# 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)

  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.

### 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.

## Writing log observers

Log observers are the basis of the Twisted logging system. An example of a log observer in Twisted is the `FileLogObserver` used by `startLogging` that 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 raise an exception from a log observer. If your log observer raises an exception, it will be removed.
- Never block in a log observer, as it may run in main Twisted thread. This means you can't use socket or syslog Python-logging backends.
- The observer needs to be thread safe if you anticipate using threads in your program.

Index

Version: 2.1.0