OK, I get it now :-)

See my comments on the PR - I think the renaming strategy might need rethinking in the presence of a "namer" routine being set. See bpo-43344, bpo-44753. This area (TimedRotatingFileHandler) is unfortunately not yet well tested, because you would need mocking to avoid delays in test runs. But ideally, we would test in the absence and presence of a renamer routine that moved the ".log" to the end of the filename.