [-] Log opened.
21:17 EST [-] twistd SVN-Trunk (/usr/bin/python2.4 2.4.1) starting up
21:17 EST [-] reactor class: twisted.internet.selectreactor.SelectReactor
21:17 EST [-] Loading smtpclient-6.tac...
21:17 EST [-] Loaded.
21:17 EST [-] Starting factory <__builtin__.SMTPClientFactory instance
at 0xb77fd68c>
21:17 EST [-] Enabling Multithreading.
21:17 EST [ESMTPClient,client] Traceback (most recent call last):
File "twisted/python/log.py", line 56, in callWithLogger
return callWithContext({"system": lp}, func, *args, **kw)
File "twisted/python/log.py", line 41, in callWithContext
return context.call({ILogContext: newCtx}, func, *args, **kw)
File "twisted/python/context.py", line 52, in callWithContext
return self.currentContext().callWithContext(ctx, func, *args, **kw)
File "twisted/python/context.py", line 31, in callWithContext
return func(*args,**kw)
--- <exception caught here> ---
File "twisted/internet/selectreactor.py", line 139, in _doReadOrWrite
why = getattr(selectable, method)()
File "twisted/internet/tcp.py", line 351, in doRead
return self.protocol.dataReceived(data)
File "twisted/protocols/basic.py", line 221, in dataReceived
why = self.lineReceived(line)
File "twisted/mail/smtp.py", line 1039, in lineReceived
why = self._okresponse(self.code,'\n'.join(self.resp))
File "twisted/mail/smtp.py", line 1281, in esmtpState_serverConfig
self.tryTLS(code, resp, items)
File "twisted/mail/smtp.py", line 1294, in tryTLS
self.authenticate(code, resp, items)
File "twisted/mail/smtp.py", line 1343, in authenticate
self.smtpState_from(code, resp)
File "twisted/mail/smtp.py", line 1062, in smtpState_from
self._from = self.getMailFrom()
File "twisted/mail/smtp.py", line 1137, in getMailFrom
raise NotImplementedError
exceptions.NotImplementedError:
21:17 EST [ESMTPClient,client] Stopping factory
<__builtin__.SMTPClientFactory instance at 0xb77fd68c>
21:17 EST [-] Received SIGINT, shutting down.
21:17 EST [-] Main loop terminated.
21:17 EST [-] Server Shut Down.
exarkun@boson:~/doc/mail/tutorial/smtpclient$
What we have accomplished with this iteration of the example is to navigate far enough into an SMTP transaction that Twisted is now interested in calling back to application-level code to determine what its next step should be. In the next example, we’ll see how to provide that information to it.
SMTP Client 7¶
SMTP Client 7 is the first version of our SMTP client which
actually includes message data to transmit. For simplicity’s sake,
the message is defined as part of a new class. In a useful program
which sent email, message data might be pulled in from the filesystem,
a database, or be generated based on
user-input. smtpclient-7.tac , however,
defines a new class, SMTPTutorialClient , with three class
attributes (mailFrom , mailTo ,
and mailData ):
class SMTPTutorialClient(smtp.ESMTPClient):
mailFrom = "tutorial_sender@example.com"
mailTo = "tutorial_recipient@example.net"
mailData = '''\
Date: Fri, 6 Feb 2004 10:14:39 -0800
From: Tutorial Guy <tutorial_sender@example.com>
To: Tutorial Gal <tutorial_recipient@example.net>
Subject: Tutorate!
Hello, how are you, goodbye.
'''
This statically defined data is accessed later in the class
definition by three of the methods which are part of the
SMTPClient callback API . Twisted expects each of the three
methods below to be defined and to return an object with a particular
meaning. First, getMailFrom :
def getMailFrom(self):
result = self.mailFrom
self.mailFrom = None
return result
This method is called to determine the reverse-path ,
otherwise known as the envelope from , of the message. This
value will be used when sending the MAIL FROM SMTP
command. The method must return a string which conforms to the RFC 2821 definition
of a reverse-path . In simpler terms, it should be a string
like "alice@example.com" . Only one envelope from is allowed by the SMTP protocol, so it cannot be a list of
strings or a comma separated list of addresses. Our implementation
of getMailFrom does a little bit more than just return a
string; we’ll get back to this in a little bit.
The next method is getMailTo :
def getMailTo(self):
return [self.mailTo]
getMailTo is similar to getMailFrom . It
returns one or more RFC 2821 addresses (this time a
forward-path , or envelope to ). Since SMTP allows
multiple recipients, getMailTo returns a list of these
addresses. The list must contain at least one address, and even if
there is exactly one recipient, it must still be in a list.
The final callback we will define to provide information to
Twisted is getMailData :
def getMailData(self):
return StringIO.StringIO(self.mailData)
This one is quite simple as well: it returns a file or a file-like
object which contains the message contents. In our case, we return
a StringIO since we already have a string containing our
message. If the contents of the file returned
by getMailData span multiple lines (as email messages
often do), the lines should be \n delimited (as they
would be when opening a text file in the "rt" mode):
necessary newline translation will be performed
by SMTPClient automatically.
There is one more new callback method defined in smtpclient-7.tac. This one isn’t for providing information about the messages to Twisted, but for Twisted to provide information about the success or failure of the message transmission to the application:
def sentMail(self, code, resp, numOk, addresses, log):
print 'Sent', numOk, 'messages'
Each of the arguments to sentMail provides some
information about the success or failure of the message transmission
transaction. code is the response code from the ultimate
command. For successful transactions, it will be 250. For transient
failures (those which should be retried), it will be between 400 and
499, inclusive. For permanent failures (this which will never work,
no matter how many times you retry them), it will be between 500 and
599.
SMTP Client 8¶
Thus far we have succeeded in creating a Twisted client application
which starts up, connects to a (possibly) remote host, transmits some
data, and disconnects. Notably missing, however, is application
shutdown. Hitting ^C is fine during development, but it’s not exactly
a long-term solution. Fortunately, programmatic shutdown is extremely
simple. smtpclient-8.tac
extends sentMail with these two lines:
from twisted.internet import reactor
reactor.stop()
The stop method of the reactor causes the main event
loop to exit, allowing a Twisted server to shut down. With this
version of the example, we see that the program actually terminates
after sending the message, without user-intervention:
exarkun@boson:~/doc/mail/tutorial/smtpclient$ twistd -ny smtpclient-8.tac
19:52 EST [-] Log opened.
19:52 EST [-] twistd SVN-Trunk (/usr/bin/python2.4 2.4.1) starting up
19:52 EST [-] reactor class: twisted.internet.selectreactor.SelectReactor
19:52 EST [-] Loading smtpclient-8.tac...
19:52 EST [-] Loaded.
19:52 EST [-] Starting factory <__builtin__.SMTPClientFactory instance
at 0xb791beec>
19:52 EST [-] Enabling Multithreading.
19:52 EST [SMTPTutorialClient,client] Sent 1 messages
19:52 EST [SMTPTutorialClient,client] Stopping factory
<__builtin__.SMTPClientFactory instance at 0xb791beec>
19:52 EST [-] Main loop terminated.
19:52 EST [-] Server Shut Down.
exarkun@boson:~/doc/mail/tutorial/smtpclient$
SMTP Client 9¶
One task remains to be completed in this tutorial SMTP client: instead of always sending mail through a well-known host, we will look up the mail exchange server for the recipient address and try to deliver the message to that host.
In smtpclient-9.tac , we’ll take the
first step towards this feature by defining a function which returns
the mail exchange host for a particular domain:
def getMailExchange(host):
return 'localhost'
Obviously this doesn’t return the correct mail exchange host yet
(in fact, it returns the exact same host we have been using all
along), but pulling out the logic for determining which host to
connect to into a function like this is the first step towards our
ultimate goal. Now that we have getMailExchange , we’ll
call it when constructing our TCPClient service:
smtpClientService = internet.TCPClient(
getMailExchange('example.net'), 25, smtpClientFactory)
We’ll expand on the definition of getMailExchange in
the next example.
SMTP Client 10¶
In the previous example we defined getMailExchange to
return a string representing the mail exchange host for a particular
domain. While this was a step in the right direction, it turns out
not to be a very big one. Determining the mail exchange host for a
particular domain is going to involve network traffic (specifically,
some DNS requests). These might take an arbitrarily large amount of
time, so we need to introduce a Deferred to represent the
result of getMailExchange . smtpclient-10.tac redefines it
thusly:
def getMailExchange(host):
return defer.succeed('localhost')
defer.succeed is a function which creates a
new Deferred which already has a result, in this
case 'localhost' . Now we need to adjust
our TCPClient -constructing code to expect and properly
handle this Deferred :
def cbMailExchange(exchange):
smtpClientFactory = SMTPClientFactory()
smtpClientService = internet.TCPClient(exchange, 25, smtpClientFactory)
smtpClientService.setServiceParent(application)
getMailExchange('example.net').addCallback(cbMailExchange)
An in-depth exploration of Deferred s is beyond the
scope of this document. For such a look, see
the Deferred Reference TCPClient until the Deferred
returned by getMailExchange fires. Once it does, we
proceed normally through the creation of
our SMTPClientFactory and TCPClient , as well
as set the TCPClient ‘s service parent, just as we did in
the previous examples.
SMTP Client 11¶
At last we’re ready to perform the mail exchange lookup. We do
this by calling on an object provided specifically for this
task, twisted.mail.relaymanager.MXCalculator :
def getMailExchange(host):
def cbMX(mxRecord):
return str(mxRecord.name)
return relaymanager.MXCalculator().getMX(host).addCallback(cbMX)
Because getMX returns a Record_MX object
rather than a string, we do a little bit of post-processing to get the
results we want. We have already converted the rest of the tutorial
application to expect a Deferred
from getMailExchange , so no further changes are
required. smtpclient-11.tac completes
this tutorial by being able to both look up the mail exchange host for
the recipient domain, connect to it, complete an SMTP transaction,
report its results, and finally shut down the reactor.
- Examples: short code examples using Twisted Mail
- Developer Guides: documentation on using Twisted Mail
- Twisted Mail Tutorial: Building an SMTP Client from Scratch
Twisted Names¶
Developer Guides¶
A Guided Tour of twisted.names.client¶
Twisted Names provides a layered selection of client APIs.
In this section you will learn:
- about the high level client API,
- about how you can use the client API interactively from the Python shell (useful for DNS debugging and diagnostics),
- about the IResolverSimple and the IResolver interfaces,
- about various implementations of those interfaces and when to use them,
- how to customise how the reactor carries out hostname resolution,
- and finally, you will also be introduced to some of the low level APIs.
Using the Global Resolver¶
The easiest way to issue DNS queries from Twisted is to use the module level functions in names.client.
Here’s an example showing some DNS queries generated in an interactive twisted.conch shell.
Note
The twisted.conch shell starts a reactor so that asynchronous operations can be run interactively and it prints the current result of deferreds which have fired.
You’ll notice that the deferreds returned in the following examples do not immediately have a result – they are waiting for a response from the DNS server.
So we type _ (the default variable) a little later, to display the value of the deferred after an answer has been received and the deferred has fired.
$ python -m twisted.conch.stdio
>>> from twisted.names import client
>>> client.getHostByName('www.example.com')
<Deferred at 0xf5c5a8 waiting on Deferred at 0xf5cb90>
>>> _
<Deferred at 0xf5c5a8 current result: '2606:2800:220:6d:26bf:1447:1097:aa7'>
>>> client.lookupMailExchange('twistedmatrix.com')
<Deferred at 0xf5cd40 waiting on Deferred at 0xf5cea8>
>>> _
<Deferred at 0xf5cd40 current result: ([<RR name=twistedmatrix.com type=MX class=IN ttl=1s auth=False>], [], [])>
All the IResolverSimple and IResolver methods are asynchronous and therefore return deferreds.
getHostByName (part of IResolverSimple) returns an IP address whereas lookupMailExchange returns three lists of DNS records. These three lists contain answer records, authority records, and additional records.
Note
getHostByName may return an IPv6 address; unlike its stdlib equivalent (
socket.gethostbyname())IResolver contains separate functions for looking up each of the common DNS record types.
IResolver includes a lower level
queryfunction for issuing arbitrary queries.The names.client module
directlyProvidesboth the IResolverSimple and the IResolver interfaces.createResolver constructs a global resolver which performs queries against the same DNS sources and servers used by the underlying operating system.
That is, it will use the DNS server IP addresses found in a local
resolv.conffile (if the operating system provides such a file) and it will use an OS specifichostsfile path.
A simple example¶
In this section you will learn how the IResolver interface can be used to write a utility for performing a reverse DNS lookup for an IPv4 address. dig can do this too, so lets start by examining its output:
$ dig -x 127.0.0.1
...
;; QUESTION SECTION:
;1.0.0.127.in-addr.arpa. IN PTR
;; ANSWER SECTION:
1.0.0.127.in-addr.arpa. 86400 IN PTR localhost.
...
As you can see, dig has performed a DNS query with the following attributes:
- Name:
1.0.0.127.in-addr.arpa. - Class:
IN - Type:
PTR
The name is a reverse domain name and is derived by reversing an IPv4 address and prepending it to the special in-addr.arpa parent domain name. So, lets write a function to create a reverse domain name from an IP address.
def reverseNameFromIPAddress(address):
return '.'.join(reversed(address.split('.'))) + '.in-addr.arpa'
We can test the output from a python shell:
>>> reverseNameFromIPAddress('192.0.2.100')
'100.2.0.192.in-addr.arpa'
We’re going to use twisted.names.client.lookupPointer to perform the actual DNS lookup.
So lets examine the output of lookupPointer so that we can design a function to format and print its results in a style similar to dig.
Note
lookupPointer is an asynchronous function, so we’ll use an interactive twisted.conch shell here.
$ python -m twisted.conch.stdio
>>> from twisted.names import client
>>> from reverse_lookup import reverseNameFromIPAddress
>>> d = client.lookupPointer(name=reverseNameFromIPAddress('127.0.0.1'))
>>> d
<Deferred at 0x286b170 current result: ([<RR name=1.0.0.127.in-addr.arpa type=PTR class=IN ttl=86400s auth=False>], [], [])>
>>> d.result
([<RR name=1.0.0.127.in-addr.arpa type=PTR class=IN ttl=86400s auth=False>], [], [])
The deferred result of lookupPointer is a tuple containing three lists of records; answers, authority, and additional.
The actual record is a Record_PTR instance which can be reached via the RRHeader.payload attribute.
>>> recordHeader = d.result[0][0]
>>> recordHeader.payload
<PTR name=localhost ttl=86400>
So, now we’ve found the information we need, lets create a function that extracts the first answer and prints the domain name and the record payload.
def printResult(result):
answers, authority, additional = result
if answers:
a = answers[0]
print('{} IN {}'.format(a.name.name, a.payload))
And lets test the output:
>>> from twisted.names import dns
>>> printResult(([dns.RRHeader(name='1.0.0.127.in-addr.arpa', type=dns.PTR, payload=dns.Record_PTR('localhost'))], [], []))
1.0.0.127.in-addr.arpa IN <PTR name=localhost ttl=None>
Fine! Now we can assemble the pieces in a main function, which we’ll call using twisted.internet.task.react.
Here’s the complete script.
listings/names/reverse_lookup.py
import sys
from twisted.internet import task
from twisted.names import client
def reverseNameFromIPAddress(address):
return '.'.join(reversed(address.split('.'))) + '.in-addr.arpa'
def printResult(result):
answers, authority, additional = result
if answers:
a = answers[0]
print('{} IN {}'.format(a.name.name, a.payload))
def main(reactor, address):
d = client.lookupPointer(name=reverseNameFromIPAddress(address=address))
d.addCallback(printResult)
return d
task.react(main, sys.argv[1:])
The output looks like this:
$ python reverse_lookup.py 127.0.0.1
1.0.0.127.in-addr.arpa IN <PTR name=localhost ttl=86400>
Note
- You can read more about reverse domain names in RFC 1034#section-5.2.1.
- We’ve ignored IPv6 addresses in this example, but you can read more about reverse IPv6 domain names in RFC 3596#section-2.5 and the example could easily be extended to support these.
- You might also consider using netaddr, which can generate reverse domain names and which also includes sophisticated IP network and IP address handling.
- This script only prints the first answer, but sometimes you’ll get multiple answers due to CNAME indirection, for example in the case of classless reverse zones.
- All lookups and responses are handled asynchronously, so the script could be extended to perform thousands of reverse DNS lookups in parallel.
Next you should study ../examples/multi_reverse_lookup.py which extends this example to perform both IPv4 and IPv6 addresses and which can perform multiple reverse DNS lookups in parallel.
Creating a New Resolver¶
Now suppose we want to create a DNS client which sends its queries to a specific server (or servers).
In this case, we use client.Resolver directly and pass it a list of preferred server IP addresses and ports.
For example, suppose we want to lookup names using the free Google DNS servers:
$ python -m twisted.conch.stdio
>>> from twisted.names import client
>>> resolver = client.createResolver(servers=[('8.8.8.8', 53), ('8.8.4.4', 53)])
>>> resolver.getHostByName('example.com')
<Deferred at 0x9dcfbac current result: '93.184.216.119'>
Here we are using the Google DNS server IP addresses and the standard DNS port (53).
Installing a Resolver in the Reactor¶
You can also install a custom resolver into the reactor using the IReactorPluggable interface.
The reactor uses its installed resolver whenever it needs to resolve hostnames; for example, when you supply a hostname to connectTCP.
Here’s a short example that shows how to install an alternative resolver for the global reactor:
from twisted.internet import reactor
from twisted.names import client
reactor.installResolver(client.createResolver(servers=[('8.8.8.8', 53), ('8.8.4.4', 53)]))
After this, all hostname lookups requested by the reactor will be sent to the Google DNS servers; instead of to the local operating system.
Note
- By default the reactor uses the POSIX
gethostbynamefunction provided by the operating system, - but
gethostbynameis a blocking function, so it has to be called in a thread pool. - Check out ThreadedResolver if you’re interested in learning more about how the default threaded resolver works.
Lower Level APIs¶
Here’s an example of how to use the DNSDatagramProtocol directly.
from twisted.internet import task
from twisted.names import dns
def main(reactor):
proto = dns.DNSDatagramProtocol(controller=None)
reactor.listenUDP(0, proto)
d = proto.query(('8.8.8.8', 53), [dns.Query('www.example.com', dns.AAAA)])
d.addCallback(printResult)
return d
def printResult(res):
print 'ANSWERS: ', [a.payload for a in res.answers]
task.react(main)
The disadvantage of working at this low level is that you will need to handle query failures yourself, by manually re-issuing queries or by issuing followup TCP queries using the stream based dns.DNSProtocol.
These things are handled automatically by the higher level APIs in client.
Also notice that in this case, the deferred result of dns.DNSDatagramProtocol.query is a dns.Message object, rather than a list of DNS records.
Further Reading¶
Check out the Twisted Names Examples which demonstrate how the client APIs can be used to create useful DNS diagnostic tools.
Creating and working with a names (DNS) server¶
A Names server can be perform three basic operations:
- act as a recursive server, forwarding queries to other servers
- perform local caching of recursively discovered records
- act as the authoritative server for a domain
Creating a non-authoritative server¶
The first two of these are easy, and you can create a server that performs them
with the command twistd -n dns --recursive --cache .
You may wish to run this as root since it will try to bind to UDP port 53. Try
performing a lookup with it, dig twistedmatrix.com @127.0.0.1 .
Creating an authoritative server¶
To act as the authority for a domain, two things are necessary: the address of the machine on which the domain name server will run must be registered as a nameserver for the domain; and the domain name server must be configured to act as the authority. The first requirement is beyond the scope of this howto and will not be covered.
To configure Names to act as the authority
for example-domain.com , we first create a zone file for
this domain.
zone = [
SOA(
# For whom we are the authority
'example-domain.com',
# This nameserver's name
mname = "ns1.example-domain.com",
# Mailbox of individual who handles this
rname = "root.example-domain.com",
# Unique serial identifying this SOA data
serial = 2003010601,
# Time interval before zone should be refreshed
refresh = "1H",
# Interval before failed refresh should be retried
retry = "1H",
# Upper limit on time interval before expiry
expire = "1H",
# Minimum TTL
minimum = "1H"
),
A('example-domain.com', '127.0.0.1'),
NS('example-domain.com', 'ns1.example-domain.com'),
CNAME('www.example-domain.com', 'example-domain.com'),
CNAME('ftp.example-domain.com', 'example-domain.com'),
MX('example-domain.com', 0, 'mail.example-domain.com'),
A('mail.example-domain.com', '123.0.16.43')
]
Next, run the command twistd -n dns --pyzone example-domain.com . Now try querying the domain locally (again, with
dig): dig -t any example-domain.com @127.0.0.1 .
Names can also read a traditional, BIND-syntax zone file. Specify these
with the --bindzone parameter. The $GENERATE and $INCLUDE
directives are not yet supported.
Creating a custom server¶
The builtin DNS server plugin is useful, but the beauty of Twisted Names is that you can build your own custom servers and clients using the names components.
- In this section you will learn about the components required to build a simple DNS server.
- You will then learn how to create a custom DNS server which calculates responses dynamically.
A simple forwarding DNS server¶
Lets start by creating a simple forwarding DNS server, which forwards all requests to an upstream server (or servers).
