Python logging multiple modules logger not working outside main program

Question

My goal is to log from multiple modules, while only configuring the logger in one place - in the main program. As shown in this answer, one should include

logging.config.fileConfig('/path/to/logging.conf') 

in the main program, then in all other modules include

logger = logging.getLogger(__name__)

I believe that's what I've done below, yet I get unexpected behaviour.

c.py

# c.py
import logging
import logging.config
import d

logging.config.fileConfig("logging.conf")
logger = logging.getLogger(__name__)

logger.warning('logging from c')
d.foo()

d.py

# d.py
import logging

logger = logging.getLogger(__name__)

# this will print when d is imported
logger.warning('logging from d on import')

def foo():
    # this does not print
    logger.warning("logging from d on call foo()")

Output

$ python c.py
logging from d on import
logging from c

What I would expect, is that when d.foo() executes in c.py, that a message would be logged from d, however, that's not the case. This is confusing because when the logger is called from module level in d, it logs a message to the console, but when called from inside foo() it doesn't.

Config file

[loggers]
keys=root

[handlers]
keys=consoleHandler

[formatters]
keys=simpleFormatter

[logger_root]
level=DEBUG
handlers=consoleHandler

[handler_consoleHandler]
class=StreamHandler
level=DEBUG
formatter=simpleFormatter
args=(sys.stdout,)

[formatter_simpleFormatter]
format=%(message)s
datefmt=

Further analysis

So I've noticed that if I remove the line

logging.config.fileConfig("logging.conf")

from c.py, then logging from d.foo() works as expected. So there's either something wrong in the configuration file or due to the fact that I provide a configuration file something causes the logger in d.py to get messed up.

Question

1. Why does a message get logged from d when called from module level, but not from inside d.foo()?
2. And how can I achieve the goal of logging from multiple modules, while only configuring the logging in the main program?

Answer 1

The problem lies in the fact that

import d

comes before

logging.config.fileConfig("logging.conf")

as pointed out by @davidejones. Here's why:

As stated in the docs, when logging.config.fileConfig() is called, its default behaviour is to disable any existing loggers. So when import d happens, logger is initialised in d, then when logging.config.fileConfig() is called, the logger in d is disabled, which is why we didn't see any logging when d.foo() was called.

Solution

logging.config.fileConfig() takes an argument, disable_existing_loggers, which is True by default. Use

logging.config.fileConfig("logging.conf", disable_existing_loggers=False)

And output becomes

>>> python c.py
logging from d on import
logging from c
logging from d on call foo()

As expected.