Using logger across multiple modules / functions

Question

I have the following code for creating a logger:

import logging
import a as a


def main():

    # create logger
    logger = logging.getLogger('cli_logger')
    logger.setLevel(logging.DEBUG)

    #Create file handler
    fh = logging.FileHandler('cli_log.log')
    fh.setLevel(logging.DEBUG)

    # create console handler
    ch = logging.StreamHandler()
    ch.setLevel(logging.INFO)

    # create formatter and add it to the handlers
    formatter = logging.Formatter('\n%(asctime)s - %(module)s - %(funcName)s()\n%(message)s\n')
    fh.setFormatter(formatter)
    ch.setFormatter(formatter)

    # add the handlers to the logger
    logger.addHandler(fh)
    logger.addHandler(ch)

    logger.info("This should print to console and log file!")
    logger.debug("This should print to the log file!")

    #Do module stuff
    a.write_something1()
    a.write_something2()
    ...

if __name__ == '__main__':
    main()

Does this mean that - for each function in the module a, I have to get the logger again?

i.e

module a
import logging

def write_something1():
    logger = logging.getLogger('cli_logger')

    logger.info('Module A1: Console and Debug Log')
    logger.debug('Module A1: Debug Log')

def write_something2():
    logger = logging.getLogger('cli_logger')

    logger.info('Module A2: Console and Debug Log')
    logger.debug('Module A2: Debug Log')

user3159253 · Accepted Answer · 2014-02-07 11:50:27Z

2

First of all, I would avoid "in-program" configuration of the logger, and move all configuration stuff to an external file as described in http://docs.python.org/2/howto/logging.html#configuring-logging.

Also, in every module I would create a module-wide logger instance specific for that particular module . Thus you'll get a possibility to increase verbosity of a particular module. Also you won't need explicit labels at the beginning of log messages.

Regarding code duplication. Actually it's only one line:

# this is my.library.module
import logging
logger = logging.getLogger("my.library.module")

logger.debug("module loaded")
...

edited Feb 7, 2014 at 11:50

answered Feb 7, 2014 at 10:40

user3159253

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4 Comments

TomSelleck Over a year ago

Is there any benefit to each module having it's own logger? I would have thought a central point would have made more sense. Seem's like there would be a lot of code duplication if each module had it's own logger. Also, then I would need to initialise the Logger for each module before it could log anything right?

user3159253 Over a year ago

Well, you'll get the central point (root logger). All other loggers are "derived" from that logger, so you can easily tune them all from one common place. Or configure specific one. From the documentation mentioned above:

Each instance has a name, and they are conceptually arranged in a namespace hierarchy using dots (periods) as separators. For example, a logger named ‘scan’ is the parent of loggers ‘scan.text’, ‘scan.html’ and ‘scan.pdf’

. Certainly there should be a reasonable limit and probably some modules of your package may share a logger if they perform a logically unsplittable task.

user3159253 Over a year ago

Also it's worth mentioning that Python logging facility is modelled after Java log4j and other related works, so you may easily adapt your previous experience to the Python environment

Vinay Sajip Over a year ago

The documented recommendation (docs.python.org/2/howto/logging.html#advanced-logging-tutorial) is to use __name__ as the logger name in each module.

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