# Copyright (c) Twisted Matrix Laboratories.
# See LICENSE for details.
"""
An example of a simple non-authoritative DNS server.
"""
from twisted.internet import reactor
from twisted.names import client, dns, server
def main():
"""
Run the server.
"""
factory = server.DNSServerFactory(
clients=[client.Resolver(resolv='/etc/resolv.conf')]
)
protocol = dns.DNSDatagramProtocol(controller=factory)
reactor.listenUDP(10053, protocol)
reactor.listenTCP(10053, factory)
reactor.run()
if __name__ == '__main__':
raise SystemExit(main())
In this example we are passing a client.Resolver instance
to the DNSServerFactory
and we are configuring that client to use the upstream DNS servers which are specified in a local resolv.conf file.
Also note that we start the server listening on both UDP and TCP ports. This is a standard requirement for DNS servers.
You can test the server using dig.
For example:
$ dig -p 10053 @127.0.0.1 example.com SOA +short
sns.dns.icann.org. noc.dns.icann.org. 2013102791 7200 3600 1209600 3600
A server which computes responses dynamically¶
Now suppose we want to create a bespoke DNS server which responds to certain hostname queries by dynamically calculating the resulting IP address, while passing all other queries to another DNS server. Queries for hostnames matching the pattern workstation{0-9}+ will result in an IP address where the last octet matches the workstation number.
We’ll write a custom resolver which we insert before the standard client resolver. The custom resolver will be queried first.
Here’s the code:
# Copyright (c) Twisted Matrix Laboratories.
# See LICENSE for details.
"""
An example demonstrating how to create a custom DNS server.
The server will calculate the responses to A queries where the name begins with
the word "workstation".
Other queries will be handled by a fallback resolver.
eg
python doc/names/howto/listings/names/override_server.py
$ dig -p 10053 @localhost workstation1.example.com A +short
172.0.2.1
"""
from twisted.internet import reactor, defer
from twisted.names import client, dns, error, server
class DynamicResolver(object):
"""
A resolver which calculates the answers to certain queries based on the
query type and name.
"""
_pattern = 'workstation'
_network = '172.0.2'
def _dynamicResponseRequired(self, query):
"""
Check the query to determine if a dynamic response is required.
"""
if query.type == dns.A:
labels = query.name.name.split('.')
if labels[0].startswith(self._pattern):
return True
return False
def _doDynamicResponse(self, query):
"""
Calculate the response to a query.
"""
name = query.name.name
labels = name.split('.')
parts = labels[0].split(self._pattern)
lastOctet = int(parts[1])
answer = dns.RRHeader(
name=name,
payload=dns.Record_A(address=b'%s.%s' % (self._network, lastOctet)))
answers = [answer]
authority = []
additional = []
return answers, authority, additional
def query(self, query, timeout=None):
"""
Check if the query should be answered dynamically, otherwise dispatch to
the fallback resolver.
"""
if self._dynamicResponseRequired(query):
return defer.succeed(self._doDynamicResponse(query))
else:
return defer.fail(error.DomainError())
def main():
"""
Run the server.
"""
factory = server.DNSServerFactory(
clients=[DynamicResolver(), client.Resolver(resolv='/etc/resolv.conf')]
)
protocol = dns.DNSDatagramProtocol(controller=factory)
reactor.listenUDP(10053, protocol)
reactor.listenTCP(10053, factory)
reactor.run()
if __name__ == '__main__':
raise SystemExit(main())
Notice that DynamicResolver.query returns a Deferred.
On success, it returns three lists of DNS records (answers, authority, additional),
which will be encoded by dns.Message and returned to the client.
On failure, it returns a DomainError,
which is a signal that the query should be dispatched to the next client resolver in the list.
Note
The fallback behaviour is actually handled by ResolverChain.
ResolverChain is a proxy for other resolvers. It takes a list of IResolver providers and queries each one in turn until it receives an answer, or until the list is exhausted.
Each IResolver in the chain may return a deferred DomainError, which is a signal that ResolverChain should query the next chained resolver.
The DNSServerFactory constructor takes a list of authoritative resolvers, caches and client resolvers and ensures that they are added to the ResolverChain in the correct order.
Let’s use dig to see how this server responds to requests that match the pattern we specified:
$ dig -p 10053 @127.0.0.1 workstation1.example.com A +short
172.0.2.1
$ dig -p 10053 @127.0.0.1 workstation100.example.com A +short
172.0.2.100
And if we issue a request that doesn’t match the pattern:
$ dig -p 10053 @localhost www.example.com A +short
93.184.216.119
Further Reading¶
For simplicity, the examples above use the reactor.listenXXX APIs.
But your application will be more flexible if you use the Twisted Application APIs,
along with the Twisted plugin system and twistd.
Read the source code of names.tap to see how the twistd names plugin works.
Examples¶
DNS (Twisted Names)¶
testdns.py- Prints the results of an Address record lookup, Mail-Exchanger record lookup, and Nameserver record lookup for the given domain name.dns-service.py- Searches for SRV records in DNS.gethostbyname.py- Returns the IP address for a given hostname.
Twisted Names is a library of DNS components for building DNS servers and clients.
It includes a client resolver API, with which you can generate queries for all the standard record types.
The client API also includes a replacement for the blocking gethostbyname() function provided by the Python stdlib socket module.
Twisted Names provides a twistd DNS server plugin which can:
- Act as a master authoritative server which can read most BIND-syntax zone files as well as a simple Python-based configuration format.
- Act as a secondary authoritative DNS server, which retrieves its records from a master server by zone transfer.
- Act as a caching / forwarding nameserver which forwards requests to one or more upstream recursive nameservers and caches the results.
- Or any combination of these.
The following developer guides, example scripts and API documentation will demonstrate how to use these components and provide you with all the information you need to build your own custom DNS client or server using Twisted Names.
- Developer guides: documentation on using Twisted Names to develop your own applications
- Examples: short code examples using Twisted Names
- API documentation: Detailed API documentation for all the Twisted Names components
Twisted Pair¶
Developer Guides¶
Twisted Pair: Tunnels And Network Taps¶
On Linux, Twisted Pair supports the special tun and tap network interface types. This functionality allows you to interact with raw sockets (for example, to send or receive ICMP or ARP traffic). It also allows the creation of simulated networks. This document will not cover the details of these platform-provided features, but it will explain how to use the Twisted Pair APIs which interact with them. Before reading this document, you may want to familiarize yourself with Linux tuntap if you have not already done so (good online resources, are a little scarce, but you may find the linux tuntap tutorial google results helpful).
Tuntap Ports¶
The twisted.pair.tuntap.TuntapPort class is the entry point into the tun/tap functionality. This class is initialized with an application-supplied protocol object and associates that object with a tun or tap device on the system. If the protocol provides twisted.pair.ethernet.IEthernetProtocol then it is associated with a tap device. Otherwise the protocol must provide twisted.pair.raw.IRawPacketProtocol and it will be associated with a tun device.
from zope.interface import implementer
from twisted.pair.tuntap import TuntapPort
from twisted.pair.ethernet import EthernetProtocol
from twisted.pair.rawudp import RawUDPProtocol
from twisted.internet import reactor
# Note that you must run this example as a user with permission to open this
# device. That means run it as root or pre-configure tap0 and assign ownership
# of it to a non-root user. The same goes for tun0 below.
tap = TuntapPort(b"tap0", EthernetProtocol(), reactor=reactor)
tap.startListening()
tun = TuntapPort(b"tun0", RawUDPProtocol(), reactor=reactor)
tun.startListening()
In the above example two protocols are attached to the network: one to a tap device and the other to a tun device.
The EthernetProtocol used in this example is a very simple implementation of IEthernetProtocol which does nothing more than dispatch to some other protocol based on the protocol found in the header of each ethernet frame it receives. RawUDPProtocol is similar - it dispatches to other protocols based on the UDP port of IP datagrams it received.
This example won’t do anything since no application protocols have been added to either the EthernetProtocol or RawUDPProtocol instances
(not to mention the reactor isn’t being started).
However, it should give you some idea of how tun/tap functionality fits into a Twisted application.
By the behaviors of these two protocols you can see the primary difference between tap and tun devices.
The lower level of the two, tap devices, is hooked in to the network stack at the ethernet layer.
When a TuntapPort is associated with a tap device, it delivers whole ethernet frames to its protocol.
The higher level version, tun devices, strips off the ethernet layer before delivering data to the application.
This means that a TuntapPort associated with a tun device most commonly delivers IP datagrams to its protocol (though if your network is being used to convey non-IP datagrams then it may deliver those instead).
Both IEthernetProtocol and IRawSocketProtocol are similar to twisted.internet.protocol.DatagramProtocol .
Datagrams, either ethernet or otherwise, are delivered to the protocol’s datagramReceived method.
Conversely the protocol is associated with a transport with a write method that accepts datagrams for injection into the network.
You can see an example of some of this functionality in the ../examples/pairudp.py example.
Twisted Pair: Device Configuration¶
Twisted Pair’s Test Suite¶
Certain system configuration is required before the full Twisted Pair test suite can be run. Without this setup the test suite will lack the permission necessary to access tap and tun devices. Some tests will still run but the integration tests which verify Twisted Pair can successfully read from and write to real devices will be skipped.
The following shell script creates two tun devices and two tap devices and grants permission to use them to whatever user the shell script is run as. Run it to configure your system so that you can perform a complete run of the Twisted Pair test suite.
# Needs to be short enough so that with prefix and suffix, fits into 16 bytes
IDENTIFIER="twtest"
# A tap device without protocol information
sudo ip tuntap add dev tap-${IDENTIFIER} mode tap user $(id -u -n) group $(id -g -n)
sudo ip link set up dev tap-${IDENTIFIER}
sudo ip addr add 172.16.0.1/24 dev tap-${IDENTIFIER}
sudo ip neigh add 172.16.0.2 lladdr de:ad:be:ef:ca:fe dev tap-${IDENTIFIER}
# A tap device with protocol information
sudo ip tuntap add dev tap-${IDENTIFIER}-pi mode tap user $(id -u -n) group $(id -g -n) pi
sudo ip link set up dev tap-${IDENTIFIER}-pi
sudo ip addr add 172.16.1.1/24 dev tap-${IDENTIFIER}-pi
sudo ip neigh add 172.16.1.2 lladdr de:ad:ca:fe:be:ef dev tap-${IDENTIFIER}-pi
# A tun device without protocol information
sudo ip tuntap add dev tun-${IDENTIFIER} mode tun user $(id -u -n) group $(id -g -n)
sudo ip link set up dev tun-${IDENTIFIER}
# A tun device with protocol information
sudo ip tuntap add dev tun-${IDENTIFIER}-pi mode tun user $(id -u -n) group $(id -g -n) pi
sudo ip link set up dev tun-${IDENTIFIER}-pi
There are two things to keep in mind about this configuration. First, it uses addresses from the 172.16.0.0/12 private use range. If your network is configured to use these already then running the script may cause problems for your network. These addresses are hard-coded into the Twisted Pair test suite so this problem is not easily avoided. Second, the changes are not persistent across reboots. If you want this network configuration to be available even after a reboot you will need to integrate the above into your system’s init scripts somehow (the details of this for different systems is beyond the scope of this document).
Certain platforms may also require a modification to their firewall rules in order to allow the traffic the test suite wants to transmit. Adding firewall rules which allowing traffic destined for the addresses used by the test suite should address this problem. If you encounter timeouts when running the Twisted Pair test suite then this may apply to you. For example, to configure an iptables firewall to allow this traffic:
iptables -I INPUT --dest 172.16.1.1 -j ACCEPT
iptables -I INPUT --dest 172.16.2.1 -j ACCEPT
- Twisted Pair Documentation
Examples¶
Miscellaneous¶
pairudp.py- UDP implemented with a TUN/TAP device
- Developer guides: documentation on using Twisted Pair to develop your own applications
- Code Examples: short code examples using Twisted Pair
Twisted Web¶
Developer Guides¶
Overview of Twisted Web¶
Introduction¶
Twisted Web is a web application server written in pure Python, with APIs at multiple levels of abstraction to facilitate different kinds of web programming.
Twisted Web’s Structure¶
When the Web Server receives a request from a Client, it creates a Request object and passes it on to the Resource system. The Resource system dispatches to the appropriate Resource object based on what path was requested by the client. The Resource is asked to render itself, and the result is returned to the client.
Resources¶
Resources are the lowest-level abstraction for applications
in the Twisted web server. Each Resource is a 1:1 mapping with
a path that is requested: you can think of a Resource as a
single “page” to be rendered. The interface for making
Resources is very simple; they must have a method named
render which takes a single argument, which is the
Request object (an instance of twisted.web.server.Request ). This render
method must return a string, which will be returned to the web
browser making the request. Alternatively, they can return a
special constant, twisted.web.server.NOT_DONE_YET , which tells
the web server not to close the connection; you must then use
request.write(data) to render the
page, and call request.finish()
whenever you’re done.
Configuring and Using the Twisted Web Server¶
Twisted Web Development¶
Twisted Web serves Python objects that implement the interface IResource.
Main Concepts¶
- Site Objects are responsible for
creating
HTTPChannelinstances to parse the HTTP request, and begin the object lookup process. They contain the root Resource, the resource which represents the URL/on the site. - Resource objects represent a single URL segment. The IResource interface describes the methods a Resource object must implement in order to participate in the object publishing process.
- Resource trees are arrangements of Resource objects into a Resource tree. Starting at the root Resource object, the tree of Resource objects defines the URLs which will be valid.
- .rpy scripts are python scripts which the twisted.web static file server will execute, much like a CGI. However, unlike CGI they must create a Resource object which will be rendered when the URL is visited.
- Resource rendering occurs when Twisted Web locates a leaf Resource object. A Resource can either return an html string or write to the request object.
- Session objects allow you to store information across multiple requests. Each individual browser using the system has a unique Session instance.
The Twisted Web server is started through the Twisted Daemonizer, as in:
% twistd web
Site Objects¶
Site objects serve as the glue between a port to listen for HTTP requests on, and a root Resource object.
When using twistd -n web --path /foo/bar/baz , a Site object is created with a root Resource that serves files out of the given path.
You can also create a Site instance by hand, passing
it a Resource object which will serve as the root of the
site:
from twisted.web import server, resource
from twisted.internet import reactor
class Simple(resource.Resource):
isLeaf = True
def render_GET(self, request):
return "<html>Hello, world!</html>"
site = server.Site(Simple())
reactor.listenTCP(8080, site)
reactor.run()
Resource objects¶
Resource objects represent a single URL segment of a site. During URL parsing, getChild is called on the current Resource to produce the next Resource object.
When the leaf Resource is reached, either because there were no more URL segments or a Resource had isLeaf set to True, the leaf Resource is rendered by calling render(request) . See “Resource Rendering” below for more about this.
During the Resource location process, the URL segments which have already been processed and those which have not yet been processed are available in request.prepath and request.postpath .
A Resource can know where it is in the URL tree by looking at request.prepath , a list of URL segment strings.
A Resource can know which path segments will be processed after it by looking at request.postpath .
If the URL ends in a slash, for example http://example.com/foo/bar/ , the final URL segment will be an empty string. Resources can thus know if they were requested with or without a final slash.
Here is a simple Resource object:
from twisted.web.resource import Resource
class Hello(Resource):
isLeaf = True
def getChild(self, name, request):
if name == '':
return self
return Resource.getChild(self, name, request)
def render_GET(self, request):
return "Hello, world! I am located at %r." % (request.prepath,)
resource = Hello()
Resource Trees¶
Resources can be arranged in trees using putChild . putChild puts a Resource instance into another Resource instance, making it available at the given path segment name:
root = Hello()
root.putChild('fred', Hello())
root.putChild('bob', Hello())
If this root resource is served as the root of a Site instance, the following URLs will all be valid:
http://example.com/http://example.com/fredhttp://example.com/bobhttp://example.com/fred/http://example.com/bob/
.rpy scripts¶
Files with the extension .rpy are python scripts which, when placed in a directory served by Twisted Web, will be executed when visited through the web.
An .rpy script must define a variable, resource , which is the Resource object that will render the request.
.rpy files are very convenient for rapid development and prototyping. Since they are executed on every web request, defining a Resource subclass in an .rpy will make viewing the results of changes to your class visible simply by refreshing the page:
from twisted.web.resource import Resource
class MyResource(Resource):
def render_GET(self, request):
return "<html>Hello, world!</html>"
resource = MyResource()
However, it is often a better idea to define Resource subclasses in Python modules. In order for changes in modules to be visible, you must either restart the Python process, or reload the module:
import myresource
## Comment out this line when finished debugging
reload(myresource)
resource = myresource.MyResource()
Creating a Twisted Web server which serves a directory is easy:
% twistd -n web --path /Users/dsp/Sites
Resource rendering¶
Resource rendering occurs when Twisted Web locates a leaf Resource object to handle a web request. A Resource’s render method may do various things to produce output which will be sent back to the browser:
- Return a string
- Call
request.write("stuff")as many times as desired, then callrequest.finish()and returnserver.NOT_DONE_YET(This is deceptive, since you are in fact done with the request, but is the correct way to do this) - Request a
Deferred, returnserver.NOT_DONE_YET, and callrequest.write("stuff")andrequest.finish()later, in a callback on theDeferred.
The Resource
class, which is usually what one’s Resource classes subclass, has a
convenient default implementation
of render . It will call a method
named self.render_METHOD
where “METHOD” is whatever HTTP method was used to request this
resource. Examples: request_GET, request_POST, request_HEAD, and so
on. It is recommended that you have your resource classes
subclass Resource
and implement render_METHOD methods as
opposed to render itself. Note that for
certain resources, request_POST = request_GET may be desirable in case one wants to process
arguments passed to the resource regardless of whether they used GET
(?foo=bar&baz=quux , and so forth) or POST.
Request encoders¶
When using a Resource , one can specify wrap it using a EncodingResourceWrapper and passing a list of encoder factories. The encoder factories are called when a request is processed and potentially return an encoder. By default twisted provides GzipEncoderFactory which manages standard gzip compression. You can use it this way:
from twisted.web.server import Site, GzipEncoderFactory
from twisted.web.resource import Resource, EncodingResourceWrapper
from twisted.internet import reactor
class Simple(Resource):
isLeaf = True
def render_GET(self, request):
return "<html>Hello, world!</html>"
resource = Simple()
wrapped = EncodingResourceWrapper(resource, [GzipEncoderFactory()])
site = Site(wrapped)
reactor.listenTCP(8080, site)
reactor.run()
Using compression on SSL served resources where the user can influence the content can lead to information leak, so be careful which resources use request encoders.
Note that only encoder can be used per request: the first encoder factory returning an object will be used, so the order in which they are specified matters.
Session¶
HTTP is a stateless protocol; every request-response is treated as an individual unit, distinguishable from any other request only by the URL requested. With the advent of Cookies in the mid nineties, dynamic web servers gained the ability to distinguish between requests coming from different browser sessions by sending a Cookie to a browser. The browser then sends this cookie whenever it makes a request to a web server, allowing the server to track which requests come from which browser session.
Twisted Web provides an abstraction of this browser-tracking behavior called the Session object . Calling request.getSession() checks to see if a session cookie has been set; if not, it creates a unique session id, creates a Session object, stores it in the Site, and returns it. If a session object already exists, the same session object is returned. In this way, you can store data specific to the session in the session object.
Proxies and reverse proxies¶
A proxy is a general term for a server that functions as an intermediary between clients and other servers.
Twisted supports two main proxy variants: a Proxy and a ReverseProxy .
A proxy forwards requests made by a client to a destination server. Proxies typically sit on the internal network for a client or out on the internet, and have many uses, including caching, packet filtering, auditing, and circumventing local access restrictions to web content.
Here is an example of a simple but complete web proxy:
from twisted.web import proxy, http
from twisted.internet import reactor
class ProxyFactory(http.HTTPFactory):
def buildProtocol(self, addr):
return proxy.Proxy()
reactor.listenTCP(8080, ProxyFactory())
reactor.run()
With this proxy running, you can configure your web browser to use localhost:8080 as a proxy. After doing so, when browsing the web
all requests will go through this proxy.
Proxy inherits
from http.HTTPChannel . Each client
request to the proxy generates a ProxyRequest from the proxy to the destination
server on behalf of the client. ProxyRequest uses
a ProxyClientFactory to create
an instance of the ProxyClient
protocol for the connection. ProxyClient inherits
from http.HTTPClient . Subclass ProxyRequest to
customize the way requests are processed or logged.
A reverse proxy retrieves resources from other servers on behalf of a client. Reverse proxies typically sit inside the server’s internal network and are used for caching, application firewalls, and load balancing.
Here is an example of a basic reverse proxy:
from twisted.internet import reactor
from twisted.web import proxy, server
site = server.Site(proxy.ReverseProxyResource('www.yahoo.com', 80, ''))
reactor.listenTCP(8080, site)
reactor.run()
With this reverse proxy running locally, you can
visit http://localhost:8080 in your web browser, and the reverse
proxy will proxy your connection to www.yahoo.com.
In this example we use server.Site to serve
a ReverseProxyResource directly. There is
also a ReverseProxy family of classes
in twisted.web.proxy mirroring those of the Proxy
family:
Like Proxy , ReverseProxy inherits
from http.HTTPChannel . Each client request to the reverse proxy
generates a ReverseProxyRequest to the destination
server. Like ProxyRequest , ReverseProxyRequest uses a ProxyClientFactory to create an instance of
the ProxyClient protocol for
the connection.
