Propagated logs with lower level get logged in the parent logger

[frostming]

Bug report

Bug description:

import logging

logger = logging.getLogger(__name__)
logger.setLevel(logging.DEBUG)
logger.addHandler(logging.NullHandler())

logging.basicConfig()  # set formatter and handler for root logger(WARNING level)

logger.debug("This is a debug message")  # Should this be logged?

As title, if I logged a message with matching level on the child logger, the message will also be propagated to the parent, even if the parent logger has a higher level configured.

Actual Result

DEBUG:__main__:This is a debug message

Output log from the root logger.

Expected Result

The message should not be printed since root logger has WARNING level

CPython versions tested on:

3.14

Operating systems tested on:

macOS

Linked PRs

• gh-139878: check logger level when propagating logs #139879

[picnixz]

Correct me if I'm wrong, but here, logging.basicConfig() wouldn't change anything here. logger = logging.getLogger(__name__) already returns the main logger. If this is the case, then the root logger won't be reconfigured because there is no force=True when calling basicConfig(). Now, I seem to be unable to see the effect of force=True though but that may be because of the REPL session.

• Could you check if the logger you created is not already the root logger?
• Can you check the logging level of your logger before and after calling basicConfig()?
• Can you check the handlers it contains as well?
• Could you check if using a non-NULL handler changes something if you call logging.basicConfig(force=True) instead?

[frostming]

Correct me if I'm wrong, but here, logging.basicConfig() wouldn't change anything here. logger = logging.getLogger(__name__) already returns the main logger. If this is the case, then the root logger won't be reconfigured because there is no force=True when calling basicConfig().

That is not correct, logger = logging.getLogger(__name__) creates a logger with name=__main__, while basicConfig() configures the root logger. So these two statements create a logger hierarchy as root -> __main__

• Could you check if the logger you created is not already the root logger?

  No.

• Can you check the logging level of your logger before and after calling basicConfig()?

  The logging level is always DEBUG for logger and WARNING for root logger.

• Can you check the handlers it contains as well?

  basicConfig() adds a StreamHandler(stderr) to the root logger and doesn't touch my logger.

• Could you check if using a non-NULL handler changes something if you call logging.basicConfig(force=True) instead?

  The same, if using a non-NULL handler for my logger it will be logged twice, the first one comes from this handler and the second comes from root logger handler

[frostming]

Hmm, I feel like this is by design, so that users can just "declare" a bare logger and reuse the handlers from the root logger.

If so, feel free to close this.