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 withrepr
and logged.If you pass nothing, it will construct a Failure from the currently active exception, which makes it convenient to use in an
except
clause:try: x = 1 / 0 except BaseException: 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,
startLogging
will also redirect anything written tosys.stdout
andsys.stderr
to the log. You can disable this by passingsetStdout=False
tostartLogging
.
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 to
log.msg
or the message in the failure passed tolog.err
.
isError
This is a boolean that will be true if this event came from a call to
log.err
. If this is set, there may be afailure
item 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 from
sys.stdout
, i.e. this message came from aisError
is 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.