Additional examples of proxies and reverse proxies can be found in the Twisted web examples
Advanced Configuration¶
Non-trivial configurations of Twisted Web are achieved with Python
configuration files. This is a Python snippet which builds up a
variable called application. Usually,
a twisted.application.internet.TCPServer
instance will be used to make the application listen on a TCP port
(80, in case direct web serving is desired), with the listener being
a twisted.web.server.Site . The resulting file
can then be run with twistd -y . Alternatively a reactor object can be used directly to make
a runnable script.
The Site will wrap a Resource object – the
root.
from twisted.application import internet, service
from twisted.web import static, server
root = static.File("/var/www/htdocs")
application = service.Application('web')
site = server.Site(root)
sc = service.IServiceCollection(application)
i = internet.TCPServer(80, site)
i.setServiceParent(sc)
Most advanced configurations will be in the form of tweaking the root resource object.
Adding Children¶
Usually, the root’s children will be based on the filesystem’s contents.
It is possible to override the filesystem by explicit putChild
methods.
Here are two examples. The first one adds a /doc child
to serve the documentation of the installed packages, while the second
one adds a cgi-bin directory for CGI scripts.
from twisted.internet import reactor
from twisted.web import static, server
root = static.File("/var/www/htdocs")
root.putChild("doc", static.File("/usr/share/doc"))
reactor.listenTCP(80, server.Site(root))
reactor.run()
from twisted.internet import reactor
from twisted.web import static, server, twcgi
root = static.File("/var/www/htdocs")
root.putChild("cgi-bin", twcgi.CGIDirectory("/var/www/cgi-bin"))
reactor.listenTCP(80, server.Site(root))
reactor.run()
Modifying File Resources¶
File resources, be they root object or children
thereof, have two important attributes that often need to be
modified: indexNames
and processors . indexNames determines which
files are treated as “index files” – served up when a directory
is rendered. processors determine how certain file
extensions are treated.
Here is an example for both, creating a site where all .rpy
extensions are Resource Scripts, and which renders directories by
searching for a index.rpy file.
from twisted.application import internet, service
from twisted.web import static, server, script
root = static.File("/var/www/htdocs")
root.indexNames=['index.rpy']
root.processors = {'.rpy': script.ResourceScript}
application = service.Application('web')
sc = service.IServiceCollection(application)
site = server.Site(root)
i = internet.TCPServer(80, site)
i.setServiceParent(sc)
File objects also have a method called ignoreExt .
This method can be used to give extension-less URLs to users, so that
implementation is hidden. Here is an example:
from twisted.application import internet, service
from twisted.web import static, server, script
root = static.File("/var/www/htdocs")
root.ignoreExt(".rpy")
root.processors = {'.rpy': script.ResourceScript}
application = service.Application('web')
sc = service.IServiceCollection(application)
site = server.Site(root)
i = internet.TCPServer(80, site)
i.setServiceParent(sc)
Now, a URL such as /foo might be served from a Resource
Script called foo.rpy , if no file by the name of foo
exists.
Virtual Hosts¶
Virtual hosting is done via a special resource, that should be used
as the root resource
– NameVirtualHost . NameVirtualHost has an
attribute named default , which holds the default
website. If a different root for some other name is desired,
the addHost method should be called.
from twisted.application import internet, service
from twisted.web import static, server, vhost, script
root = vhost.NameVirtualHost()
# Add a default -- htdocs
root.default=static.File("/var/www/htdocs")
# Add a simple virtual host -- foo.com
root.addHost("foo.com", static.File("/var/www/foo"))
# Add a simple virtual host -- bar.com
root.addHost("bar.com", static.File("/var/www/bar"))
# The "baz" people want to use Resource Scripts in their web site
baz = static.File("/var/www/baz")
baz.processors = {'.rpy': script.ResourceScript}
baz.ignoreExt('.rpy')
root.addHost('baz', baz)
application = service.Application('web')
sc = service.IServiceCollection(application)
site = server.Site(root)
i = internet.TCPServer(80, site)
i.setServiceParent(sc)
Advanced Techniques¶
Since the configuration is a Python snippet, it is possible to use the full power of Python. Here are some simple examples:
# No need for configuration of virtual hosts -- just make sure
# a directory /var/vhosts/<vhost name> exists:
from twisted.web import vhost, static, server
from twisted.application import internet, service
root = vhost.NameVirtualHost()
root.default = static.File("/var/www/htdocs")
for dir in os.listdir("/var/vhosts"):
root.addHost(dir, static.File(os.path.join("/var/vhosts", dir)))
application = service.Application('web')
sc = service.IServiceCollection(application)
site = server.Site(root)
i = internet.TCPServer(80, site)
i.setServiceParent(sc)
# Determine ports we listen on based on a file with numbers:
from twisted.web import vhost, static, server
from twisted.application import internet, service
root = static.File("/var/www/htdocs")
site = server.Site(root)
application = service.Application('web')
serviceCollection = service.IServiceCollection(application)
for num in map(int, open("/etc/web/ports").read().split()):
serviceCollection.addCollection(internet.TCPServer(num, site))
Running a Twisted Web Server¶
In many cases, you’ll end up repeating common usage patterns of twisted.web. In those cases you’ll probably want to use Twisted’s pre-configured web server setup.
The easiest way to run a Twisted Web server is with the Twisted Daemonizer. For example, this command will run a web server which serves static files from a particular directory:
% twistd web --path /path/to/web/content
If you just want to serve content from your own home directory, the following will do:
% twistd web --path ~/public_html/
You can stop the server at any time by going back to the directory you started it in and running the command:
% kill `cat twistd.pid`
Some other configuration options are available as well:
--port: Specify the port for the web server to listen on. This defaults to 8080.--logfile: Specify the path to the log file.
The full set of options that are available can be seen with:
% twistd web --help
Serving Flat HTML¶
Twisted Web serves flat HTML files just as it does any other flat file.
Resource Scripts¶
A Resource script is a Python file ending with the extension .rpy , which is required to create an instance of a (subclass of a) twisted.web.resource.Resource .
Resource scripts have 3 special variables:
__file__: The name of the .rpy file, including the full path. This variable is automatically defined and present within the namespace.registry: An object of class static.Registry . It can be used to access and set persistent data keyed by a class.resource: The variable which must be defined by the script and set to the resource instance that will be used to render the page.
A very simple Resource Script might look like:
from twisted.web import resource
class MyGreatResource(resource.Resource):
def render_GET(self, request):
return "<html>foo</html>"
resource = MyGreatResource()
A slightly more complicated resource script, which accesses some persistent data, might look like:
from twisted.web import resource
from SillyWeb import Counter
counter = registry.getComponent(Counter)
if not counter:
registry.setComponent(Counter, Counter())
counter = registry.getComponent(Counter)
class MyResource(resource.Resource):
def render_GET(self, request):
counter.increment()
return "you are visitor %d" % counter.getValue()
resource = MyResource()
This is assuming you have the SillyWeb.Counter module,
implemented something like the following:
class Counter:
def __init__(self):
self.value = 0
def increment(self):
self.value += 1
def getValue(self):
return self.value
Web UIs¶
The Nevow framework, available as part of the Quotient project, is an advanced system for giving Web UIs to your application. Nevow uses Twisted Web but is not itself part of Twisted.
Spreadable Web Servers¶
One of the most interesting applications of Twisted Web is the distributed webserver; multiple servers can all answer requests on the same port, using the twisted.spread package for “spreadable” computing. In two different directories, run the commands:
% twistd web --user
% twistd web --personal [other options, if you desire]
Once you’re running both of these instances, go to http://localhost:8080/your_username.twistd/ – you will see the front page from the server you created with the --personal option. What’s happening here is that the request you’ve sent is being relayed from the central (User) server to your own (Personal) server, over a PB connection. This technique can be highly useful for small “community” sites; using the code that makes this demo work, you can connect one HTTP port to multiple resources running with different permissions on the same machine, on different local machines, or even over the internet to a remote site.
By default, a personal server listens on a UNIX socket in the owner’s home
directory. The --port option can be used to make
it listen on a different address, such as a TCP or SSL server or on a UNIX
server in a different location. If you use this option to make a personal
server listen on a different address, the central (User) server won’t be
able to find it, but a custom server which uses the same APIs as the central
server might. Another use of the --port option
is to make the UNIX server robust against system crashes. If the server
crashes and the UNIX socket is left on the filesystem, the personal server
will not be able to restart until it is removed. However, if --port unix:/home/username/.twistd-web-pb:wantPID=1 is
supplied when creating the personal server, then a lockfile will be used to
keep track of whether the server socket is in use and automatically delete
it when it is not.
Serving PHP/Perl/CGI¶
Everything related to CGI is located in
the twisted.web.twcgi , and it’s here you’ll find the
classes that you need to subclass in order to support the language of
your (or somebody elses) taste. You’ll also need to create your own
kind of resource if you are using a non-unix operating system (such as
Windows), or if the default resources has wrong pathnames to the
parsers.
The following snippet is a .rpy that serves perl-files. Look at twisted.web.twcgi
for more examples regarding twisted.web and CGI.
from twisted.web import static, twcgi
class PerlScript(twcgi.FilteredScript):
filter = '/usr/bin/perl' # Points to the perl parser
resource = static.File("/perlsite") # Points to the perl website
resource.processors = {".pl": PerlScript} # Files that end with .pl will be
# processed by PerlScript
resource.indexNames = ['index.pl']
Serving WSGI Applications¶
WSGI is the Web Server Gateway Interface. It is a specification for web servers and application servers to communicate with Python web applications. All modern Python web frameworks support the WSGI interface.
The easiest way to get started with WSGI application is to use the twistd command:
% twistd -n web --wsgi=helloworld.application
This assumes that you have a WSGI application called application in your helloworld module/package, which might look like this:
def application(environ, start_response):
"""Basic WSGI Application"""
start_response('200 OK', [('Content-type','text/plain')])
return ['Hello World!']
The above setup will be suitable for many applications where all that is
needed is to server the WSGI application at the site’s root. However, for
greater control, Twisted provides support for using WSGI applications as
resources twisted.web.wsgi.WSGIResource .
Here is an example of a WSGI application being served as the root resource for a site, in the following tac file:
from twisted.web import server
from twisted.web.wsgi import WSGIResource
from twisted.python.threadpool import ThreadPool
from twisted.internet import reactor
from twisted.application import service, strports
# Create and start a thread pool,
wsgiThreadPool = ThreadPool()
wsgiThreadPool.start()
# ensuring that it will be stopped when the reactor shuts down
reactor.addSystemEventTrigger('after', 'shutdown', wsgiThreadPool.stop)
def application(environ, start_response):
"""A basic WSGI application"""
start_response('200 OK', [('Content-type','text/plain')])
return ['Hello World!']
# Create the WSGI resource
wsgiAppAsResource = WSGIResource(reactor, wsgiThreadPool, application)
# Hooks for twistd
application = service.Application('Twisted.web.wsgi Hello World Example')
server = strports.service('tcp:8080', server.Site(wsgiAppAsResource))
server.setServiceParent(application)
This can then be run like any other .tac file:
% twistd -ny myapp.tac
Because of the synchronous nature of WSGI, each application call (for
each request) is called within a thread, and the result is written back to the
web server. For this, a twisted.python.threadpool.ThreadPool
instance is used.
Using VHostMonster¶
It is common to use one server (for example, Apache) on a site with multiple
names which then uses reverse proxy (in Apache, via mod_proxy ) to different
internal web servers, possibly on different machines. However, naive
configuration causes miscommunication: the internal server firmly believes it
is running on “internal-name:port” , and will generate URLs to that effect,
which will be completely wrong when received by the client.
While Apache has the ProxyPassReverse directive, it is really a hack and is nowhere near comprehensive enough. Instead, the recommended practice in case the internal web server is Twisted Web is to use VHostMonster.
From the Twisted side, using VHostMonster is easy: just drop a file named
(for example) vhost.rpy containing the following:
from twisted.web import vhost
resource = vhost.VHostMonsterResource()
Make sure the web server is configured with the correct processors
for the rpy extensions (the web server twistd web --path generates by default is so configured).
From the Apache side, instead of using the following ProxyPass directive:
<VirtualHost ip-addr>
ProxyPass / http://localhost:8538/
ServerName example.com
</VirtualHost>
Use the following directive:
<VirtualHost ip-addr>
ProxyPass / http://localhost:8538/vhost.rpy/http/example.com:80/
ServerName example.com
</VirtualHost>
Here is an example for Twisted Web’s reverse proxy:
from twisted.application import internet, service
from twisted.web import proxy, server, vhost
vhostName = 'example.com'
reverseProxy = proxy.ReverseProxyResource('internal', 8538,
'/vhost.rpy/http/'+vhostName+'/')
root = vhost.NameVirtualHost()
root.addHost(vhostName, reverseProxy)
site = server.Site(root)
application = service.Application('web-proxy')
sc = service.IServiceCollection(application)
i = internet.TCPServer(80, site)
i.setServiceParent(sc)
Rewriting URLs¶
Sometimes it is convenient to modify the content of
the Request object
before passing it on. Because this is most often used to rewrite
either the URL, the similarity to Apache’s mod_rewrite
has inspired the twisted.web.rewrite
module. Using this module is done via wrapping a resource with
a twisted.web.rewrite.RewriterResource which
then has rewrite rules. Rewrite rules are functions which accept a
request object, and possible modify it. After all rewrite rules run,
the child resolution chain continues as if the wrapped resource,
rather than the RewriterResource , was the child.
Here is an example, using the only rule currently supplied by Twisted itself:
default_root = rewrite.RewriterResource(default, rewrite.tildeToUsers)
This causes the URL /~foo/bar.html to be treated
like /users/foo/bar.html . If done after setting
default’s users child to a distrib.UserDirectory , it gives a
configuration similar to the classical configuration of web server,
common since the first NCSA servers.
Knowing When We’re Not Wanted¶
Sometimes it is useful to know when the other side has broken the connection. Here is an example which does that:
from twisted.web.resource import Resource
from twisted.web import server
from twisted.internet import reactor
from twisted.python.util import println
class ExampleResource(Resource):
def render_GET(self, request):
request.write("hello world")
d = request.notifyFinish()
d.addCallback(lambda _: println("finished normally"))
d.addErrback(println, "error")
reactor.callLater(10, request.finish)
return server.NOT_DONE_YET
resource = ExampleResource()
This will allow us to run statistics on the log-file to see how many users are frustrated after merely 10 seconds.
As-Is Serving¶
Sometimes, you want to be able to send headers and status
directly. While you can do this with a ResourceScript , an easier way is to
use ASISProcessor .
Use it by, for example, adding it as a processor for
the .asis extension. Here is a sample file:
HTTP/1.0 200 OK
Content-Type: text/html
Hello world
Web Application Development¶
Code layout¶
The development of a Twisted Web application should be orthogonal to its
deployment. This means is that if you are developing a web application, it
should be a resource with children, and internal links. Some of the children
might use Nevow , some
might be resources manually using .write , and so on. Regardless,
the code should be in a Python module, or package, outside the web
tree.
You will probably want to test your application as you develop it. There are
many ways to test, including dropping an .rpy which looks
like:
from mypackage import toplevel
resource = toplevel.Resource(file="foo/bar", color="blue")
into a directory, and then running:
% twistd web --path=/directory
You can also write a Python script like:
#!/usr/bin/env python
from twisted.web import server
from twisted.internet import reactor
from mypackage import toplevel
reactor.listenTCP(8080,
server.Site(toplevel.Resource(file="foo/bar", color="blue")))
reactor.run()
Web application deployment¶
Which one of these development strategies you use is not terribly important,
since (and this is the important part) deployment is orthogonal .
Later, when you want users to actually use your code, you should worry
about what to do – or rather, don’t. Users may have widely different needs.
Some may want to run your code in a different process, so they’ll use
distributed web (twisted.web.distrib ). Some may be
using the twisted-web Debian package, and will drop in:
% cat > /etc/local.d/99addmypackage.py
from mypackage import toplevel
default.putChild("mypackage", toplevel.Resource(file="foo/bar", color="blue"))
^D
If you want to be friendly to your users, you can supply many examples in
your package, like the above .rpy and the Debian-package drop-in.
But the ultimate friendliness is to write a useful resource which does
not have deployment assumptions built in.
Understanding resource scripts (.rpy files)¶
Twisted Web is not PHP – it has better tools for organizing code Python
modules and packages, so use them. In PHP, the only tool for organizing code is
a web page, which leads to silly things like PHP pages full of functions that
other pages import, and so on. If you were to write your code this way with
Twisted Web, you would do web development using many .rpy files,
all importing some Python module. This is a bad idea – it mashes
deployment with development, and makes sure your users will be tied to
the file-system.
We have .rpy s because they are useful and necessary.
But using them incorrectly leads to horribly unmaintainable
applications. The best way to ensure you are using them correctly is
to not use them at all, until you are on your final
deployment stages. You should then find your .rpy files
will be less than 10 lines, because you will not have more
than 10 lines to write.
HTML Templating with twisted.web.template¶
A Very Quick Introduction To Templating In Python¶
HTML templating is the process of transforming a template document (one which describes style and structure, but does not itself include any content) into some HTML output which includes information about objects in your application. There are many, many libraries for doing this in Python: to name a few, jinja2 , django templates , and clearsilver . You can easily use any of these libraries in your Twisted Web application, either by running them as WSGI applications or by calling your preferred templating system’s APIs to produce their output as strings, and then writing those strings to Request.write .
Before we begin explaining how to use it, I’d like to stress that you
don’t need to use Twisted’s templating system if you prefer some other
way to generate HTML. Use it if it suits your personal style or your
application, but feel free to use other things. Twisted includes templating for
its own use, because the twisted.web server needs to produce HTML
in various places, and we didn’t want to add another large dependency for that.
Twisted is not in any way incompatible with other systems, so that has
nothing to do with the fact that we use our own.
twisted.web.template - Why And How you Might Want to Use It¶
Twisted includes a templating system, twisted.web.template . This can be convenient for Twisted applications that want to produce some basic HTML for a web interface without an additional dependency.
twisted.web.template also includes
support for Deferred s, so
you can incrementally render the output of a page based on the results of Deferred s that your application
has returned. This feature is fairly unique among templating libraries.
In twisted.web.template , templates are XHTML files which also contain a special namespace for indicating dynamic portions of the document. For example:
<html xmlns:t="http://twistedmatrix.com/ns/twisted.web.template/0.1">
<body>
<div t:render="header" />
<div id="content">
<p>Content goes here.</p>
</div>
<div t:render="footer" />
</body>
</html>
The basic unit of templating is twisted.web.template.Element . An Element is given a way of
loading a bit of markup like the above example, and knows how to
correlate render attributes within that markup to Python methods
exposed with twisted.web.template.renderer :
from twisted.web.template import Element, renderer, XMLFile
from twisted.python.filepath import FilePath
class ExampleElement(Element):
loader = XMLFile(FilePath('template-1.xml'))
@renderer
def header(self, request, tag):
return tag('Header.')
@renderer
def footer(self, request, tag):
return tag('Footer.')
In order to combine the two, we must render the element. For this simple example, we can use the flattenString API, which will convert a single template object - such as an Element - into a Deferred which fires with a single string, the HTML output of the rendering process.
from twisted.web.template import flattenString
from element_1 import ExampleElement
def renderDone(output):
print output
flattenString(None, ExampleElement()).addCallback(renderDone)
This short program cheats a little bit; we know that there are no Deferred s in the template which
require the reactor to eventually fire; therefore, we can simply add a callback
which outputs the result. Also, none of the renderer functions
require the request object, so it’s acceptable to
pass None through here. (The ‘request’ object here is used only to
relay information about the rendering process to each renderer, so you may
always use whatever object makes sense for your application. Note, however,
that renderers from library code may require an IRequest .)
If you run it yourself, you can see that it produces the following output:
<html>
<body>
<div>Header.</div>
<div id="content">
<p>Content goes here.</p>
</div>
<div>Footer.</div>
</body>
</html>
The third parameter to a renderer method is a Tag object which represents the XML element
with the t:render attribute in the template. Calling a Tag adds children to the element
in the DOM, which may be strings, more Tag s, or other renderables such as Element s.
For example, to make the header and footer bold:
from twisted.web.template import Element, renderer, XMLFile, tags
from twisted.python.filepath import FilePath
class ExampleElement(Element):
loader = XMLFile(FilePath('template-1.xml'))
@renderer
def header(self, request, tag):
return tag(tags.b('Header.'))
@renderer
def footer(self, request, tag):
return tag(tags.b('Footer.'))
Rendering this in a similar way to the first example would produce:
<html>
<body>
<div><b>Header.</b></div>
<div id="content">
<p>Content goes here.</p>
</div>
<div><b>Footer.</b></div>
</body>
</html>
In addition to adding children, call syntax can be used to set attributes on a
tag. For example, to change the id on the div while
adding children:
from twisted.web.template import Element, renderer, XMLFile, tags
from twisted.python.filepath import FilePath
class ExampleElement(Element):
loader = XMLFile(FilePath('template-1.xml'))
@renderer
def header(self, request, tag):
return tag(tags.p('Header.'), id='header')
@renderer
def footer(self, request, tag):
return tag(tags.p('Footer.'), id='footer')
And this would produce the following page:
<html>
<body>
<div id="header"><p>Header.</p></div>
<div id="content">
<p>Content goes here.</p>
</div>
<div id="footer"><p>Footer.</p></div>
</body>
</html>
Calling a tag mutates it, it and returns the tag itself, so you can pass it
forward and call it multiple times if you have multiple children or attributes
to add to it. twisted.web.template also exposes some
convenient objects for building more complex markup structures from within
renderer methods in the tags object. In the examples above, we’ve
only used tags.p and tags.b , but there should be a tags.x for each x which is a valid HTML tag. There may be
some omissions, but if you find one, please feel free to file a bug.
