Python logging: Set handlers for all loggers of used modules

Question

I have my main script that interprets cli commands with argparse and then starts the application by calling the according stuff form an other module (made by myself).

My question now is how I can attach a handler to the logger from that module. The logger is retrieved with

logger = logging.getLogger(__name__)

I hence put following in my main script:

consoleHandler = logging.StreamHandler()
logger = logging.getLogger('MyModule')
logger.addHandler(consoleHandler)

However there is 0 logging output from 'MyModule'. Log levels are correct, eg there should be an output.

In MyModule I have the following:

logging.getLogger(__name__).addHandler(logging.NullHandler())

However removing that makes no difference.

So how can I correctly attach a handler to the logger of MyModule ?

Answer 1

The best example of adding handler to logger is the one from Docs , See below.

Did you added consoleHandler with setLevel() and the Formatter() ?

As for this line: logging.getLogger(__name__).addHandler(logging.NullHandler())

Why are you using it this way? I would recommend to follow the next code for adding an handler the right way.

import logging

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

# create console handler and set level to debug
ch = logging.StreamHandler()
ch.setLevel(logging.DEBUG)

# create formatter
formatter = logging.Formatter('%(asctime)s - %(name)s - %(levelname)s - %(message)s')

# add formatter to ch
ch.setFormatter(formatter)

# add ch to logger
logger.addHandler(ch)

# 'application' code
logger.debug('debug message')
logger.info('info message')
logger.warn('warn message')
logger.error('error message')
logger.critical('critical message')

Answer 2

The reason you're getting no output is most likely that you haven't set a level on the logger, so it will default to WARNING . Try doing logger.setLevel(logging.DEBUG) and see if that causes output to be produced. I know you said that log levels are correct, but I can't see any other reason why no output would be produced. You can try posting a small self-contained script that demonstrates the problem, if you like.

NullHandler only needs to be added for library modules to cater for their use in an application that doesn't configure logging. It is not needed in modules of an application that configures logging.

In general, you can add a handler to the root logger and it will be used by loggers in all modules.

Update: The comment to Kobi K's answer indicates that a level has been set on the handler - this is not enough, as you need to set a level on the logger, whose level is checked first. Only if the event is allowed through by the logger's level will the handlers (and their levels) come into play.