Example of logging formatter factory usage with fileConfig()

custfmt.py

import logging

class CustomFormatter(logging.Formatter):
    def __init__(self, default):
        self.default = default

    def format(self, record):
        if record.levelno in (logging.WARNING,
                              logging.ERROR,
                              logging.CRITICAL):
            record.msg = '[%s] %s' % (record.levelname, record.msg)
        return self.default.format(record)

def factory(fmt, datefmt):
    default = logging.Formatter(fmt, datefmt)
    return CustomFormatter(default)

fcfgtest.py

from io import StringIO
import logging
import logging.config

CONFIG = u'''
[loggers]
keys=root

[handlers]
keys=console

[handler_console]
class=logging.StreamHandler
args=(sys.stderr,)
formatter=custom

[formatters]
keys=custom

[logger_root]
level=DEBUG
handlers=console

[formatter_custom]
format=%(asctime)s %(message)s
datefmt=%X
class=custfmt.factory
'''

cfg = StringIO(CONFIG)
logging.config.fileConfig(cfg)
logging.debug('debug message')
logging.info('info message')
logging.warning('warning message')
logging.error('error message')
logging.critical('critical message')

output.log

20:17:59 debug message
20:17:59 info message
20:17:59 [WARNING] warning message
20:17:59 [ERROR] error message
20:17:59 [CRITICAL] critical message