Template Attributes¶
t:attr tags allow you to set HTML attributes
(like href in an <a href="... ) on an enclosing
element.
Slots¶
t:slot tags allow you to specify “slots” which you can
conveniently fill with multiple pieces of data straight from your Python
program.
The following example demonstrates both t:attr
and t:slot in action. Here we have a layout which displays a person’s
profile on your snazzy new Twisted-powered social networking site. We use
the t:attr tag to drop in the “src” attribute on the profile picture,
where the actual value of src attribute gets specified by a t:slot
tag within the t:attr tag. Confused? It should make more
sense when you see the code:
<div xmlns:t="http://twistedmatrix.com/ns/twisted.web.template/0.1"
t:render="person_profile"
class="profile">
<img><t:attr name="src"><t:slot name="profile_image_url" /></t:attr></img>
<p><t:slot name="person_name" /></p>
</div>
from twisted.web.template import Element, renderer, XMLFile
from twisted.python.filepath import FilePath
class ExampleElement(Element):
loader = XMLFile(FilePath('slots-attributes-1.xml'))
@renderer
def person_profile(self, request, tag):
# Note how convenient it is to pass these attributes in!
tag.fillSlots(person_name='Luke',
profile_image_url='http://example.com/user.png')
return tag
<div class="profile">
<img src="http://example.com/user.png" />
<p>Luke</p>
</div>
Iteration¶
Often, you will have a sequence of things, and want to render each of them,
repeating a part of the template for each one. This can be done by
cloning tag in your renderer:
<ul xmlns:t="http://twistedmatrix.com/ns/twisted.web.template/0.1">
<li t:render="widgets"><t:slot name="widgetName"/></li>
</ul>
from twisted.web.template import Element, renderer, XMLFile, flattenString
from twisted.python.filepath import FilePath
class WidgetsElement(Element):
loader = XMLFile(FilePath('iteration-1.xml'))
widgetData = ['gadget', 'contraption', 'gizmo', 'doohickey']
@renderer
def widgets(self, request, tag):
for widget in self.widgetData:
yield tag.clone().fillSlots(widgetName=widget)
def printResult(result):
print result
flattenString(None, WidgetsElement()).addCallback(printResult)
<ul>
<li>gadget</li><li>contraption</li><li>gizmo</li><li>doohickey</li>
</ul>
This renderer works because a renderer can return anything that can be
rendered, not just tag . In this case, we define a generator, which
returns a thing that is iterable. We also could have returned
a list . Anything that is iterable will be rendered by twisted.web.template rendering each item in it. In
this case, each item is a copy of the tag the renderer received, each filled
with the name of a widget.
Sub-views¶
Another common pattern is to delegate the rendering logic for a small part of
the page to a separate Element . For example, the widgets from the
iteration example above might be more complicated to render. You can define
an Element subclass which can render a single widget. The renderer
method on the container can then yield instances of this
new Element subclass.
<ul xmlns:t="http://twistedmatrix.com/ns/twisted.web.template/0.1">
<li t:render="widgets"><span t:render="name" /></li>
</ul>
from twisted.web.template import (
XMLFile, TagLoader, Element, renderer, flattenString)
from twisted.python.filepath import FilePath
class WidgetsElement(Element):
loader = XMLFile(FilePath('subviews-1.xml'))
widgetData = ['gadget', 'contraption', 'gizmo', 'doohickey']
@renderer
def widgets(self, request, tag):
for widget in self.widgetData:
yield WidgetElement(TagLoader(tag), widget)
class WidgetElement(Element):
def __init__(self, loader, name):
Element.__init__(self, loader)
self._name = name
@renderer
def name(self, request, tag):
return tag(self._name)
def printResult(result):
print result
flattenString(None, WidgetsElement()).addCallback(printResult)
<ul>
<li><span>gadget</span></li><li><span>contraption</span></li><li><span>gizmo</span></li><li><span>doohickey</span></li>
</ul>
TagLoader lets the portion of the overall template related to
widgets be re-used for WidgetElement , which is otherwise a
normal Element subclass not much different
from WidgetsElement . Notice that the name renderer on
the span tag in this template is satisfied
from WidgetElement , not WidgetsElement .
Transparent¶
Note how renderers, slots and attributes require you to specify a renderer on
some outer HTML element. What if you don’t want to be forced to add an element
to your DOM just to drop some content into it? Maybe it messes with your
layout, and you can’t get it to work in IE with that extra div
tag? Perhaps you need t:transparent , which allows you to drop some
content in without any surrounding “container” tag. For example:
<div xmlns:t="http://twistedmatrix.com/ns/twisted.web.template/0.1">
<!-- layout decision - these things need to be *siblings* -->
<t:transparent t:render="renderer1" />
<t:transparent t:render="renderer2" />
</div>
from twisted.web.template import Element, renderer, XMLFile
from twisted.python.filepath import FilePath
class ExampleElement(Element):
loader = XMLFile(FilePath('transparent-1.xml'))
@renderer
def renderer1(self, request, tag):
return tag("hello")
@renderer
def renderer2(self, request, tag):
return tag("world")
<div>
<!-- layout decision - these things need to be *siblings* -->
hello
world
</div>
Quoting¶
twisted.web.template will quote any strings that place into the DOM. This provides protection against XSS attacks , in addition to just generally making it easy to put arbitrary strings onto a web page, without worrying about what they might have in them. This can easily be demonstrated with an element using the same template from our earlier examples. Here’s an element that returns some “special” characters in HTML (‘<’, ‘>’, and ‘”’, which is special in attribute values):
from twisted.web.template import Element, renderer, XMLFile
from twisted.python.filepath import FilePath
class ExampleElement(Element):
loader = XMLFile(FilePath('template-1.xml'))
@renderer
def header(self, request, tag):
return tag('<<<Header>>>!')
@renderer
def footer(self, request, tag):
return tag('>>>"Footer!"<<<', id='<"fun">')
Note that they are all safely quoted in the output, and will appear in a web browser just as you returned them from your Python method:
<html>
<body>
<div><<<Header>>>!</div>
<div id="content">
<p>Content goes here.</p>
</div>
<div id="<"fun">">>>>"Footer!"<<<</div>
</body>
</html>
Deferreds¶
Finally, a simple demonstration of Deferred support, the unique feature of twisted.web.template . Simply put, any renderer may return a Deferred which fires with some template content instead of the template content itself. As shown above, flattenString will return a Deferred that fires with the full content of the string. But if there’s a lot of content, you might not want to wait before starting to send some of it to your HTTP client: for that case, you can use flatten . It’s difficult to demonstrate this directly in a browser-based application; unless you insert very long delays before firing your Deferreds, it just looks like your browser is instantly displaying everything. Here’s an example that just prints out some HTML template, with markers inserted for where certain events happen:
import sys
from twisted.web.template import XMLString, Element, renderer, flatten
from twisted.internet.defer import Deferred
sample = XMLString(
"""
<div xmlns:t="http://twistedmatrix.com/ns/twisted.web.template/0.1">
Before waiting ...
<span t:render="wait"></span>
... after waiting.
</div>
""")
class WaitForIt(Element):
def __init__(self):
Element.__init__(self, loader=sample)
self.deferred = Deferred()
@renderer
def wait(self, request, tag):
return self.deferred.addCallback(
lambda aValue: tag("A value: " + repr(aValue)))
def done(ignore):
print("[[[Deferred fired.]]]")
print('[[[Rendering the template.]]]')
it = WaitForIt()
flatten(None, it, sys.stdout.write).addCallback(done)
print('[[[In progress... now firing the Deferred.]]]')
it.deferred.callback("<value>")
print('[[[All done.]]]')
If you run this example, you should get the following output:
[[[Rendering the template.]]]
<div>
Before waiting ...
[[[In progress... now firing the Deferred.]]]
<span>A value: '<value>'</span>
... after waiting.
</div>[[[Deferred fired.]]]
[[[All done.]]]
This demonstrates that part of the output (everything up to
“[[[In progress... ”) is written out immediately as it’s rendered.
But once it hits the Deferred, WaitForIt ‘s rendering needs to pause
until .callback(...) is called on that Deferred. You can see that
no further output is produced until the message indicating that the Deferred is
being fired is complete. By returning Deferreds and using flatten , you can avoid buffering large
amounts of data.
A Brief Note on Formats and DOCTYPEs¶
The goal of twisted.web.template is to emit both valid HTML or XHTML .
However, in order to get the maximally standards-compliant output format you
desire, you have to know which one you want, and take a few simple steps to emit
it correctly. Many browsers will probably work with most output if you ignore
this section entirely, but the HTML specification recommends that you specify an appropriate DOCTYPE .
As a DOCTYPE declaration in your template would describe the
template itself, rather than its output, it won’t be included in your output.
If you wish to annotate your template output with a DOCTYPE, you will have to
write it to the browser out of band. One way to do this would be to simply
do request.write('<!DOCTYPE html>\n') when you are ready to
begin emitting your response. The same goes for an XML DOCTYPE
declaration.
twisted.web.template will remove the xmlns attributes
used to declare
the http://twistedmatrix.com/ns/twisted.web.template/0.1 namespace,
but it will not modify other namespace declaration attributes. Therefore if you
wish to serialize in HTML format, you should not use other namespaces; if you
wish to serialize to XML, feel free to insert any namespace declarations that
are appropriate, and they will appear in your output.
Note
This relaxed approach is correct in many cases. However, in certain contexts -
especially <script> and <style> tags - quoting rules differ in
significant ways between HTML and XML, and between different browsers’ parsers
in HTML. If you want to generate dynamic content inside a script or stylesheet,
the best option is to load the resource externally so you don’t have to worry
about quoting rules. The second best option is to strictly configure your
content-types and DOCTYPE declarations for XML, whose quoting rules are simple
and compatible with the approach that twisted.web.template takes.
And, please remember: regardless of how you put it there, any user input placed
inside a <script> or <style> tag is a potential security issue.
A Bit of History¶
Those of you who used Divmod Nevow may notice some
similarities. twisted.web.template is in fact derived from the
latest version of Nevow, but includes only the latest components from Nevow’s
rendering pipeline, and does not have any of the legacy compatibility layers
that Nevow grew over time. This should make
using twisted.web.template a similar experience for many long-time
users of Twisted who have previously used Nevow for its twisted-friendly
templating, but more straightforward for new users.
Creating XML-RPC Servers and Clients with Twisted¶
Introduction¶
XML-RPC is a simple request/reply protocol that runs over HTTP. It is simple, easy to implement and supported by most programming languages. Twisted’s XML-RPC support is implemented using the`xmlrpclib <http://docs.python.org/library/xmlrpclib.html>`_ library that is included with Python 2.2 and later.
Creating a XML-RPC server¶
Making a server is very easy - all you need to do is inherit from twisted.web.xmlrpc.XMLRPC .
You then create methods beginning with xmlrpc_ . The methods’
arguments determine what arguments it will accept from XML-RPC clients.
The result is what will be returned to the clients.
Methods published via XML-RPC can return all the basic XML-RPC
types, such as strings, lists and so on (just return a regular python
integer, etc). They can also raise exceptions or return Failure instances to indicate an
error has occurred, or Binary , Boolean or DateTime
instances (all of these are the same as the respective classes in xmlrpclib. In
addition, XML-RPC published methods can return Deferred instances whose results are one of the above. This allows
you to return results that can’t be calculated immediately, such as database queries.
See the Deferred documentation for more
details.
XMLRPC instances
are Resource objects, and they can thus be published using a Site. The
following example has two methods published via XML-RPC, add(a, b) and echo(x) .
from twisted.web import xmlrpc, server
class Example(xmlrpc.XMLRPC):
"""
An example object to be published.
"""
def xmlrpc_echo(self, x):
"""
Return all passed args.
"""
return x
def xmlrpc_add(self, a, b):
"""
Return sum of arguments.
"""
return a + b
def xmlrpc_fault(self):
"""
Raise a Fault indicating that the procedure should not be used.
"""
raise xmlrpc.Fault(123, "The fault procedure is faulty.")
if __name__ == '__main__':
from twisted.internet import reactor
r = Example()
reactor.listenTCP(7080, server.Site(r))
reactor.run()
After we run this command, we can connect with a client and send commands to the server:
>>> import xmlrpclib
>>> s = xmlrpclib.Server('http://localhost:7080/')
>>> s.echo("lala")
'lala'
>>> s.add(1, 2)
3
>>> s.fault()
Traceback (most recent call last):
...
xmlrpclib.Fault: <Fault 123: 'The fault procedure is faulty.'>
>>>
If the Request object is
needed by an xmlrpc_* method, it can be made available using
the twisted.web.xmlrpc.withRequest decorator. When
using this decorator, the method will be passed the request object as the first
argument, before any XML-RPC parameters. For example:
from twisted.web.xmlrpc import XMLRPC, withRequest
from twisted.web.server import Site
class Example(XMLRPC):
@withRequest
def xmlrpc_headerValue(self, request, headerName):
return request.requestHeaders.getRawHeaders(headerName)
if __name__ == '__main__':
from twisted.internet import reactor
reactor.listenTCP(7080, Site(Example()))
reactor.run()
XML-RPC resources can also be part of a normal Twisted web server, using resource scripts. The following is an example of such a resource script:
from twisted.web import xmlrpc
import os
def getQuote():
return "What are you talking about, William?"
class Quoter(xmlrpc.XMLRPC):
def xmlrpc_quote(self):
return getQuote()
resource = Quoter()
Using XML-RPC sub-handlers¶
XML-RPC resource can be nested so that one handler calls another if
a method with a given prefix is called. For example, to add support
for an XML-RPC method date.time() to
the Example class, you could do the
following:
import time
from twisted.web import xmlrpc, server
class Example(xmlrpc.XMLRPC):
"""
An example object to be published.
"""
def xmlrpc_echo(self, x):
"""
Return all passed args.
"""
return x
def xmlrpc_add(self, a, b):
"""
Return sum of arguments.
"""
return a + b
class Date(xmlrpc.XMLRPC):
"""
Serve the XML-RPC 'time' method.
"""
def xmlrpc_time(self):
"""
Return UNIX time.
"""
return time.time()
if __name__ == '__main__':
from twisted.internet import reactor
r = Example()
date = Date()
r.putSubHandler('date', date)
reactor.listenTCP(7080, server.Site(r))
reactor.run()
By default, a period (‘.’) separates the prefix from the method
name, but you can use a different character by overriding the XMLRPC.separator data member in your base
XML-RPC server. XML-RPC servers may be nested to arbitrary depths
using this method.
Using your own procedure getter¶
Sometimes, you want to implement your own policy of getting the end implementation.
E.g. just like sub-handlers you want to divide the implementations into separate classes but
may not want to introduce XMLRPC.separator in the procedure name.
In such cases just override the lookupProcedure(self, procedurePath)
method and return the correct callable.
Raise twisted.web.xmlrpc.NoSuchFunction otherwise.
from twisted.web import xmlrpc, server
class EchoHandler:
def echo(self, x):
"""
Return all passed args
"""
return x
class AddHandler:
def add(self, a, b):
"""
Return sum of arguments.
"""
return a + b
class Example(xmlrpc.XMLRPC):
"""
An example of using you own policy to fetch the handler
"""
def __init__(self):
xmlrpc.XMLRPC.__init__(self)
self._addHandler = AddHandler()
self._echoHandler = EchoHandler()
#We keep a dict of all relevant
#procedure names and callable.
self._procedureToCallable = {
'add':self._addHandler.add,
'echo':self._echoHandler.echo
}
def lookupProcedure(self, procedurePath):
try:
return self._procedureToCallable[procedurePath]
except KeyError, e:
raise xmlrpc.NoSuchFunction(self.NOT_FOUND,
"procedure %s not found" % procedurePath)
def listProcedures(self):
"""
Since we override lookupProcedure, its suggested to override
listProcedures too.
"""
return ['add', 'echo']
if __name__ == '__main__':
from twisted.internet import reactor
r = Example()
reactor.listenTCP(7080, server.Site(r))
reactor.run()
Adding XML-RPC Introspection support¶
XML-RPC has an
informal IntrospectionAPI that specifies three methods in a system
sub-handler which allow a client to query a server about the server’s
API. Adding Introspection support to
the Example class is easy using
the XMLRPCIntrospection class:
from twisted.web import xmlrpc, server
class Example(xmlrpc.XMLRPC):
"""An example object to be published."""
def xmlrpc_echo(self, x):
"""Return all passed args."""
return x
xmlrpc_echo.signature = [['string', 'string'],
['int', 'int'],
['double', 'double'],
['array', 'array'],
['struct', 'struct']]
def xmlrpc_add(self, a, b):
"""Return sum of arguments."""
return a + b
xmlrpc_add.signature = [['int', 'int', 'int'],
['double', 'double', 'double']]
xmlrpc_add.help = "Add the arguments and return the sum."
if __name__ == '__main__':
from twisted.internet import reactor
r = Example()
xmlrpc.addIntrospection(r)
reactor.listenTCP(7080, server.Site(r))
reactor.run()
Note the method attributes help
and signature which are used by the
Introspection API methods system.methodHelp
and system.methodSignature respectively. If
no help attribute is specified, the
method’s documentation string is used instead.
SOAP Support¶
From the point of view of a Twisted developer, there is little difference between XML-RPC support and SOAP support. Here is an example of SOAP usage:
from twisted.web import soap
import os
def getQuote():
return "That beverage, sir, is off the hizzy."
class Quoter(soap.SOAPPublisher):
"""Publish one method, 'quote'."""
def soap_quote(self):
return getQuote()
resource = Quoter()
Creating an XML-RPC Client¶
XML-RPC clients in Twisted are meant to look as something which will be
familiar either to xmlrpclib or to Perspective Broker users,
taking features from both, as appropriate. There are two major deviations
from the xmlrpclib way which should be noted:
- No implicit
/RPC2. If the services uses this path for the XML-RPC calls, then it will have to be given explicitly. - No magic
__getattr__: calls must be made by an explicitcallRemote.
The interface Twisted presents to XML-RPC client is that of a proxy object: twisted.web.xmlrpc.Proxy . The constructor for the object receives a URL: it must be an HTTP or HTTPS URL. When an XML-RPC service is described, the URL to that service will be given there.
Having a proxy object, one can just call the callRemote method,
which accepts a method name and a variable argument list (but no named
arguments, as these are not supported by XML-RPC). It returns a deferred,
which will be called back with the result. If there is any error, at any
level, the errback will be called. The exception will be the relevant Twisted
error in the case of a problem with the underlying connection (for example,
a timeout), IOError containing the status and message in the case
of a non-200 status or a xmlrpclib.Fault in the case of an
XML-RPC level problem.
from twisted.web.xmlrpc import Proxy
from twisted.internet import reactor
def printValue(value):
print repr(value)
reactor.stop()
def printError(error):
print 'error', error
reactor.stop()
proxy = Proxy('http://advogato.org/XMLRPC')
proxy.callRemote('test.sumprod', 3, 5).addCallbacks(printValue, printError)
reactor.run()
prints:
[8, 15]
Serving SOAP and XML-RPC simultaneously¶
twisted.web.xmlrpc.XMLRPC and twisted.web.soap.SOAPPublisher are both Resource s. So, to serve both XML-RPC and SOAP in the one web server, you can use the putChild method of Resource.
The following example uses an empty resource.Resource as the root resource for
a Site , and then
adds /RPC2 and /SOAP paths to it.
from twisted.web import soap, xmlrpc, resource, server
import os
def getQuote():
return "Victory to the burgeois, you capitalist swine!"
class XMLRPCQuoter(xmlrpc.XMLRPC):
def xmlrpc_quote(self):
return getQuote()
class SOAPQuoter(soap.SOAPPublisher):
def soap_quote(self):
return getQuote()
def main():
from twisted.internet import reactor
root = resource.Resource()
root.putChild('RPC2', XMLRPCQuoter())
root.putChild('SOAP', SOAPQuoter())
reactor.listenTCP(7080, server.Site(root))
reactor.run()
if __name__ == '__main__':
main()
Refer to Twisted Web Development for more details about Resources.
Twisted Web In 60 Seconds¶
Serving Static Content From a Directory¶
The goal of this example is to show you how to serve static content from a filesystem. First, we need to import some objects:
Site, an IProtocolFactory which glues a listening server port (IListeningPort) to the HTTPChannel implementation:
from twisted.web.server import Site
File, an IResource which glues the HTTP protocol implementation to the filesystem:
from twisted.web.static import File
The reactor, which drives the whole process, actually accepting TCP connections and moving bytes into and out of them:
from twisted.internet import reactor
Next, we create an instance of the File resource pointed at the directory to serve:
resource = File("/tmp")
Then we create an instance of the Site factory with that resource:
factory = Site(resource)
Now we glue that factory to a TCP port:
reactor.listenTCP(8888, factory)
Finally, we start the reactor so it can make the program work:
reactor.run()
And that’s it. Here’s the complete program:
from twisted.web.server import Site
from twisted.web.static import File
from twisted.internet import reactor
resource = File('/tmp')
factory = Site(resource)
reactor.listenTCP(8888, factory)
reactor.run()
Bonus example!
