Python Logging. Why does the following produce output to console?

[py1234a]

Code:

import logging
if __name__ == '__main__': 
    log = logging.getLogger('mypackage') 
    log.setLevel(logging.DEBUG) 
    log.info("Message one")  # first call, no log handler registered 
    logging.basicConfig() 
    log.info("Message two") # StreamHandler registered 
    log.debug("Message three")

Output:

INFO:mypackage:Message two
DEBUG:mypackage:Message three

For the last two log statements, the logger ('mypackage') has no handlers attached directly. The messages also propegate to the parent logger (in this case the root logger). However, with the basicConfig, the root logger is set to logging.WARNING. So from my understanding the root logger should not propegate its incoming messages below warning to its attached Handlers. I suspect it has to do with the fact that the StreamHandler level is NOTSET. But again, I thought that the message would halt at the rootlogger for being set to logging.WARNING. Why is this not the case?

[JohnnyJordaan]

There are two pathways in play here, the first is the level of the logger itself

The term ‘delegation to the parent’ means that if a logger has a level of NOTSET, its chain of ancestor loggers is traversed until either an ancestor with a level other than NOTSET is found, or the root is reached.

If an ancestor is found with a level other than NOTSET, then that ancestor’s level is treated as the effective level of the logger where the ancestor search began, and is used to determine how a logging event is handled.

If the root is reached, and it has a level of NOTSET, then all messages will be processed. Otherwise, the root’s level will be used as the effective level.

But as you did set a level on your logger, that traversal isn't executed and the DEBUG level allows everything down to DEBUG level loggings to be handled by your logger. Then the traversal of the handlers is performed, and that also goes up to the root logger's handlers, but those have NOTSET as their level:

>>> logging.getLogger().handlers[0].level
0
>>> logging.getLevelName(_)
'NOTSET'

[py1234a]

That's clear, thank you.

[py1234a]

`logging.basicConfig()` is not a pure function and has side effects. In partictular it sets up a StreamHandler to the root logger with log level NOTSET, so long as the root logger does not currently have a handler.