The documentation for the logging.Filter().filter() method states:

#-----
Is the specified record to be logged? Returns 0 for no, nonzero for yes. If deemed appropriate, the record may be modified in-place.
#-----

While its implementation returns a boolean in Lib/logging/__init__.py.
Moreover the most recent version of mypy (0.790) reports it as an error if a custom class inherit from logging.Filter and implement a filter() method that returns an int as specified in the documentation:

#-----
error: Return type "int" of "filter" incompatible with return type "bool" in supertype "Filter"  [override]
#-----

P.S. As a side note, the API for filter() is quite counter-intuitive as it requires the filter() method to return False to filter out the record and True to include it.

I agree that the documentation now should use Booleans rather than ints for the return type. It's a hangover from the pre-bool days of Python 1.5.2 ...

> P.S. As a side note, the API for filter() is quite counter-intuitive

That depends on if you consider the "filter" operation as "filter in" or "filter out", but the documentation is clear on what needs to be returned for a particular outcome.

New changeset e9959c71185d0850c84e3aba0301fbc238f194a9 by Necdet Can Atesman in branch 'master':
bpo-42011: Update documentation of logging.Filter.filter() (GH-22692)
https://github.com/python/cpython/commit/e9959c71185d0850c84e3aba0301fbc238f194a9

New changeset 8f7eab788aedcf1a23ef9be767e1b3502a6bbd5c by Miss Skeleton (bot) in branch '3.8':
bpo-42011: Update documentation of logging.Filter.filter() (GH-22692) (GH-22724)
https://github.com/python/cpython/commit/8f7eab788aedcf1a23ef9be767e1b3502a6bbd5c

New changeset dff9161821032acfd2588d42d88511ebdbabaaf3 by Miss Skeleton (bot) in branch '3.9':
bpo-42011: Update documentation of logging.Filter.filter() (GH-22692) (GH-22725)
https://github.com/python/cpython/commit/dff9161821032acfd2588d42d88511ebdbabaaf3