For those times when you don’t actually want to write a new program, the above implemented functionality is one of the things the command line twistd tool can do.
In this case, the command
twistd -n web --path /tmp
will accomplish the same thing as the above server.
See helper programs in the Twisted Core documentation for more information on using twistd.
Generating a Page Dynamically¶
The goal of this example is to show you how to dynamically generate the contents of a page.
Taking care of some of the necessary imports first, we’ll import Site and the reactor :
from twisted.internet import reactor
from twisted.web.server import Site
The Site is a factory which associates a listening port with the HTTP protocol implementation. The reactor is the main loop that drives any Twisted application; we’ll use it to actually create the listening port in a moment.
Next, we’ll import one more thing from Twisted
Web: Resource . An
instance of Resource (or a subclass) represents a page
(technically, the entity addressed by a URI).
from twisted.web.resource import Resource
Since we’re going to make the demo resource a clock, we’ll also import the time module:
import time
With imports taken care of, the next step is to define
a Resource subclass which has the dynamic rendering
behavior we want. Here’s a resource which generates a page giving the
time:
class ClockPage(Resource):
isLeaf = True
def render_GET(self, request):
return "<html><body>%s</body></html>" % (time.ctime(),)
Setting isLeaf to True indicates
that ClockPage resources will never have any
children.
The render_GET method here will be called whenever the URI we
hook this resource up to is requested with the GET method. The byte
string it returns is what will be sent to the browser.
With the resource defined, we can create a Site from it:
resource = ClockPage()
factory = Site(resource)
Just as with the previous static content example, this
configuration puts our resource at the very top of the URI hierarchy,
ie at / . With that Site instance, we can
tell the reactor to create a TCP server and start servicing requests:
reactor.listenTCP(8880, factory)
reactor.run()
Here’s the code with no interruptions:
from twisted.internet import reactor
from twisted.web.server import Site
from twisted.web.resource import Resource
import time
class ClockPage(Resource):
isLeaf = True
def render_GET(self, request):
return "<html><body>%s</body></html>" % (time.ctime(),)
resource = ClockPage()
factory = Site(resource)
reactor.listenTCP(8880, factory)
reactor.run()
Static URL Dispatch¶
The goal of this example is to show you how to serve different content at different URLs.
The key to understanding how different URLs are handled with the resource
APIs in Twisted Web is understanding that any URL can be used to address a node
in a tree. Resources in Twisted Web exist in such a tree, and a request for a
URL will be responded to by the resource which that URL addresses. The
addressing scheme considers only the path segments of the URL. Starting with the
root resource (the one used to construct the Site ) and the first
path segment, a child resource is looked up. As long as there are more path
segments, this process is repeated using the result of the previous lookup and
the next path segment. For example, to handle a request
for "/foo/bar" , first the root’s "foo" child is
retrieved, then that resource’s "bar" child is retrieved, then that
resource is used to create the response.
With that out of the way, let’s consider an example that can serve a few different resources at a few different URLs.
First things first: we need to import Site , the factory for HTTP servers, Resource , a convenient base class for custom pages, and reactor , the object which implements the Twisted main loop. We’ll also import File to use as the resource at one of the example URLs.
from twisted.web.server import Site
from twisted.web.resource import Resource
from twisted.internet import reactor
from twisted.web.static import File
Now we create a resource which will correspond to the root of the URL hierarchy: all URLs are children of this resource.
root = Resource()
Here comes the interesting part of this example. We’re now going to
create three more resources and attach them to the three
URLs /foo , /bar , and /baz :
root.putChild("foo", File("/tmp"))
root.putChild("bar", File("/lost+found"))
root.putChild("baz", File("/opt"))
Last, all that’s required is to create a Site with the root
resource, associate it with a listening server port, and start the reactor:
factory = Site(root)
reactor.listenTCP(8880, factory)
reactor.run()
With this server running, http://localhost:8880/foo
will serve a listing of files
from /tmp , http://localhost:8880/bar will
serve a listing of files from /lost+found ,
and http://localhost:8880/baz will serve a listing of
files from /opt .
Here’s the whole example uninterrupted:
from twisted.web.server import Site
from twisted.web.resource import Resource
from twisted.internet import reactor
from twisted.web.static import File
root = Resource()
root.putChild("foo", File("/tmp"))
root.putChild("bar", File("/lost+found"))
root.putChild("baz", File("/opt"))
factory = Site(root)
reactor.listenTCP(8880, factory)
reactor.run()
Dynamic URL Dispatch¶
In the previous example we covered how to statically configure Twisted Web to serve different content at different URLs. The goal of this example is to show you how to do this dynamically instead. Reading the previous installment if you haven’t already is suggested in order to get an overview of how URLs are treated when using Twisted Web’s resource APIs.
Site (the object which associates a listening server port with the HTTP implementation), Resource (a convenient base class to use when defining custom pages), and reactor (the object which implements the Twisted main loop) return once again:
from twisted.web.server import Site
from twisted.web.resource import Resource
from twisted.internet import reactor
With that out of the way, here’s the interesting part of this
example. We’re going to define a resource which renders a whole-year
calendar. The year it will render the calendar for will be the year in
the request URL. So, for example, /2009 will render a
calendar for 2009. First, here’s a resource that renders a calendar
for the year passed to its initializer:
from calendar import calendar
class YearPage(Resource):
def __init__(self, year):
Resource.__init__(self)
self.year = year
def render_GET(self, request):
return "<html><body><pre>%s</pre></body></html>" % (calendar(self.year),)
Pretty simple - not all that different from the first dynamic resource
demonstrated in Generating a Page Dynamically . Now here’s the resource that handles URLs with a year in them
by creating a suitable instance of this YearPage class:
class Calendar(Resource):
def getChild(self, name, request):
return YearPage(int(name))
By implementing getChild here, we’ve just defined
how Twisted Web should find children of Calendar instances when
it’s resolving an URL into a resource. This implementation defines all integers
as the children of Calendar (and punts on error handling, more on
that later).
All that’s left is to create a Site using this resource as its
root and then start the reactor:
root = Calendar()
factory = Site(root)
reactor.listenTCP(8880, factory)
reactor.run()
And that’s all. Any resource-based dynamic URL handling is going to look
basically like Calendar.getChild . Here’s the full example code:
from twisted.web.server import Site
from twisted.web.resource import Resource
from twisted.internet import reactor
from calendar import calendar
class YearPage(Resource):
def __init__(self, year):
Resource.__init__(self)
self.year = year
def render_GET(self, request):
return "<html><body><pre>%s</pre></body></html>" % (calendar(self.year),)
class Calendar(Resource):
def getChild(self, name, request):
return YearPage(int(name))
root = Calendar()
factory = Site(root)
reactor.listenTCP(8880, factory)
reactor.run()
Error Handling¶
In this example we’ll extend dynamic dispatch to return a 404 (not found) response when a client requests a non-existent URL.
As in the previous examples, we’ll start with Site , Resource , and reactor imports:
from twisted.web.server import Site
from twisted.web.resource import Resource
from twisted.internet import reactor
Next, we’ll add one more import. NoResource is one of the pre-defined error resources provided by Twisted Web. It generates the necessary 404 response code and renders a simple html page telling the client there is no such resource.
from twisted.web.resource import NoResource
Next, we’ll define a custom resource which does some dynamic URL dispatch. This example is going to be just like the previous one , where the path segment is interpreted as a year; the difference is that this time we’ll handle requests which don’t conform to that pattern by returning the not found response:
class Calendar(Resource):
def getChild(self, name, request):
try:
year = int(name)
except ValueError:
return NoResource()
else:
return YearPage(year)
Aside from including the definition of YearPage from
the previous example, the only other thing left to do is the
normal Site and reactor setup. Here’s the
complete code for this example:
from twisted.web.server import Site
from twisted.web.resource import Resource
from twisted.internet import reactor
from twisted.web.resource import NoResource
from calendar import calendar
class YearPage(Resource):
def __init__(self, year):
Resource.__init__(self)
self.year = year
def render_GET(self, request):
return "<html><body><pre>%s</pre></body></html>" % (calendar(self.year),)
class Calendar(Resource):
def getChild(self, name, request):
try:
year = int(name)
except ValueError:
return NoResource()
else:
return YearPage(year)
root = Calendar()
factory = Site(root)
reactor.listenTCP(8880, factory)
reactor.run()
This server hands out the same calendar views as the one from the previous installment, but it will also hand out a nice error page with a 404 response when a request is made for a URL which cannot be interpreted as a year.
Custom Response Codes¶
The previous example introduced NoResource , a Twisted Web error resource which
responds with a 404 (not found) code. This example will cover the APIs
that NoResource uses to do this so that you can generate your own
custom response codes as desired.
First, the now-standard import preamble:
from twisted.web.server import Site
from twisted.web.resource import Resource
from twisted.internet import reactor
Now we’ll define a new resource class that always returns a 402 (payment required) response. This is really not very different from the resources that was defined in previous examples. The fact that it has a response code other than 200 doesn’t change anything else about its role. This will require using the request object, though, which none of the previous examples have done.
The Request object has
shown up in a couple of places, but so far we’ve ignored it. It is a parameter
to the getChild
API as well as to render methods such as render_GET . As you might
have suspected, it represents the request for which a response is to be
generated. Additionally, it also represents the response being generated. In
this example we’re going to use its setResponseCode method to - you guessed
it - set the response’s status code.
class PaymentRequired(Resource):
def render_GET(self, request):
request.setResponseCode(402)
return "<html><body>Please swipe your credit card.</body></html>"
Just like the other resources I’ve demonstrated, this one returns a
string from its render_GET method to define the body of
the response. All that’s different is the call
to setResponseCode to override the default response code,
200, with a different one.
Finally, the code to set up the site and reactor. We’ll put an instance of
the above defined resource at /buy :
root = Resource()
root.putChild("buy", PaymentRequired())
factory = Site(root)
reactor.listenTCP(8880, factory)
reactor.run()
Here’s the complete example:
from twisted.web.server import Site
from twisted.web.resource import Resource
from twisted.internet import reactor
class PaymentRequired(Resource):
def render_GET(self, request):
request.setResponseCode(402)
return "<html><body>Please swipe your credit card.</body></html>"
root = Resource()
root.putChild("buy", PaymentRequired())
factory = Site(root)
reactor.listenTCP(8880, factory)
reactor.run()
Run the server and visit http://localhost:8880/buy in your
browser. It’ll look pretty boring, but if you use Firefox’s View Page Info
right-click menu item (or your browser’s equivalent), you’ll be able to see that
the server indeed sent back a 402 response code.
Handling POSTs¶
All of the previous examples have focused on GET
requests. Unlike GET requests, POST requests can have
a request body - extra data after the request headers; for example, data
representing the contents of an HTML form. Twisted Web makes this data available
to applications via the Request object.
Here’s an example web server which renders a static HTML form and then
generates a dynamic page when that form is posted back to it. Disclaimer: While
it’s convenient for this example, it’s often not a good idea to make a resource
that POST s to itself; this isn’t about Twisted Web, but the nature
of HTTP in general; if you do this in a real application, make sure you
understand the possible negative consequences.
As usual, we start with some imports. In addition to the Twisted imports,
this example uses the cgi module to escape user-enteredcontent for inclusion in the output.
from twisted.web.server import Site
from twisted.web.resource import Resource
from twisted.internet import reactor
import cgi
Next, we’ll define a resource which is going to do two things. First, it will
respond to GET requests with a static HTML form:
class FormPage(Resource):
def render_GET(self, request):
return '<html><body><form method="POST"><input name="the-field" type="text" /></form></body></html>'
This is similar to the resource used in a previous installment . However, we’ll now add
one more method to give it a second behavior; this render_POST
method will allow it to accept POST requests:
...
def render_POST(self, request):
return '<html><body>You submitted: %s</body></html>' % (cgi.escape(request.args["the-field"][0]),)
The main thing to note here is the use
of request.args . This is a dictionary-like object that
provides access to the contents of the form. The keys in this
dictionary are the names of inputs in the form. Each value is a list
containing strings (since there can be multiple inputs with the same
name), which is why we had to extract the first element to pass
to cgi.escape . request.args will be
populated from form contents whenever a POST request is
made with a content type
of application/x-www-form-urlencoded
or multipart/form-data (it’s also populated by query
arguments for any type of request).
Finally, the example just needs the usual site creation and port setup:
root = Resource()
root.putChild("form", FormPage())
factory = Site(root)
reactor.listenTCP(8880, factory)
reactor.run()
Run the server and visit http://localhost:8880/form , submit the form, and watch it generate a page including the value you entered into the single field.
Here’s the complete source for the example:
from twisted.web.server import Site
from twisted.web.resource import Resource
from twisted.internet import reactor
import cgi
class FormPage(Resource):
def render_GET(self, request):
return '<html><body><form method="POST"><input name="the-field" type="text" /></form></body></html>'
def render_POST(self, request):
return '<html><body>You submitted: %s</body></html>' % (cgi.escape(request.args["the-field"][0]),)
root = Resource()
root.putChild("form", FormPage())
factory = Site(root)
reactor.listenTCP(8880, factory)
reactor.run()
Other Request Bodies¶
The previous example demonstrated how to accept the payload of
a POST carrying HTML form data. What about POST
requests with data in some other format? Or even PUT requests?
Here is an example which demonstrates how to get any request body,
regardless of its format - using the request’s
content attribute.
The only significant difference between this example and the previous is that
instead of accessing request.args
in render_POST , it
uses request.content to get the request’s body
directly:
...
def render_POST(self, request):
return '<html><body>You submitted: %s</body></html>' % (cgi.escape(request.content.read()),)
request.content is a file-like object, so the
body is read from it. The exact type may vary, so avoid relying on non-file
methods you may find (such as getvalue when happens
to be a StringIO instance).
Here’s the complete source for this example - again, almost identical to the
previous POST example, with
only render_POST changed:
from twisted.web.server import Site
from twisted.web.resource import Resource
from twisted.internet import reactor
import cgi
class FormPage(Resource):
def render_GET(self, request):
return '<html><body><form method="POST"><input name="the-field" type="text" /></form></body></html>'
def render_POST(self, request):
return '<html><body>You submitted: %s</body></html>' % (cgi.escape(request.content.read()),)
root = Resource()
root.putChild("form", FormPage())
factory = Site(root)
reactor.listenTCP(8880, factory)
reactor.run()
rpy scripts (or, how to save yourself some typing)¶
The goal of this installment is to show you another way to run a Twisted Web server with a custom resource which doesn’t require as much code as the previous examples.
The feature in question is called an rpy script . An rpy script
is a Python source file which defines a resource and can be loaded into a
Twisted Web server. The advantages of this approach are that you don’t have to
write code to create the site or set up a listening port with the reactor. That
means fewer lines of code that aren’t dedicated to the task you’re trying to
accomplish.
There are some disadvantages, though. An rpy script must have the
extension .rpy . This means you can’t import it using the
usual Python import statement. This means it’s hard to re-use code in
an rpy script. This also means you can’t easily unit test it. The code
in an rpy script is evaluated in an unusual context. So, while rpy
scripts may be useful for testing out ideas, they’re not recommend for
much more than that.
Okay, with that warning out of the way, let’s dive in. First, as mentioned,
rpy scripts are Python source files with the .rpy extension. So,
open up an appropriately named file (for example, example.rpy ) and
put this code in it:
import time
from twisted.web.resource import Resource
class ClockPage(Resource):
isLeaf = True
def render_GET(self, request):
return "<html><body>%s</body></html>" % (time.ctime(),)
resource = ClockPage()
You may recognize this as the resource from
the first dynamic rendering example . What’s different is what you don’t see: we didn’t
import reactor or Site . There are no calls
to listenTCP or run . Instead, and this is
the core idea for rpy scripts, we just bound the
name resource to the resource we want the script to
serve. Every rpy script must bind this name, and this name is the only
thing Twisted Web will pay attention to in an rpy script.
All that’s left is to drop this rpy script into a Twisted Web server. There
are a few ways to do this. The simplest way is with twistd :
$ twistd -n web --path .
Hit http://localhost:8080/example.rpy
to see it run. You can pass other arguments here too. twistd web
has options for specifying which port number to bind, whether to set up an HTTPS
server, and plenty more. Other options you can pass to twistd allow
you to configure logging to work differently, to select a different reactor,
etc. For a full list of options, see twistd --help and twistd web --help .
Asynchronous Responses¶
In all of the previous examples, the resource examples presented generated responses immediately. One of the features of prime interest of Twisted Web, though, is the ability to generate a response over a longer period of time while leaving the server free to respond to other requests. In other words, asynchronously. In this installment, we’ll write a resource like this.
A resource that generates a response asynchronously looks like one that generates a response synchronously in many ways. The same base class, Resource , is used either way; the same render methods are used. There are three basic differences, though.
First, instead of returning the string which will be used as the
body of the response, the resource uses Request.write . This method can be
called repeatedly. Each call appends another string to the response
body. Second, when the entire response body has been passed
to Request.write , the application must
call Request.finish . As you might expect
from the name, this ends the response. Finally, in order to make
Twisted Web not end the response as soon as the render method returns,
the render method must return NOT_DONE_YET . Consider this
example:
from twisted.web.resource import Resource
from twisted.web.server import NOT_DONE_YET
from twisted.internet import reactor
class DelayedResource(Resource):
def _delayedRender(self, request):
request.write("<html><body>Sorry to keep you waiting.</body></html>")
request.finish()
def render_GET(self, request):
reactor.callLater(5, self._delayedRender, request)
return NOT_DONE_YET
If you’re not familiar with the reactor callLater
method, all you really need to know about it to understand this
example is that the above usage of it arranges to
have self._delayedRender(request) run about 5 seconds
after callLater is invoked from this render method and
that it returns immediately.
All three of the elements mentioned earlier can be seen in this
example. The resource uses Request.write to set the
response body. It uses Request.finish after the entire
body has been specified (all with just one call to write in this
case). Lastly, it returns NOT_DONE_YET from its render
method. So there you have it, asynchronous rendering with Twisted
Web.
Here’s a complete rpy script based on this resource class (see the previous example if you need a reminder about rpy scripts):
from twisted.web.resource import Resource
from twisted.web.server import NOT_DONE_YET
from twisted.internet import reactor
class DelayedResource(Resource):
def _delayedRender(self, request):
request.write("<html><body>Sorry to keep you waiting.</body></html>")
request.finish()
def render_GET(self, request):
reactor.callLater(5, self._delayedRender, request)
return NOT_DONE_YET
resource = DelayedResource()
Drop this source into a .rpy file and fire up a server
using twistd -n web --path /directory/containing/script/.
You’ll see that loading the page takes 5 seconds. If you try to load a
second before the first completes, it will also take 5 seconds from
the time you request it (but it won’t be delayed by any other
outstanding requests).
Something else to consider when generating responses asynchronously is that the client may not wait around to get the response to its request. A subsequent example demonstrates how to detect that the client has abandoned the request and that the server shouldn’t bother to finish generating its response.
Asynchronous Responses (via Deferred)¶
The previous example had a Resource that generates its response
asynchronously rather than immediately upon the call to its render
method. Though it was a useful demonstration of the NOT_DONE_YET
feature of Twisted Web, the example didn’t reflect what a realistic application
might want to do. This example introduces Deferred , the Twisted class which is used
to provide a uniform interface to many asynchronous events, and shows you an
example of using a Deferred -returning API to generate an
asynchronous response to a request in Twisted Web.
Deferred is the result of two consequences of the
asynchronous programming approach. First, asynchronous code is
frequently (if not always) concerned with some data (in Python, an
object) which is not yet available but which probably will be
soon. Asynchronous code needs a way to define what will be done to the
object once it does exist. It also needs a way to define how to handle
errors in the creation or acquisition of that object. These two needs
are satisfied by the callbacks and errbacks of
a Deferred . Callbacks are added to
a Deferred with Deferred.addCallback ; errbacks
are added with Deferred.addErrback . When the
object finally does exist, it is passed to Deferred.callback which passes it
on to the callback added with addCallback . Similarly, if
an error occurs, Deferred.errback is called and
the error is passed along to the errback added
with addErrback . Second, the events that make
asynchronous code actually work often take many different,
incompatible forms. Deferred acts as the uniform
interface which lets different parts of an asynchronous application
interact and isolates them from implementation details they shouldn’t
be concerned with.
That’s almost all there is to Deferred . To solidify your new
understanding, now consider this rewritten version
of DelayedResource which uses a Deferred -based delay
API. It does exactly the same thing as the previous example . Only the implementation is different.
First, the example must import that new API that was just mentioned, deferLater :
from twisted.internet.task import deferLater
Next, all the other imports (these are the same as last time):
from twisted.web.resource import Resource
from twisted.web.server import NOT_DONE_YET
from twisted.internet import reactor
With the imports done, here’s the first part of
the DelayedResource implementation. Again, this part of
the code is identical to the previous version:
class DelayedResource(Resource):
def _delayedRender(self, request):
request.write("<html><body>Sorry to keep you waiting.</body></html>")
request.finish()
Next we need to define the render method. Here’s where things
change a bit. Instead of using callLater ,
We’re going to use deferLater this
time. deferLater accepts a reactor, delay (in seconds, as
with callLater ), and a function to call after the delay
to produce that elusive object discussed in the description
of Deferred s. We’re also going to
use _delayedRender as the callback to add to
the Deferred returned by deferLater . Since
it expects the request object as an argument, we’re going to set up
the deferLater call to return a Deferred
which has the request object as its result.
