Logging with twisted.python.log¶

Basic usage¶

Twisted provides a simple and flexible logging system in the twisted.python.log module. It has three commonly used functions:

msg

Logs a new message. For example:

from twisted.python import log
log.msg('Hello, world.')

err

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 repr 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:
    log.err()   # will log the ZeroDivisionError

startLogging

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 to sys.stdout and sys.stderr to the log. You can disable this by passing setStdout=False to startLogging .

Before startLogging is called, log messages will be discarded and errors will be written to stderr.

observer = log.PythonLoggingObserver()
observer.start()

log.msg("This is important!", logLevel=logging.CRITICAL)
log.msg("Don't mind", logLevel=logging.DEBUG)

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 to log.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 a failure 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 a print statement. If isError is also true, it came from sys.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.