Python logger with a callback function

Question

is there a way to configure a python logger to call a custom function and pass it the logging message whenever it loggs? Thanks!

Answer 1

Subclass logging.Handler and implement the emit method:

import logging

class MyHandler(logging.Handler):
    def emit(self, record):
        print('custom handler called with\n   ', record)

logger = logging.getLogger(__name__)
logger.addHandler(MyHandler())   # or: logger.handlers = [MyHandler()]
logger.warning('Log 1')
logger.warning('Log 2')

custom handler called with
    <LogRecord: __main__, 30, /home/jan/Dropbox/py/so_logging.py, 9, "Log 1">
custom handler called with
    <LogRecord: __main__, 30, /home/jan/Dropbox/py/so_logging.py, 10, "Log 2">

The specific message being logged can be accessed as record.msg (unformatted string) or self.format(record) (formatted string, in case you added a timestamp or loglevel)

Additionally: If you also override the logging.Handler's __init__ method, it is important to do the super call to it in the subclass.

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Standard caveats for the logging module apply:

• The default loglevel is WARNING (so DEBUG/INFO aren't passed to emit)
• A log record is passed to all handlers of a logger
  • After that, it is passed to its ancestor in the logging hierarchy recursively