...
def render_GET(self, request):
d = deferLater(reactor, 5, lambda: request)
The Deferred referenced by d now needs to
have the _delayedRender callback added to it. Once this
is done, _delayedRender will be called with the result
of d (which will be request , of course — the
result of (lambda: request)() ).
...
d.addCallback(self._delayedRender)
Finally, the render method still needs to return NOT_DONE_YET ,
for exactly the same reasons as it did in the previous version of the
example.
...
return NOT_DONE_YET
And with that, DelayedResource is now implemented
based on a Deferred . The example still isn’t very
realistic, but remember that since Deferred s offer a
uniform interface to many different asynchronous event sources, this
code now resembles a real application even more closely; you could
easily replace deferLater with
another Deferred -returning API and suddenly you might
have a resource that does something useful.
Finally, here’s the complete, uninterrupted example source, as an rpy script:
from twisted.internet.task import deferLater
from twisted.web.resource import Resource
from twisted.web.server import NOT_DONE_YET
from twisted.internet import reactor
class DelayedResource(Resource):
def _delayedRender(self, request):
request.write("<html><body>Sorry to keep you waiting.</body></html>")
request.finish()
def render_GET(self, request):
d = deferLater(reactor, 5, lambda: request)
d.addCallback(self._delayedRender)
return NOT_DONE_YET
resource = DelayedResource()
Interrupted Responses¶
The previous example had a Resource that generates its response asynchronously rather than immediately upon the call to its render method. When generating responses asynchronously, the possibility is introduced that the connection to the client may be lost before the response is generated. In such a case, it is often desirable to abandon the response generation entirely, since there is nothing to do with the data once it is produced. This example shows how to be notified that the connection has been lost.
This example will build upon the asynchronous responses example which simply (if not very realistically) generated its response after a fixed delay. We will expand that resource so that as soon as the client connection is lost, the delayed event is cancelled and the response is never generated.
The feature this example relies on is provided by another Request method: notifyFinish . This method returns a new
Deferred which will fire with None if the request is successfully
responded to or with an error otherwise - for example if the connection is lost
before the response is sent.
The example starts in a familiar way, with the requisite Twisted imports and
a resource class with the same _delayedRender used previously:
from twisted.web.resource import Resource
from twisted.web.server import NOT_DONE_YET
from twisted.internet import reactor
class DelayedResource(Resource):
def _delayedRender(self, request):
request.write("<html><body>Sorry to keep you waiting.</body></html>")
request.finish()
Before defining the render method, we’re going to define an errback
(an errback being a callback that gets called when there’s an error),
though. This will be the errback attached to the Deferred
returned by Request.notifyFinish . It will cancel the
delayed call to _delayedRender .
...
def _responseFailed(self, err, call):
call.cancel()
Finally, the render method will set up the delayed call just as it
did before, and return NOT_DONE_YET likewise. However, it
will also use Request.notifyFinish to make
sure _responseFailed is called if appropriate.
...
def render_GET(self, request):
call = reactor.callLater(5, self._delayedRender, request)
request.notifyFinish().addErrback(self._responseFailed, call)
return NOT_DONE_YET
Notice that since _responseFailed needs a reference to
the delayed call object in order to cancel it, we passed that object
to addErrback . Any additional arguments passed
to addErrback (or addCallback ) will be
passed along to the errback after the Failure instance which is always
passed as the first argument. Passing call here means it
will be passed to _responseFailed , where it is expected
and required.
That covers almost all the code for this example. Here’s the entire example without interruptions, as an rpy script :
from twisted.web.resource import Resource
from twisted.web.server import NOT_DONE_YET
from twisted.internet import reactor
class DelayedResource(Resource):
def _delayedRender(self, request):
request.write("<html><body>Sorry to keep you waiting.</body></html>")
request.finish()
def _responseFailed(self, err, call):
call.cancel()
def render_GET(self, request):
call = reactor.callLater(5, self._delayedRender, request)
request.notifyFinish().addErrback(self._responseFailed, call)
return NOT_DONE_YET
resource = DelayedResource()
Toss this into example.rpy , fire it up with twistd -n web --path . , and
hit http://localhost:8080/example.rpy . If
you wait five seconds, you’ll get the page content. If you interrupt the request
before then, say by hitting escape (in Firefox, at least), then you’ll see
perhaps the most boring demonstration ever - no page content, and nothing in the
server logs. Success!
Logging Errors¶
The previous example created a server that dealt with response errors by aborting response generation, potentially avoiding pointless work. However, it did this silently for any error. In this example, we’ll modify the previous example so that it logs each failed response.
This example will use the Twisted API for logging errors. As was
mentioned in the first example covering Deferreds , errbacks are passed an error. In the previous
example, the _responseFailed errback accepted this error
as a parameter but ignored it. The only way this example will differ
is that this _responseFailed will use that error
parameter to log a message.
This example will require all of the imports required by the previous example plus one new import:
from twisted.python.log import err
The only other part of the previous example which changes is
the _responseFailed callback, which will now log the
error passed to it:
...
def _responseFailed(self, failure, call):
call.cancel()
err(failure, "Async response demo interrupted response")
We’re passing two arguments to err here. The first is the error which is being
passed in to the callback. This is always an object of type Failure , a class which represents an
exception and (sometimes, but not always) a traceback. err will
format this nicely for the log. The second argument is a descriptive string that
tells someone reading the log what the source of the error was.
Here’s the full example with the two above modifications:
from twisted.web.resource import Resource
from twisted.web.server import NOT_DONE_YET
from twisted.internet import reactor
from twisted.python.log import err
class DelayedResource(Resource):
def _delayedRender(self, request):
request.write("<html><body>Sorry to keep you waiting.</body></html>")
request.finish()
def _responseFailed(self, failure, call):
call.cancel()
err(failure, "Async response demo interrupted response")
def render_GET(self, request):
call = reactor.callLater(5, self._delayedRender, request)
request.notifyFinish().addErrback(self._responseFailed, call)
return NOT_DONE_YET
resource = DelayedResource()
Run this server as in the previous example and interrupt a request. Unlike the previous example, where the server gave no indication that this had happened, you’ll see a message in the log output with this version.
Access Logging¶
As long as we’re on the topic of logging, this is probably a good time to mention Twisted Web’s access log support. In this example, we’ll see what Twisted Web logs for each request it processes and how this can be customized.
If you’ve run any of the previous examples and watched the output of twistd or read twistd.log then you’ve already seen some log lines like this:
2014-01-29 17:50:50-0500 [HTTPChannel,0,127.0.0.1] “127.0.0.1” - - [29/Jan/2014:22:50:50 +0000] “GET / HTTP/1.1” 200 2753 “-” “Mozilla/5.0 ...”
If you focus on the latter portion of this log message you’ll see something that looks like a standard “combined log format” message.
However, it’s prefixed with the normal Twisted logging prefix giving a timestamp and some protocol and peer addressing information.
Much of this information is redundant since it is part of the combined log format.
Site lets you produce a more compact log which omits the normal Twisted logging prefix.
To take advantage of this feature all that is necessary is to tell Site where to write this compact log.
Do this by passing logPath to the initializer:
...
factory = Site(root, logPath=b"/tmp/access-logging-demo.log")
Or if you want to change the logging behavior of a server you’re launching with twistd web then just pass the --logfile option:
$ twistd -n web --logfile /tmp/access-logging-demo.log
Apart from this, the rest of the server setup is the same.
Once you pass logPath or use --logfile on the command line the server will produce a log file containing lines like:
“127.0.0.1” - - [30/Jan/2014:00:13:35 +0000] “GET / HTTP/1.1” 200 2753 “-” “Mozilla/5.0 ...”
Any tools expecting combined log format messages should be able to work with these log files.
Site also allows the log format used to be customized using its logFormatter argument.
Twisted Web comes with one alternate formatter, proxiedLogFormatter, which is for use behind a proxy that sets the X-Forwarded-For header.
It logs the client address taken from this header rather than the network address of the client directly connected to the server.
Here’s the complete code for an example that uses both these features:
from twisted.web.http import proxiedLogFormatter
from twisted.web.server import Site
from twisted.web.static import File
from twisted.internet import reactor
resource = File('/tmp')
factory = Site(resource, logPath=b"/tmp/access-logging-demo.log", logFormatter=proxiedLogFormatter)
reactor.listenTCP(8888, factory)
reactor.run()
WSGI¶
The goal of this example is to show you how to use WSGIResource , another existing Resource subclass, to serve WSGI applications in a Twisted Web server.
Note that WSGIResource is a multithreaded WSGI container. Like
any other WSGI container, you can’t do anything asynchronous in your WSGI
applications, even though this is a Twisted WSGI container.
The first new thing in this example is the import
of WSGIResource :
from twisted.web.wsgi import WSGIResource
Nothing too surprising there. We still need one of the other usual suspects, too:
from twisted.internet import reactor
You’ll see why in a minute. Next, we need a WSGI application. Here’s a really simple one just to get things going:
def application(environ, start_response):
start_response('200 OK', [('Content-type', 'text/plain')])
return ['Hello, world!']
If this doesn’t make sense to you, take a look at one of
these fine tutorials . Otherwise,
or once you’re done with that, the next step is to create
a WSGIResource instance, as this is going to be
another rpy script example:
resource = WSGIResource(reactor, reactor.getThreadPool(), application)
Let’s dwell on this line for a minute. The first parameter passed
to WSGIResource is the reactor. Despite the fact that the
reactor is global and any code that wants it can always just import it
(as, in fact, this rpy script simply does itself), passing it around
as a parameter leaves the door open for certain future possibilities -
for example, having more than one reactor. There are also testing
implications. Consider how much easier it is to unit test a function
that accepts a reactor - perhaps a mock reactor specially constructed
to make your tests easy to write - rather than importing the real
global reactor. That’s why WSGIResource requires you to
pass the reactor to it.
The second parameter passed to WSGIResource is
a ThreadPool . WSGIResource
uses this to actually call the application object passed in to it. To keep this
example short, we’re passing in the reactor’s internal threadpool here, letting
us skip its creation and shutdown-time destruction. For finer control over how
many WSGI requests are served in parallel, you may want to create your own
thread pool to use with your WSGIResource , but for simple testing,
using the reactor’s is fine.
The final argument is the application object. This is pretty typical of how WSGI containers work.
The example, sans interruption:
from twisted.web.wsgi import WSGIResource
from twisted.internet import reactor
def application(environ, start_response):
start_response('200 OK', [('Content-type', 'text/plain')])
return ['Hello, world!']
resource = WSGIResource(reactor, reactor.getThreadPool(), application)
Up to the point where the WSGIResource instance defined here
exists in the resource hierarchy, the normal resource traversal rules
apply: getChild
will be called to handle each segment. Once the WSGIResource is
encountered, though, that process stops and all further URL handling is the
responsibility of the WSGI application. This application does nothing with the
URL, though, so you won’t be able to tell that.
Oh, and as was the case with the first static file example, there’s also a
command line option you can use to avoid a lot of this. If you just put the
above application function, without all of the WSGIResource stuff,
into a file, say, foo.py , then you can launch a roughly equivalent
server like this:
$ twistd -n web --wsgi foo.application
HTTP Authentication¶
Many of the previous examples have looked at how to serve content by using existing resource classes or implementing new ones. In this example we’ll use Twisted Web’s basic or digest HTTP authentication to control access to these resources.
guard , the Twisted Web module which provides most of the APIs that will be used in this example, helps you to add authentication and authorization to a resource hierarchy. It does this by providing a resource which implements getChild to return a dynamically selected resource . The selection is based on the authentication headers in the request. If those headers indicate that the request is made on behalf of Alice, then Alice’s resource will be returned. If they indicate that it was made on behalf of Bob, his will be returned. If the headers contain invalid credentials, an error resource is returned. Whatever happens, once this resource is returned, URL traversal continues as normal from that resource.
The resource that implements this is HTTPAuthSessionWrapper , though it is directly responsible for very little of the process. It will extract headers from the request and hand them off to a credentials factory to parse them according to the appropriate standards (eg HTTPAuthentication: Basic and Digest Access Authentication ) and then hand the resulting credentials object off to a Portal , the core of Twisted Cred , a system for uniform handling of authentication and authorization. We won’t discuss Twisted Cred in much depth here. To make use of it with Twisted Web, the only thing you really need to know is how to implement an IRealm .
You need to implement a realm because the realm is the object that
actually decides which resources are used for which users. This can be
as complex or as simple as it suitable for your application. For this
example we’ll keep it very simple: each user will have a resource
which is a static file listing of the public_html
directory in their UNIX home directory. First, we need to
import implements from zope.interface
and IRealm
from twisted.cred.portal . Together these will let me mark
this class as a realm (this is mostly - but not entirely - a
documentation thing). We’ll also need File for the actual implementation
later.
from zope.interface import implements
from twisted.cred.portal import IRealm
from twisted.web.static import File
class PublicHTMLRealm(object):
implements(IRealm)
A realm only needs to implement one method: requestAvatar . This method is called
after any successful authentication attempt (ie, Alice supplied the right
password). Its job is to return the avatar for the user who succeeded in
authenticating. An avatar is just an object that represents a user. In
this case, it will be a File . In general, with Guard ,
the avatar must be a resource of some sort.
...
def requestAvatar(self, avatarId, mind, *interfaces):
if IResource in interfaces:
return (IResource, File("/home/%s/public_html" % (avatarId,)), lambda: None)
raise NotImplementedError()
A few notes on this method:
- The
avatarIdparameter is essentially the username. It’s the job of some other code to extract the username from the request headers and make sure it gets passed here. - The
mindis alwaysNonewhen writing a realm to be used withGuard. You can ignore it until you want to write a realm for something else. Guardis always passedIResourceas theinterfacesparameter. Ifinterfacesonly contains interfaces your code doesn’t understand, raisingNotImplementedErroris the thing to do, as above. You’ll only need to worry about getting a different interface when you write a realm for something other thanGuard.- If you want to track when a user logs out, that’s what the last element of
the returned tuple is for. It will be called when this avatar logs
out.
lambda: Noneis the idiomatic no-op logout function. - Notice that the path handling code in this example is written very poorly. This example may be vulnerable to certain unintentional information disclosure attacks. This sort of problem is exactly the reason FilePath exists. However, that’s an example for another day...
We’re almost ready to set up the resource for this example. To
create an HTTPAuthSessionWrapper , though, we need two
things. First, a portal, which requires the realm above, plus at least
one credentials checker:
from twisted.cred.portal import Portal
from twisted.cred.checkers import FilePasswordDB
portal = Portal(PublicHTMLRealm(), [FilePasswordDB('httpd.password')])
FilePasswordDB is the
credentials checker. It knows how to read passwd(5) -style (loosely)
files to check credentials against. It is responsible for the authentication
work after HTTPAuthSessionWrapper extracts the credentials from the
request.
Next we need either BasicCredentialFactory or DigestCredentialFactory . The former knows how to challenge HTTP clients to do basic authentication; the latter, digest authentication. We’ll use digest here:
from twisted.web.guard import DigestCredentialFactory
credentialFactory = DigestCredentialFactory("md5", "example.org")
The two parameters to this constructor are the hash algorithm and the HTTP authentication realm which will be used. The only other valid hash algorithm is “sha” (but be careful, MD5 is more widely supported than SHA). The HTTP authentication realm is mostly just a string that is presented to the user to let them know why they’re authenticating (you can read more about this in the RFC ).
With those things created, we can finally
instantiate HTTPAuthSessionWrapper :
from twisted.web.guard import HTTPAuthSessionWrapper
resource = HTTPAuthSessionWrapper(portal, [credentialFactory])
There’s just one last thing that needs to be done
here. When rpy scripts were
introduced, it was mentioned that they are evaluated in an unusual
context. This is the first example that actually needs to take this
into account. It so happens that DigestCredentialFactory
instances are stateful. Authentication will only succeed if the same
instance is used to both generate challenges and examine the responses
to those challenges. However, the normal mode of operation for an rpy
script is for it to be re-executed for every request. This leads to a
new DigestCredentialFactory being created for every request, preventing
any authentication attempt from ever succeeding.
There are two ways to deal with this. First, and the better of the two ways,
we could move almost all of the code into a real Python module, including the
code that instantiates the DigestCredentialFactory . This would
ensure that the same instance was used for every request. Second, and the easier
of the two ways, we could add a call to cache() to the beginning of
the rpy script:
cache()
cache is part of the globals of any rpy script, so you don’t
need to import it (it’s okay to be cringing at this
point). Calling cache makes Twisted re-use the result of the first
evaluation of the rpy script for subsequent requests too - just what we want in
this case.
Here’s the complete example (with imports re-arranged to the more conventional style):
cache()
from zope.interface import implements
from twisted.cred.portal import IRealm, Portal
from twisted.cred.checkers import FilePasswordDB
from twisted.web.static import File
from twisted.web.resource import IResource
from twisted.web.guard import HTTPAuthSessionWrapper, DigestCredentialFactory
class PublicHTMLRealm(object):
implements(IRealm)
def requestAvatar(self, avatarId, mind, *interfaces):
if IResource in interfaces:
return (IResource, File("/home/%s/public_html" % (avatarId,)), lambda: None)
raise NotImplementedError()
portal = Portal(PublicHTMLRealm(), [FilePasswordDB('httpd.password')])
credentialFactory = DigestCredentialFactory("md5", "localhost:8080")
resource = HTTPAuthSessionWrapper(portal, [credentialFactory])
And voila, a password-protected per-user Twisted Web server.
Session Basics¶
Sessions are the most complicated topic covered in this series of examples, and because of that it is going to take a few examples to cover all of the different aspects. This first example demonstrates the very basics of the Twisted Web session API: how to get the session object for the current request and how to prematurely expire a session.
Before diving into the APIs, let’s look at the big picture of
sessions in Twisted Web. Sessions are represented by instances
of Session . The Site creates a new instance
of Session the first time an application asks for it for
a particular session. Session instances are kept on
the Site instance until they expire (due to inactivity or
because they are explicitly expired). Each time after the first that a
particular session’s Session object is requested, it is
retrieved from the Site .
With the conceptual underpinnings of the upcoming API in place, here comes the example. This will be a very simple rpy script which tells a user what its unique session identifier is and lets it prematurely expire the session.
First, we’ll import Resource so we can define a couple of subclasses of it:
from twisted.web.resource import Resource
Next we’ll define the resource which tells the client what its session identifier is. This is done easily by first getting the session object using Request.getSession and then getting the session object’s uid attribute:
class ShowSession(Resource):
def render_GET(self, request):
return 'Your session id is: ' + request.getSession().uid
To let the client expire its own session before it times out, we’ll define another resource which expires whatever session it is requested with. This is done using the Session.expire method:
class ExpireSession(Resource):
def render_GET(self, request):
request.getSession().expire()
return 'Your session has been expired.'
Finally, to make the example an rpy script, we’ll make an instance
of ShowSession and give it an instance
of ExpireSession as a child using Resource.putChild :
resource = ShowSession()
resource.putChild("expire", ExpireSession())
And that is the complete example. You can fire this up and load the top
page. You’ll see a (rather opaque) session identifier that remains the same
across reloads (at least until you flush the TWISTED_SESSION cookie
from your browser or enough time passes). You can then visit
the expire child and go back to the top page and see that you have
a new session.
Here’s the complete source for the example:
from twisted.web.resource import Resource
class ShowSession(Resource):
def render_GET(self, request):
return 'Your session id is: ' + request.getSession().uid
class ExpireSession(Resource):
def render_GET(self, request):
request.getSession().expire()
return 'Your session has been expired.'
resource = ShowSession()
resource.putChild("expire", ExpireSession())
Storing Objects in the Session¶
This example shows you how you can persist objects across requests in the session object.
As was discussed previously , instances
of Session last as long as
the notional session itself does. Each time Request.getSession is called, if the session
for the request is still active, then the same Session instance is
returned as was returned previously. Because of this, Session
instances can be used to keep other objects around for as long as the session
exists.
It’s easier to demonstrate how this works than explain it, so here’s an example:
>>> from zope.interface import Interface, Attribute, implements
>>> from twisted.python.components import registerAdapter
>>> from twisted.web.server import Session
>>> class ICounter(Interface):
... value = Attribute("An int value which counts up once per page view.")
...
>>> class Counter(object):
... implements(ICounter)
... def __init__(self, session):
... self.value = 0
...
>>> registerAdapter(Counter, Session, ICounter)
>>> ses = Session(None, None)
>>> data = ICounter(ses)
>>> print data
<__main__.Counter object at 0x8d535ec>
>>> print data is ICounter(ses)
True
>>>
What? , I hear you say.
What’s shown in this example is the interface and adaption-based
API which Session exposes for persisting state. There are
several critical pieces interacting here:
ICounteris an interface which serves several purposes. Like all interfaces, it documents the API of some class of objects (in this case, just thevalueattribute). It also serves as a key into what is basically a dictionary within the session object: the interface is used to store or retrieve a value on the session (theCounterinstance, in this case).Counteris the class which actually holds the session data in this example. It implementsICounter(again, mostly for documentation purposes). It also has avalueattribute, as the interface declared.- The registerAdapter call sets up the relationship between its three arguments so that adaption will do what we want in this case.
