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.
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.
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(x): """ 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. """ d = defer.Deferred() # simulate a delayed result by asking the reactor to fire the # Deferred in 2 seconds time with the result x * 3 reactor.callLater(2, d.callback, x * 3) return d def printData(d): """ Data handling function to be added as a callback: handles the data by printing the result """ print d d = getDummyData(3) d.addCallback(printData) # 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 reactor.run()
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 printData(d): print d def printError(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(printData) d.addErrback(printError) # this series of callbacks and errbacks will print "Result: 12" g = Getter() d = g.getDummyData(4) d.addCallback(printData) d.addErrback(printError) reactor.callLater(4, reactor.stop) reactor.run()
- Pay particular attention to the handling of
gotResultsmethod. Before the
Deferredis fired with a result or an error, the attribute is set to
Noneso that the
Getterinstance no longer has a reference to the
Deferredabout to be fired. This has several benefits. First, it avoids any chance
Getter.gotResultswill accidentally fire the same
Deferredmore than once (which would result in an
AlreadyCalledErrorexception). Second, it allows a callback on that
Getter.getDummyData(which sets a new value for the
dattribute) without causing problems. Third, it makes the Python garbage collector’s job easier by eliminating a reference cycle.
- 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
.callback(result)if the operation succeeded,
.errback(failure)if it failed. Note that
failureis typically an instance of a twisted.python.failure.Failure instance.
- Deferred object triggers previously-added (call/err)back
failure. 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
- If an errback doesn’t raise an exception or return a twisted.python.failure.Failure instance, switch to callback.
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
of ordinary Python code.
Unless you explicitly
raise an error in an except
Exception is caught and stops
propagating, and normal execution continues. The same thing happens with
errbacks: unless you explicitly
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
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
match the error encapsulated in that
Failure , then
it re-raises the error.
There’s another potential “gotcha” here. There’s a
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
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
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
it will not do any handling of 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
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,
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
isValidUser or an
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 (
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
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
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
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
isValidUser , ensuring that the result of
isValidUser is a Deferred,
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)
isValidUser could be either
It is also possible to modify
synchronousIsValidUser to return
a Deferred, see Generating Deferreds for more
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
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 interruptable: 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
.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:
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.
Deferredmay already have a result, and so there’s nothing to cancel.
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
Deferred s that support cancellation should
document what they do when cancelled, if they are uncancellable in certain edge
If the cancelled
Deferred is waiting on another
Deferred , the cancellation will be forwarded to the other
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.
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.
operationDonehas not yet been invoked, then
operationwill be immediately errbacked with a
However, once it’s cancelled, there’s no way to tell
operationDonenot to run; it will eventually call
operation.callbacklater. In normal operation, issuing
Deferredthat has already called back results in an
AlreadyCalledError, and this would cause an ugly traceback that could not be caught. Therefore,
.callbackcan be invoked exactly once, causing a no-op, on a
Deferredwhich has been cancelled but has no canceller. If you call it multiple times, you will still get an
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
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.
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
True . See below for more details about this and other
flags which modify the behavior of DeferredList.
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')]
DeferredList accepts three keyword arguments that modify its behaviour:
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).
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.
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  . Passing a true value
consumeErrors parameter will not change the behavior of
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 printResult(result): print result d.addCallback(printResult) d1.callback("one") # nothing is printed yet; d is still awaiting completion of d2 d2.callback("two") # printResult prints ["one", "two"]
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.
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 tuple
callbackArgswill 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
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.
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.
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 to
otherDeferred.callback. Further additions to my callback chain do not affect
This is the same as
- Generating Deferreds , an introduction to writing asynchronous functions that return Deferreds.
|||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.|