How to use python logging in multiple modules

Question

I was wondering what the standard set up is for performing logging from within a Python app.

I am using the Logging class, and I\'ve written my own logger class that

Answer 1

Try using logging.getLogger() to get your logging object instance:

http://docs.python.org/3/library/logging.html#logging.getLogger

All calls to this function with a given name return the same logger instance. This means that logger instances never need to be passed between different parts of an application.

UPDATE:

The recommended way to do this is to use the getLogger() function and configure it (setting a handler, formatter, etc...):

# main.py
import logging
import lib


def main():
    logger = logging.getLogger('custom_logger')
    logger.setLevel(logging.INFO)
    logger.addHandler(logging.FileHandler('test.log'))
    logger.info('logged from main module')
    lib.log()

if __name__ == '__main__':
    main()

# lib.py
import logging


def log():
    logger = logging.getLogger('custom_logger')
    logger.info('logged from lib module')

If you really need to extend the logger class take a look at logging.setLoggerClass(klass)

UPDATE 2:

Example on how to add a custom logging level without changing the Logging class:

# main.py
import logging
import lib


# Extend Logger class
CUSTOM_LEVEL_NUM = 9
logging.addLevelName(CUSTOM_LEVEL_NUM, 'CUSTOM')
def custom(self, msg, *args, **kwargs):
    self._log(CUSTOM_LEVEL_NUM, msg, args, **kwargs)
logging.Logger.custom = custom

# Do global logger instance setup
logger = logging.getLogger('custom_logger')
logger.setLevel(logging.INFO)
logger.addHandler(logging.FileHandler('test.log'))


def main():
    logger = logging.getLogger('custom_logger')
    logger.custom('logged from main module')
    lib.log()

if __name__ == '__main__':
    main()

Note that adding custom level is not recommended: http://docs.python.org/2/howto/logging.html#custom-levels

Defining a custom handler and maybe using more than one logger may do the trick for your other requirement: optional output to stderr.