- Adaption is performed by the expression
ICounter(ses). This is read as : adaptsestoICounter. Because of theregisterAdaptercall, it is roughly equivalent toCounter(ses). However (because of certain thingsSessiondoes), it also saves theCounterinstance created so that it will be returned the next time this adaption is done. This is why the last statement producesTrue.
If you’re still not clear on some of the details there, don’t worry about it
and just remember this: ICounter(ses) gives you an object you can
persist state on. It can be as much or as little state as you want, and you can
use as few or as many different Interface classes as you want on a
single Session instance.
With those conceptual dependencies out of the way, it’s a very short step to actually getting persistent state into a Twisted Web application. Here’s an example which implements a simple counter, re-using the definitions from the example above:
from twisted.web.resource import Resource
class CounterResource(Resource):
def render_GET(self, request):
session = request.getSession()
counter = ICounter(session)
counter.value += 1
return "Visit #%d for you!" % (counter.value,)
Pretty simple from this side, eh? All this does is
use Request.getSession and the adaption from above, plus some
integer math to give you a session-based visit counter.
Here’s the complete source for an rpy script based on this example:
cache()
from zope.interface import Interface, Attribute, implements
from twisted.python.components import registerAdapter
from twisted.web.server import Session
from twisted.web.resource import Resource
class ICounter(Interface):
value = Attribute("An int value which counts up once per page view.")
class Counter(object):
implements(ICounter)
def __init__(self, session):
self.value = 0
registerAdapter(Counter, Session, ICounter)
class CounterResource(Resource):
def render_GET(self, request):
session = request.getSession()
counter = ICounter(session)
counter.value += 1
return "Visit #%d for you!" % (counter.value,)
resource = CounterResource()
One more thing to note is the cache() call at the top
of this example. As with the previous example where this came up, this rpy script is stateful. This
time, it’s the ICounter definition and
the registerAdapter call that need to be executed only
once. If we didn’t use cache , every request would define
a new, different interface named ICounter . Each of these
would be a different key in the session, so the counter would never
get past one.
Session Endings¶
The previous two examples introduced Twisted Web’s session APIs. This included accessing the session object, storing state on it, and retrieving it later, as well as the idea that the Session object has a lifetime which is tied to the notional session it represents. This example demonstrates how to exert some control over that lifetime and react when it expires.
The lifetime of a session is controlled by the sessionTimeout
attribute of the Session class. This attribute gives the number of
seconds a session may go without being accessed before it expires. The default
is 15 minutes. In this example we’ll change that to a different value.
One way to override the value is with a subclass:
from twisted.web.server import Session
class ShortSession(Session):
sessionTimeout = 60
To have Twisted Web actually make use of this session class, rather
than the default, it is also necessary to override
the sessionFactory attribute of Site . We could do this with another
subclass, but we could also do it to just one instance
of Site :
from twisted.web.server import Site
factory = Site(rootResource)
factory.sessionFactory = ShortSession
Sessions given out for requests served by this Site will
use ShortSession and only last one minute without activity.
You can have arbitrary functions run when sessions expire, too. This can be useful for cleaning up external resources associated with the session, tracking usage statistics, and more. This functionality is provided via Session.notifyOnExpire . It accepts a single argument: a function to call when the session expires. Here’s a trivial example which prints a message whenever a session expires:
from twisted.web.resource import Resource
class ExpirationLogger(Resource):
sessions = set()
def render_GET(self, request):
session = request.getSession()
if session.uid not in self.sessions:
self.sessions.add(session.uid)
session.notifyOnExpire(lambda: self._expired(session.uid))
return ""
def _expired(self, uid):
print "Session", uid, "has expired."
self.sessions.remove(uid)
Keep in mind that using a method as the callback will keep the instance (in
this case, the ExpirationLogger resource) in memory until the
session expires.
With those pieces in hand, here’s an example that prints a message whenever a session expires, and uses sessions which last for 5 seconds:
from twisted.web.server import Site, Session
from twisted.web.resource import Resource
from twisted.internet import reactor
class ShortSession(Session):
sessionTimeout = 5
class ExpirationLogger(Resource):
sessions = set()
def render_GET(self, request):
session = request.getSession()
if session.uid not in self.sessions:
self.sessions.add(session.uid)
session.notifyOnExpire(lambda: self._expired(session.uid))
return ""
def _expired(self, uid):
print "Session", uid, "has expired."
self.sessions.remove(uid)
rootResource = Resource()
rootResource.putChild("logme", ExpirationLogger())
factory = Site(rootResource)
factory.sessionFactory = ShortSession
reactor.listenTCP(8080, factory)
reactor.run()
Since Site customization is required, this example can’t be
rpy-based, so it brings back the manual reactor.listenTCP
and reactor.run calls. Run it and visit /logme to see
it in action. Keep visiting it to keep your session active. Stop visiting it for
five seconds to see your session expiration message.
This set of examples contains short, complete applications of twisted.web. For subjects not covered here, see the Twisted Web tutorial and the API documentation.
- Serving static content from a directory
- Generating a page dynamically
- Static URL dispatch
- Dynamic URL dispatch
- Error handling
- Custom response codes
- Handling POSTs
- Other request bodies
- rpy scripts (or, how to save yourself some typing)
- Asynchronous responses
- Asynchronous responses (via Deferred)
- Interrupted responses
- Logging errors
- Access logging
- WSGIs
- HTTP authentication
- Session basics
- Storing objects in the session
- Session endings
Light Weight Templating With Resource Templates¶
Overview¶
While high-level templating systems can be used with Twisted (for example, DivmodNevow , sometimes one needs a less file-heavy system which lets one directly write HTML. While ResourceScript is available, it has a high coding overhead, and requires some boring string arithmetic. ResourceTemplate fills the space between Nevow and ResourceScript using Quixote’s PTL (Python Templating Language).
ResourceTemplates need Quixote
installed. In Debian , that means
installing the python-quixote package
(apt-get install python-quixote ). Other operating systems
require other ways to install Quixote, or it can be done manually.
Configuring Twisted Web¶
The easiest way to get Twisted Web to support ResourceTemplates is to
bind them to some extension using the web tap’s --processor
flag. Here is an example:
% twistd web --path=/var/www \
--processor=.rtl=twisted.web.script.ResourceTemplate
The above command line binds the rtl extension to use the
ResourceTemplate processor. Other ways are possible, but would require
more Python coding and are outside the scope of this HOWTO.
Using ResourceTemplate¶
ResourceTemplates are coded in an extension of Python called the”Python Templating Language” . Complete documentation of the PTL
is available
at the quixote web site . The web server will expect the PTL source file
to define a variable named resource . This should be
a twisted.web.resource.Resource ,
whose .render method be called. Usually, you would want
to define render using the keyword template
rather than def .
Here is a simple example for a resource template.
from twisted.web.resource import Resource
def getQuote():
return "An apple a day keeps the doctor away."
class QuoteResource(Resource):
template render(self, request):
"""\
<html>
<head><title>Quotes Galore</title></head>
<body><h1>Quotes</h1>"""
getQuote()
"</body></html>"
resource = QuoteResource()
Using the Twisted Web Client¶
Overview¶
This document describes how to use the HTTP client included in Twisted Web. After reading it, you should be able to make HTTP and HTTPS requests using Twisted Web. You will be able to specify the request method, headers, and body and you will be able to retrieve the response code, headers, and body.
A number of higher-level features are also explained, including proxying, automatic content encoding negotiation, and cookie handling.
Prerequisites¶
This document assumes that you are familiar with Deferreds and Failures , and producers and consumers . It also assumes you are familiar with the basic concepts of HTTP, such as requests and responses, methods, headers, and message bodies. The HTTPS section of this document also assumes you are somewhat familiar with SSL and have read about using SSL in Twisted .
The Agent¶
Issuing Requests¶
The twisted.web.client.Agent class is the entry point into the client API. Requests are issued using the request method, which takes as parameters a request method, a request URI, the request headers, and an object which can produce the request body (if there is to be one). The agent is responsible for connection setup. Because of this, it requires a reactor as an argument to its initializer. An example of creating an agent and issuing a request using it might look like this:
from twisted.internet import reactor
from twisted.web.client import Agent
from twisted.web.http_headers import Headers
agent = Agent(reactor)
d = agent.request(
'GET',
'http://example.com/',
Headers({'User-Agent': ['Twisted Web Client Example']}),
None)
def cbResponse(ignored):
print 'Response received'
d.addCallback(cbResponse)
def cbShutdown(ignored):
reactor.stop()
d.addBoth(cbShutdown)
reactor.run()
As may be obvious, this issues a new GET request for /
to the web server on example.com . Agent is
responsible for resolving the hostname into an IP address and connecting
to it on port 80 (for HTTP URIs), port 443 (for HTTPS
URIs), or on the port number specified in the URI itself. It is also
responsible for cleaning up the connection afterwards. This code sends
a request which includes one custom header, User-Agent . The
last argument passed to Agent.request is None ,
though, so the request has no body.
Sending a request which does include a body requires passing an object
providing twisted.web.iweb.IBodyProducer
to Agent.request . This interface extends the more general
IPushProducer
by adding a new length attribute and adding several
constraints to the way the producer and consumer interact.
- The length attribute must be a non-negative integer or the constant
twisted.web.iweb.UNKNOWN_LENGTH. If the length is known, it will be used to specify the value for the Content-Length header in the request. If the length is unknown the attribute should be set toUNKNOWN_LENGTH. Since more servers support Content-Length , if a length can be provided it should be. - An additional method is required on
IBodyProducerimplementations:startProducing. This method is used to associate a consumer with the producer. It should return aDeferredwhich fires when all data has been produced. IBodyProducerimplementations should never call the consumer’sunregisterProducermethod. Instead, when it has produced all of the data it is going to produce, it should only fire theDeferredreturned bystartProducing.
For additional details about the requirements of IBodyProducer implementations, see the API documentation.
Here’s a simple IBodyProducer implementation which
writes an in-memory string to the consumer:
from zope.interface import implements
from twisted.internet.defer import succeed
from twisted.web.iweb import IBodyProducer
class StringProducer(object):
implements(IBodyProducer)
def __init__(self, body):
self.body = body
self.length = len(body)
def startProducing(self, consumer):
consumer.write(self.body)
return succeed(None)
def pauseProducing(self):
pass
def stopProducing(self):
pass
This producer can be used to issue a request with a body:
from twisted.internet import reactor
from twisted.web.client import Agent
from twisted.web.http_headers import Headers
from stringprod import StringProducer
agent = Agent(reactor)
body = StringProducer("hello, world")
d = agent.request(
'GET',
'http://example.com/',
Headers({'User-Agent': ['Twisted Web Client Example'],
'Content-Type': ['text/x-greeting']}),
body)
def cbResponse(ignored):
print 'Response received'
d.addCallback(cbResponse)
def cbShutdown(ignored):
reactor.stop()
d.addBoth(cbShutdown)
reactor.run()
If you want to upload a file or you just have some data in a string, you
don’t have to copy StringProducer though. Instead, you can
use FileBodyProducer .
This IBodyProducer implementation works with any file-like
object (so use it with a StringIO if your upload data is
already in memory as a string); the idea is the same
as StringProducer from the previous example, but with a
little extra code to only send data as fast as the server will take it.
from StringIO import StringIO
from twisted.internet import reactor
from twisted.web.client import Agent
from twisted.web.http_headers import Headers
from twisted.web.client import FileBodyProducer
agent = Agent(reactor)
body = FileBodyProducer(StringIO("hello, world"))
d = agent.request(
'GET',
'http://example.com/',
Headers({'User-Agent': ['Twisted Web Client Example'],
'Content-Type': ['text/x-greeting']}),
body)
def cbResponse(ignored):
print 'Response received'
d.addCallback(cbResponse)
def cbShutdown(ignored):
reactor.stop()
d.addBoth(cbShutdown)
reactor.run()
FileBodyProducer closes the file when it no longer needs it.
If the connection or the request take too much time, you can cancel the
Deferred returned by the Agent.request method.
This will abort the connection, and the Deferred will errback
with CancelledError .
Receiving Responses¶
So far, the examples have demonstrated how to issue a request. However,
they have ignored the response, except for showing that it is a
Deferred which seems to fire when the response has been
received. Next we’ll cover what that response is and how to interpret
it.
Agent.request , as with most Deferred -returning
APIs, can return a Deferred which fires with a
Failure . If the request fails somehow, this will be
reflected with a failure. This may be due to a problem looking up the
host IP address, or it may be because the HTTP server is not accepting
connections, or it may be because of a problem parsing the response, or
any other problem which arises which prevents the response from being
received. It does not include responses with an error status.
If the request succeeds, though, the Deferred will fire with
a Response . This
happens as soon as all the response headers have been received. It
happens before any of the response body, if there is one, is processed.
The Response object has several attributes giving the
response information: its code, version, phrase, and headers, as well as
the length of the body to expect. In addition to these, the
Response also contains a reference to the request that it is
a response to; one particularly useful attribute on the request is absoluteURI :
The absolute URI to which the request was made. The
Response object has a method which makes the response body
available: deliverBody . Using the
attributes of the response object and this method, here’s an example
which displays part of the response to a request:
from pprint import pformat
from twisted.internet import reactor
from twisted.internet.defer import Deferred
from twisted.internet.protocol import Protocol
from twisted.web.client import Agent
from twisted.web.http_headers import Headers
class BeginningPrinter(Protocol):
def __init__(self, finished):
self.finished = finished
self.remaining = 1024 * 10
def dataReceived(self, bytes):
if self.remaining:
display = bytes[:self.remaining]
print 'Some data received:'
print display
self.remaining -= len(display)
def connectionLost(self, reason):
print 'Finished receiving body:', reason.getErrorMessage()
self.finished.callback(None)
agent = Agent(reactor)
d = agent.request(
'GET',
'http://example.com/',
Headers({'User-Agent': ['Twisted Web Client Example']}),
None)
def cbRequest(response):
print 'Response version:', response.version
print 'Response code:', response.code
print 'Response phrase:', response.phrase
print 'Response headers:'
print pformat(list(response.headers.getAllRawHeaders()))
finished = Deferred()
response.deliverBody(BeginningPrinter(finished))
return finished
d.addCallback(cbRequest)
def cbShutdown(ignored):
reactor.stop()
d.addBoth(cbShutdown)
reactor.run()
The BeginningPrinter protocol in this example is passed to
Response.deliverBody and the response body is then delivered
to its dataReceived method as it arrives. When the body has
been completely delivered, the protocol’s connectionLost
method is called. It is important to inspect the Failure
passed to connectionLost . If the response body has been
completely received, the failure will wrap a twisted.web.client.ResponseDone exception. This
indicates that it is known that all data has been received. It
is also possible for the failure to wrap a twisted.web.http.PotentialDataLoss exception: this
indicates that the server framed the response such that there is no way
to know when the entire response body has been received. Only
HTTP/1.0 servers should behave this way. Finally, it is possible for
the exception to be of another type, indicating guaranteed data loss for
some reason (a lost connection, a memory error, etc).
Just as protocols associated with a TCP connection are given a transport,
so will be a protocol passed to deliverBody . Since it makes
no sense to write more data to the connection at this stage of the
request, though, the transport only provides IPushProducer . This allows the
protocol to control the flow of the response data: a call to the
transport’s pauseProducing method will pause delivery; a
later call to resumeProducing will resume it. If it is
decided that the rest of the response body is not desired,
stopProducing can be used to stop delivery permanently;
after this, the protocol’s connectionLost method will be
called.
An important thing to keep in mind is that the body will only be read
from the connection after Response.deliverBody is called.
This also means that the connection will remain open until this is done
(and the body read). So, in general, any response with a body
must have that body read using deliverBody . If the
application is not interested in the body, it should issue a
HEAD request or use a protocol which immediately calls
stopProducing on its transport.
If the body of the response isn’t going to be consumed incrementally, then readBody can be used to get the body as a byte-string.
This function returns a Deferred that fires with the body after the request has been completed; cancelling this Deferred will close the connection to the HTTP server immediately.
from sys import argv
from pprint import pformat
from twisted.internet.task import react
from twisted.web.client import Agent, readBody
from twisted.web.http_headers import Headers
def cbRequest(response):
print 'Response version:', response.version
print 'Response code:', response.code
print 'Response phrase:', response.phrase
print 'Response headers:'
print pformat(list(response.headers.getAllRawHeaders()))
d = readBody(response)
d.addCallback(cbBody)
return d
def cbBody(body):
print 'Response body:'
print body
def main(reactor, url=b"http://example.com/"):
agent = Agent(reactor)
d = agent.request(
'GET', url,
Headers({'User-Agent': ['Twisted Web Client Example']}),
None)
d.addCallback(cbRequest)
return d
react(main, argv[1:])
HTTP over SSL¶
Everything you’ve read so far applies whether the scheme of the request URI is HTTP or HTTPS . However, to control the SSL negotiation performed when an HTTPS URI is requested, there’s one extra object to pay attention to: the SSL context factory.
Agent ‘s constructor takes an optional second argument, a
context factory. This is an object like the context factory described
in Using SSL in Twisted but has
one small difference. The getContext method of this factory
accepts the address from the URL being requested. This allows it to
return a context object which verifies that the server’s certificate
matches the URL being requested.
Here’s an example which shows how to use Agent to request
an HTTPS URL with no certificate verification.
from twisted.python.log import err
from twisted.web.client import Agent
from twisted.internet import reactor
from twisted.internet.ssl import ClientContextFactory
class WebClientContextFactory(ClientContextFactory):
def getContext(self, hostname, port):
return ClientContextFactory.getContext(self)
def display(response):
print "Received response"
print response
def main():
contextFactory = WebClientContextFactory()
agent = Agent(reactor, contextFactory)
d = agent.request("GET", "https://example.com/")
d.addCallbacks(display, err)
d.addCallback(lambda ignored: reactor.stop())
reactor.run()
if __name__ == "__main__":
main()
The important point to notice here is that getContext now
accepts two arguments, a hostname and a port number. These two arguments,
a str and an int , give the address to which a
connection is being established to request an HTTPS URL. Because an agent
might make multiple requests over a single connection,
getContext may not be called once for each request. A second
or later request for a URL with the same hostname as a previous request
may re-use an existing connection, and therefore will re-use the
previously returned context object.
To configure SSL options or enable certificate verification or hostname checking, provide a context factory which creates suitably configured context objects.
HTTP Persistent Connection¶
HTTP persistent connections use the same TCP connection to send and receive multiple HTTP requests/responses. This reduces latency and TCP connection establishment overhead.
The constructor of twisted.web.client.Agent
takes an optional parameter pool, which should be an instance
of HTTPConnectionPool , which will be used
to manage the connections. If the pool is created with the
parameter persistent set to True (the
default), it will not close connections when the request is done, and
instead hold them in its cache to be re-used.
Here’s an example which sends requests over a persistent connection:
from twisted.internet import reactor
from twisted.internet.defer import Deferred, DeferredList
from twisted.internet.protocol import Protocol
from twisted.web.client import Agent, HTTPConnectionPool
class IgnoreBody(Protocol):
def __init__(self, deferred):
self.deferred = deferred
def dataReceived(self, bytes):
pass
def connectionLost(self, reason):
self.deferred.callback(None)
def cbRequest(response):
print 'Response code:', response.code
finished = Deferred()
response.deliverBody(IgnoreBody(finished))
return finished
pool = HTTPConnectionPool(reactor)
agent = Agent(reactor, pool=pool)
def requestGet(url):
d = agent.request('GET', url)
d.addCallback(cbRequest)
return d
# Two requests to the same host:
d = requestGet('http://localhost:8080/foo').addCallback(
lambda ign: requestGet("http://localhost:8080/bar"))
def cbShutdown(ignored):
reactor.stop()
d.addCallback(cbShutdown)
reactor.run()
Here, the two requests are to the same host, one after the each other. In most cases, the same connection will be used for the second request, instead of two different connections when using a non-persistent pool.
Multiple Connections to the Same Server¶
twisted.web.client.HTTPConnectionPool instances
have an attribute
called maxPersistentPerHost which limits the
number of cached persistent connections to the same server. The default
value is 2. This is effective only when the persistent option is
True. You can change the value like bellow:
from twisted.web.client import HTTPConnectionPool
pool = HTTPConnectionPool(reactor, persistent=True)
pool.maxPersistentPerHost = 1
With the default value of 2, the pool keeps around two connections to
the same host at most. Eventually the cached persistent connections will
be closed, by default after 240 seconds; you can change this timeout
value with the cachedConnectionTimeout
attribute of the pool. To force all connections to close use
the closeCachedConnections
method.
If a request fails without getting a response, and the request is
something that hopefully can be retried without having any side-effects
(e.g. a request with method GET), it will be retried automatically when
sending a request over a previously-cached persistent connection. You can
disable this behavior by setting retryAutomatically
to False . Note that each request will only be retried
once.
Following redirects¶
By itself, Agent doesn’t follow HTTP redirects (responses
with 301, 302, 303, 307 status codes and a location header
field). You need to use the twisted.web.client.RedirectAgent class to do so. It
implements a rather strict behavior of the RFC, meaning it will redirect
301 and 302 as 307, only on GET and HEAD
requests.
