Python logging configuration

logging.py

import logging
import logging.handlers
from functools import wraps

LOGGER_NAME = "APPLOGGER"
LOGGER_HANDLER_NAME = "APPHANDLER"
LOGGER_FILENAME = "log.out"
LOGGER_FORMATTER = "[%(asctime)s] %(levelname)s : %(user)s : %(funcName)s : %(message)s"
LOGGER_DATEFMT = "%Y-%m-%d %H:%M:%S"
LOGGER_USER = "app"


def make_logger():
    """ make and return the logger instance """
    formatter = logging.Formatter(LOGGER_FORMATTER, LOGGER_DATEFMT)
    handler = logging.handlers.RotatingFileHandler(LOGGER_FILENAME, maxBytes=200000, backupCount=50)
    handler.setFormatter(formatter)
    handler.setLevel(logging.DEBUG)
    handler.name = LOGGER_HANDLER_NAME

    logger = logging.getLogger(LOGGER_NAME)
    logger.addHandler(handler)
    logger.setLevel(logging.DEBUG)
    logger.addFilter(DefaultUserLoggingFilter())
    return logger


def log_this(f):
    """ decorater to implement the default app logger in case no logger is passed """
    @wraps(f)
    def decorated(*args, **kwargs):
        if "logger" in kwargs and kwargs["logger"]:
            pass
        else:
            kwargs["logger"] = logging.getLogger(LOGGER_NAME)
        return f(*args, **kwargs)
    return decorated


class DefaultUserLoggingFilter(logging.Filter):
    """ Create a custom filter to accept `user`.
    if no `user` is passed, consider a default value
    """
    def filter(self, log_record):
        if hasattr(log_record, "user") and log_record.user:
            return True
        log_record.user = LOGGER_USER
        return True


####################################################################
# and somewhere in this realm, a service is exposed to the Universe #
####################################################################
@log_this
def some_service(foo, bar, user=None, logger=None):
    logger.info((foo, bar, user), extra={"user":user})


if __name__ == "__main__":

    l = make_logger()
    l.info("Test message with default user")
    l.info("Test message with custom user", extra={"user":"saveshodhan"})

    some_service("foo", "bar")
    some_service("foo", "bar", "saveshodhan")


##########
# OUTPUT #
##########
"""
[2018-08-31 12:01:30] INFO : app : <module> : Test message with default user
[2018-08-31 12:01:30] INFO : saveshodhan : <module> : Test message with custom user
[2018-08-31 12:01:30] INFO : app : some_service : ('foo', 'bar', None)
[2018-08-31 12:01:30] INFO : saveshodhan : some_service : ('foo', 'bar', 'saveshodhan')
"""

To check if the logger exists (along with the handler set):

import logging.handlers
def logger_exists():
    """ checks if the app logger exists with its handler set
    its necessary to check the handler as well, because,
    if we simply call `logging.getLogger(LOGGER_NAME)` we will
    still get the logger instance, but it may or may not have the handler set
    """
    logger_dict = logging.Logger.manager.loggerDict
    try:
        assert any([LOGGER_HANDLER_NAME in x.name for x in logger_dict[LOGGER_NAME].handlers])
    except:
        return False
    return True

Courtesy:
Official documentation
A StackOverflow answer