The following example shows how to have a redirect-enabled agent.
from twisted.python.log import err
from twisted.web.client import Agent, RedirectAgent
from twisted.internet import reactor
def display(response):
print "Received response"
print response
def main():
agent = RedirectAgent(Agent(reactor))
d = agent.request("GET", "http://example.com/")
d.addCallbacks(display, err)
d.addCallback(lambda ignored: reactor.stop())
reactor.run()
if __name__ == "__main__":
main()
In contrast, twisted.web.client.BrowserLikeRedirectAgent implements
more lenient behaviour that closely emulates what web browsers do; in
other words 301 and 302 POST redirects are treated like 303,
meaning the method is changed to GET before making the redirect
request.
As mentioned previously, Response contains a reference to both
the request that it is a response
to, and the previously received response , accessible by previousResponse .
In most cases there will not be a previous response, but in the case of
RedirectAgent the response history can be obtained by
following the previous responses from response to response.
Using a HTTP proxy¶
To be able to use HTTP proxies with an agent, you can use the twisted.web.client.ProxyAgent class.
It supports the same interface as Agent, but takes the endpoint of the proxy as initializer argument.
This is specifically intended for talking to servers that implement the proxying variation of the HTTP protocol; for other types of proxies you will want Agent.usingEndpointFactory (see documentation below).
Here’s an example demonstrating the use of an HTTP proxy running on localhost:8000.
from twisted.python.log import err
from twisted.web.client import ProxyAgent
from twisted.internet import reactor
from twisted.internet.endpoints import TCP4ClientEndpoint
def display(response):
print "Received response"
print response
def main():
endpoint = TCP4ClientEndpoint(reactor, "localhost", 8000)
agent = ProxyAgent(endpoint)
d = agent.request("GET", "https://example.com/")
d.addCallbacks(display, err)
d.addCallback(lambda ignored: reactor.stop())
reactor.run()
if __name__ == "__main__":
main()
Please refer to the endpoints documentation for more information about how they work and the twisted.internet.endpoints API documentation to learn what other kinds of endpoints exist.
Handling HTTP cookies¶
An existing agent instance can be wrapped with
twisted.web.client.CookieAgent to automatically
store, send and track HTTP cookies. A CookieJar
instance, from the Python standard library module
cookielib , is
used to store the cookie information. An example of using
CookieAgent to perform a request and display the collected
cookies might look like this:
from cookielib import CookieJar
from twisted.internet import reactor
from twisted.python import log
from twisted.web.client import Agent, CookieAgent
def displayCookies(response, cookieJar):
print 'Received response'
print response
print 'Cookies:', len(cookieJar)
for cookie in cookieJar:
print cookie
def main():
cookieJar = CookieJar()
agent = CookieAgent(Agent(reactor), cookieJar)
d = agent.request('GET', 'http://www.google.com/')
d.addCallback(displayCookies, cookieJar)
d.addErrback(log.err)
d.addCallback(lambda ignored: reactor.stop())
reactor.run()
if __name__ == "__main__":
main()
Automatic Content Encoding Negotiation¶
twisted.web.client.ContentDecoderAgent adds
support for sending Accept-Encoding request headers and
interpreting Content-Encoding response headers. These headers
allow the server to encode the response body somehow, typically with some
compression scheme to save on transfer
costs. ContentDecoderAgent provides this functionality as a
wrapper around an existing agent instance. Together with one or more
decoder objects (such as
twisted.web.client.GzipDecoder ), this wrapper
automatically negotiates an encoding to use and decodes the response body
accordingly. To application code using such an agent, there is no visible
difference in the data delivered.
from twisted.python import log
from twisted.internet import reactor
from twisted.internet.defer import Deferred
from twisted.internet.protocol import Protocol
from twisted.web.client import Agent, ContentDecoderAgent, GzipDecoder
class BeginningPrinter(Protocol):
def __init__(self, finished):
self.finished = finished
self.remaining = 1024 * 10
def dataReceived(self, bytes):
if self.remaining:
display = bytes[:self.remaining]
print 'Some data received:'
print display
self.remaining -= len(display)
def connectionLost(self, reason):
print 'Finished receiving body:', reason.type, reason.value
self.finished.callback(None)
def printBody(response):
finished = Deferred()
response.deliverBody(BeginningPrinter(finished))
return finished
def main():
agent = ContentDecoderAgent(Agent(reactor), [('gzip', GzipDecoder)])
d = agent.request('GET', 'http://www.yahoo.com/')
d.addCallback(printBody)
d.addErrback(log.err)
d.addCallback(lambda ignored: reactor.stop())
reactor.run()
if __name__ == "__main__":
main()
Implementing support for new content encodings is as simple as writing a
new class like GzipDecoder that can decode a response using
the new encoding. As there are not many content encodings in widespread
use, gzip is the only encoding supported by Twisted itself.
Connecting To Non-standard Destinations¶
Typically you want your HTTP client to open a TCP connection directly to the web server.
Sometimes however it’s useful to be able to connect some other way, e.g. making an HTTP request over a SOCKS proxy connection or connecting to a server listening on a UNIX socket.
For this reason, there is an alternate constructor called Agent.usingEndpointFactory that takes an endpointFactory argument.
This argument must provide the twisted.web.iweb.IAgentEndpointFactory interface.
Note that when talking to a HTTP proxy, i.e. a server that implements the proxying-specific variant of HTTP you should use ProxyAgent - see documentation above.
# Copyright (c) Twisted Matrix Laboratories.
# See LICENSE for details.
"""
Send a HTTP request to Docker over a Unix socket.
Will probably need to be run as root.
Usage:
$ sudo python endpointconstructor.py [<docker API path>]
"""
from __future__ import print_function
from sys import argv
from zope.interface import implementer
from twisted.internet.endpoints import UNIXClientEndpoint
from twisted.internet.task import react
from twisted.web.iweb import IAgentEndpointFactory
from twisted.web.client import Agent, readBody
@implementer(IAgentEndpointFactory)
class DockerEndpointFactory(object):
"""
Connect to Docker's Unix socket.
"""
def __init__(self, reactor):
self.reactor = reactor
def endpointForURI(self, uri):
return UNIXClientEndpoint(self.reactor, b"/var/run/docker.sock")
def main(reactor, path=b"/containers/json?all=1"):
agent = Agent.usingEndpointFactory(reactor, DockerEndpointFactory(reactor))
d = agent.request(b'GET', b"unix://localhost" + path)
d.addCallback(readBody)
d.addCallback(print)
return d
react(main, argv[1:])
Conclusion¶
You should now understand the basics of the Twisted Web HTTP client. In particular, you should understand:
- How to issue requests with arbitrary methods, headers, and bodies.
- How to access the response version, code, phrase, headers, and body.
- How to store, send, and track cookies.
- How to control the streaming of the response body.
- How to enable the HTTP persistent connection, and control the number of connections.
Glossary¶
This glossary is very incomplete. Contributions are welcome.
resource
An object accessible via HTTP at one or more URIs. In Twisted Web, a resource is represented by an object which provides twisted.web.resource.IResource and most often is a subclass of twisted.web.resource.Resource . For example, here is a resource which represents a simple HTML greeting.
from twisted.web.resource import Resource class Greeting(Resource): def render_GET(self, request): return "
- Introduction
- Web Applications
- Other
- Appendix
Examples¶
twisted.web.client¶
getpage.py- usetwisted.web.client.getPageto download a web page.dlpage.py- add callbacks totwisted.web.client.downloadPageto display errors that occur when downloading a web page
XML-RPC¶
xmlrpc.pyXML-RPC server with several methods, including echoing, faulting, returning deferreds and failed deferredsxmlrpcclient.py- usetwisted.web.xmlrpc.Proxyto call remote XML-RPC methodsadvogato.py- usetwisted.web.xmlrpcto post a diary entry to advogato.org; requires an advogato account
Virtual hosts and proxies¶
proxy.py- usetwisted.web.proxy.Proxyto make the simplest proxylogging-proxy.py- example of subclassing the core classes oftwisted.web.proxyto log requests through a proxyreverse-proxy.py- usetwisted.web.proxy.ReverseProxyResourceto make any HTTP request to the proxy port get applied to a specified websiterootscript.py- example use oftwisted.web.vhost.NameVirtualHostweb.py- an example of both using theprocessorsattribute to set how certain file types are treated and usingtwisted.web.vhost.VHostMonsterResourceto reverse proxy
.rpys and ResourceTemplate¶
hello.rpy.py- usetwisted.web.staticto create a static resource to servefortune.rpy.py- create a resource that returns the output of a process run on the serverreport.rpy.py- display various properties of a resource, including path, host, and portusers.rpy.py- usetwisted.web.distribto publish user directories as for a “community web site”simple.rtl- example use oftwisted.web.resource.ResourceTemplate
Miscellaneous¶
webguard.py- pairingtwisted.webwithtwisted.credto guard resources against unauthenticated userssilly-web.py- bare-bones distributed web setup with a master and slave usingtwisted.web.distribandtwisted.spread.pbsoap.py- usetwisted.web.soapto publish SOAP methods
- Developer guides: documentation on using Twisted Web to develop your own applications
- Examples: short code examples using Twisted Web
Twisted Words¶
Developer Guides¶
Overview of Twisted IM¶
Twisted IM (Instance Messenger) is a multi-protocol chat framework, based on the Twisted framework we’ve all come to know and love. It’s fairly simple and extensible in two directions - it’s pretty easy to add new protocols, and it’s also quite easy to add new front-ends.
Code flow¶
AccountManager¶
The control flow starts at the relevant subclass of baseaccount.AccountManager . The AccountManager is responsible for, well, managing accounts - remembering what accounts are available, their settings, adding and removal of accounts, and making accounts log on at startup.
This would be a good place to start your interface, load a
list of accounts from disk and tell them to login. Most of the
method names in AccountManager
are pretty self-explanatory, and your subclass can override
whatever it wants, but you need to override __init__ . Something like
this:
...
def __init__(self):
self.chatui = ... # Your subclass of basechat.ChatUI
self.accounts = ... # Load account list
for a in self.accounts:
a.logOn(self.chatui)
ChatUI¶
Account objects talk to the user via a subclass of basechat.ChatUI . This class keeps track of all the various conversations that are currently active, so that when an account receives and incoming message, it can put that message in its correct context.
How much of this class you need to override depends on what
you need to do. You will need to override
getConversation (a one-on-one conversation, like
an IRC DCC chat) and getGroupConversation (a
multiple user conversation, like an IRC channel). You might
want to override getGroup and
getPerson .
The main problem with the default versions of the above
routines is that they take a parameter, Class ,
which defaults to an abstract implementation of that class -
for example, getConversation has a
Class parameter that defaults to basechat.Conversation which
raises a lot of NotImplementedError s. In your
subclass, override the method with a new method whose Class
parameter defaults to your own implementation of
Conversation , that simply calls the parent
class’ implementation.
Conversation and GroupConversation¶
These classes are where your interface meets the chat
protocol. Chat protocols get a message, find the appropriate
Conversation or GroupConversation
object, and call its methods when various interesting things
happen.
Override whatever methods you want to get the information
you want to display. You must override the hide
and show methods, however - they are called
frequently and the default implementation raises
NotImplementedError .
Accounts¶
An account is an instance of a subclass of basesupport.AbstractAccount .
For more details and sample code, see the various
*support files in twisted.words.im .
Using the Twisted IRC Client¶
A complete howto would explain how to actually use the IRC client. However, until that howto is written, here is a howto that explains how to do text formatting for IRC.
Text formatting¶
The text formatting support in Twisted Words is based on the widely used mIRC format which supports bold, underline, reverse video and colored text; nesting these attributes is also supported.
Creating formatted text¶
The API used for creating formatted text in the IRC client is almost the same as that used by Twisted insults . Text attributes are built up by accessing and indexing attributes on a special module-level attribute, twisted.words.protocols.irc.attributes , multiple values can be passed when indexing attributes to mix text with nested text attributes. The resulting object can then be serialized to formatted text, with twisted.words.protocols.irc.assembleFormattedText , suitable for use with any of the IRC client messaging functions.
Bold, underline and reverse video attributes¶
Bold, underline and reverse video attributes are just flags and are the
simplest text attributes to apply. They are accessed by the names
bold , underline and reverseVideo ,
respectively, on
twisted.words.protocols.irc.attributes . For
example, messaging someone the bold and underlined text “Hello world!”:
from twisted.words.protocols.irc import assembleFormattedText, attributes as A
# Message "someone" the bold and underlined text "Hello world!"
anIRCClient.msg('someone', assembleFormattedText(
A.bold[
A.underline['Hello world!']])
The “normal” attribute¶
At first glance a text attribute called “normal” that does not apply any unusual text attributes may not seem that special but it can be quite useful, both as a container:
A.normal[
'This is normal text. ',
A.bold['This is bold text! '],
'Back to normal',
A.underline['This is underlined text!']]
And also as a way to temporarily disable text attributes without having to close and respecify all text attributes for a brief piece of text:
A.normal[
A.reverseVideo['This is reverse, ', A.normal['except for this'], ', text']]
It is worth noting that assembled text will always begin with the control code to disable other attributes for the sake of correctness.
Color attributes¶
Since colors for both the foreground and background can be specified with
IRC text formatting another level of attribute access is introduced.
Firstly the foreground or background, through the
fg and bg attribute names respectively, is
accessed and then the color name is accessed. The available color
attribute names are:
- white
- black
- blue
- green
- lightRed
- red
- magenta
- orange
- yellow
- lightGreen
- cyan
- lightCyan
- lightBlue
- lightMagenta
- gray
- lightGray
It is possible to nest foreground and background colors to alter both for a single piece of text. For example to display black on green text:
A.fg.black[A.bg.green['Like a terminal!']]
Parsing formatted text¶
Most IRC clients format text so it is logical that you may want to parse
this formatted text.
twisted.words.protocols.irc.parseFormattedText
will parse text into structured text attributes. It is worth noting that
while feeding the output of parseFormattedText back to
assembleFormattedText will produce the same final result,
the actual structure of the parsed text will differ. Color codes are
mapped from 0 to 15, codes greater than 15 will begin to wrap around.
Removing formatting¶
In some cases, such as an automaton handling user input from IRC, it is desirable to have all formatting stripped from text. This can be accomplished with twisted.words.protocols.irc.stripFormatting .
Communicating With IRC Clients¶
Communicating with clients is the whole point of an IRC server, so you want to make sure you’re doing it properly. Today, we’ll be looking at receiving messages from a client and sending messages to the client.
Representing Clients in Twisted¶
Users in Twisted IRC are represented as subclasses of the IRC class. This works as the protocol for your Factory class. It will also give you IRC features (like automatically parsing incoming lines) without you having to implement them yourself. The rest of this guide assumes this setup.
Sending Messages¶
Messages are sent to users using the user object’s sendMessage method.
Sending Basic Messages¶
The basic syntax for sending messages to users is as follows:
user.sendCommand("COMMAND", (param1, param2), server.name)
The prefix keyword argument is optional, and it may be omitted to send a message without a prefix (for example, the ERROR command). The command is whatever command you plan to send, e.g. “PRIVMSG”, “MODE”, etc. All arguments following the command are the parameters you want to send for the command. If the last argument needs to be prefixed with a colon (because it has spaces in it, e.g. a PRIVMSG message), you must add the colon to the beginning of the parameter yourself. For example: .. code-block:: python
user.sendCommand(“PRIVMSG”, (user.nickname, ”:{}”.format(message)), sendingUser.hostmask)
Sending Messages with Tags¶
Twisted also allows sending message tags as specified in IRCv3.
Let’s say, for example, that your server has a feature to play back a little bit of previous channel content when someone joins a channel. You want a way to tell people when this message occurred. The best way to provide this information is through the server-time specification.
Let’s say you’re storing past messages in a channel object in some structure like this:
channel.pastMessages = [
("I sent some text!", "author!ident@host", datetime object representing the when the message was sent),
("I did, too!", "someone-else!ident@host", another datetime object)
]
Your actual implementation may vary. I went with something simple here. The times of the messages would be generated using something like datetime.utcnow() when the message was received.
Tags are passed as a list of tuples. If you’re sending a number of tags, you may have an existing tag dictionary. You can simply add to it (assuming message is the loop variable for channel.pastMessages above):
sendingTags["server-time"] = "{}Z".format(message[2].isoformat()[:-3])
This will generate the required time format and add it to the tag dictionary. The last three characters that we remove are the microseconds; removing the last three digits changes the precision to milliseconds.
Once your tags are collected, you can send the message. The tag dictionary is passed using the tags argument (in the same loop as above):
user.sendCommand("PRIVMSG", (user.nickname, message[0]), message[1], sendingTags)
Receiving Messages¶
Twisted Words will handle receiving messages and parsing lines into tokens. The parsed messages are passed into your command through the user’s handleCommand method.
Handling Commands¶
The default IRC handleCommand method calls the irc_COMMAND method when it receives the command COMMAND, and it calls irc_unknown if the method for the command received isn’t defined.
from twisted.words.protocols import irc
class IRCUser(irc.IRC):
# possibly other definitions here
def irc_unknown(self, prefix, command, params):
self.sendCommand(irc.ERR_UNKNOWNCOMMAND, (command, ":Unknown command"), server.name)
def irc_PRIVMSG(self, prefix, params):
# do some stuff to handle PRIVMSG for your server's setup
# lots of other command definitions
If you have a server setup that doesn’t allow you to do this (e.g. a modular server program), you may, of course, override the handleCommand function to route commands to your own handlers.
Receiving Messages with Tags¶
This has not yet been implemented.
Examples¶
ircLogBot.py- connects to an IRC server and logs all messagesminchat.py- log bot using twisted.immsn_example.pyoscardemo.pyjabber_client.pypb_client.pyxmpp_client.pycursesclient.py- trivial curses-based IRC client
- Developer guides: documentation on using Twisted Words to develop your own applications
- Examples: short code examples using Twisted Words
Historical Documents¶
Here are documents which contain no pertinent information or documentation. People from the Twisted team have published them, and they serve as interesting land marks and thoughts. Please don’t look here for documentation – however, if you are interested in the history of Twisted, or want to quote from these documents, feel free. Remember, however – the documents here may contain wrong information – they are not updated as Twisted is, to keep their historical value intact.
2003¶
Python Community Conference¶
These papers were part of the Python Community Conference (PyCon) in March of 2003.
Generalization of Deferred Execution in Python
A deceptively simple architectural challenge faced by many multi-tasking applications is gracefully doing nothing. Systems that must wait for the results of a long-running process, network message, or database query while continuing to perform other tasks must establish conventions for the semantics of waiting. The simplest of these is blocking in a thread, but it has significant scalability problems. In asynchronous frameworks, the most common approach is for long-running methods to accept a callback that will be executed when the command completes. These callbacks will have different signatures depending on the nature of the data being requested, and often, a great deal of code is necessary to glue one portion of an asynchronous networking system to another. Matters become even more complicated when a developer wants to wait for two different events to complete, requiring the developer to “juggle” the callbacks and create a third, mutually incompatible callback type to handle the final result.
This paper describes the mechanism used by the Twisted framework for waiting for the results of long-running operations. This mechanism, the
Deferred, handles the often-neglected problems of error handling, callback juggling, inter-system communication and code readability.
Applications of the Twisted Framework
Two projects developed using the Twisted framework are described; one, Twisted.names, which is included as part of the Twisted distribution, a domain name server and client API, and one, Pynfo, which is packaged separately, a network information robot.
Twisted Conch: SSH in Python with Twisted
Conch is an implementation of the Secure Shell Protocol (currently in the IETF standarization process). Secure Shell (or SSH) is a popular protocol for remote shell access, file management and port forwarding protected by military-grade security. SSH supports multiple encryption and compression protocols for the wire transports, and a flexible system of multiplexed channels on top. Conch uses the Twisted networking framework to supply a library which can be used to implement both SSH clients and servers. In addition, it also contains several ready made client programs, including a drop-in replacement for the OpenSSH program from the OpenBSD project.
The Lore Document Generation Framework
Lore is a documentation generation system which uses a limited subset of XHTML, together with some class attributes, as its source format. This allows for lower barrier of entry than many other similar systems, since HTML authoring tools are plentiful as is knowledge of HTML writing. As an added advantage, the source format is viewable directly, so that even if Lore is not available the documentation is useful. It currently outputs LaTeX and HTML, which allows for most use-cases.
Perspective Broker: “Translucent” Remote Method calls in Twisted
One of the core services provided by the Twisted networking framework is “Perspective Broker” , which provides a clean, secure, easy-to-use Remote Procedure Call (RPC) mechanism. This paper explains the novel features of PB, describes the security model and its implementation, and provides brief examples of usage.
Managing the Release of a Large Python Project
Twisted is a Python networking framework. At last count, the project contains nearly 60,000 lines of effective code (not comments or blank lines). When preparing a release, many details must be checked, and many steps must be followed. We describe here the technologies and tools we use, and explain how we built tools on top of them which help us make releasing as painless as possible.
Twisted Reality: A Flexible Framework for Virtual Worlds
Flexibly modelling virtual worlds in object-oriented languages has historically been difficult; the issues arising from multiple inheritance and order-of-execution resolution have limited the sophistication of existing object-oriented simulations. Twisted Reality avoids these problems by reifying both actions and relationships, and avoiding inheritance in favor of automated composition through adapters and interfaces.