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vstinner@DESKTOP-DK7VBIL C:\vstinner\python\master>python -m test test_logging -m test_namer_rotator_inheritance
Running Debug|x64 interpreter...
0:00:00 Run tests sequentially
0:00:00 [1/1] test_logging
--- Logging error ---
Traceback (most recent call last):
 File "C:\vstinner\python\master\lib\logging\handlers.py", line 74, in emit
 self.doRollover()
 File "C:\vstinner\python\master\lib\logging\handlers.py", line 179, in doRollover
 self.rotate(self.baseFilename, dfn)
 File "C:\vstinner\python\master\lib\logging\handlers.py", line 117, in rotate
 self.rotator(source, dest)
 File "C:\vstinner\python\master\lib\test\test_logging.py", line 5222, in rotator
 os.rename(source, dest + ".rotated")
FileExistsError: [WinError 183] Cannot create a file when that file already exists: 'C:\\Users\\vstinner\\AppData\\Local\\Temp\\test_logging-2-tj71b
t8k.log' -> 'C:\\Users\\vstinner\\AppData\\Local\\Temp\\test_logging-2-tj71bt8k.log.1.test.rotated'
Call stack:
 File "C:\vstinner\python\master\lib\runpy.py", line 196, in _run_module_as_main
 return _run_code(code, main_globals, None,
 File "C:\vstinner\python\master\lib\runpy.py", line 86, in _run_code
 exec(code, run_globals)
 File "C:\vstinner\python\master\lib\test\__main__.py", line 2, in <module>
 main()
 File "C:\vstinner\python\master\lib\test\libregrtest\main.py", line 719, in main
 Regrtest().main(tests=tests, **kwargs)
 File "C:\vstinner\python\master\lib\test\libregrtest\main.py", line 641, in main
 self._main(tests, kwargs)
 File "C:\vstinner\python\master\lib\test\libregrtest\main.py", line 694, in _main
 self.run_tests()
 File "C:\vstinner\python\master\lib\test\libregrtest\main.py", line 521, in run_tests
 self.run_tests_sequential()
 File "C:\vstinner\python\master\lib\test\libregrtest\main.py", line 423, in run_tests_sequential
 result = runtest(self.ns, test_name)
 File "C:\vstinner\python\master\lib\test\libregrtest\runtest.py", line 194, in runtest
 return _runtest(ns, test_name)
 File "C:\vstinner\python\master\lib\test\libregrtest\runtest.py", line 154, in _runtest
 result = _runtest_inner(ns, test_name,
 File "C:\vstinner\python\master\lib\test\libregrtest\runtest.py", line 282, in _runtest_inner
 refleak = _runtest_inner2(ns, test_name)
 File "C:\vstinner\python\master\lib\test\libregrtest\runtest.py", line 246, in _runtest_inner2
 test_runner()
 File "C:\vstinner\python\master\lib\test\support\__init__.py", line 682, in inner
 return func(*args, **kwds)
 File "C:\vstinner\python\master\lib\test\test_logging.py", line 5503, in test_main
 support.run_unittest(*tests)
 File "C:\vstinner\python\master\lib\test\support\__init__.py", line 1082, in run_unittest
 _run_suite(suite)
 File "C:\vstinner\python\master\lib\test\support\__init__.py", line 959, in _run_suite
 result = runner.run(suite)
 File "C:\vstinner\python\master\lib\test\support\testresult.py", line 169, in run
 test(self.result)
 File "C:\vstinner\python\master\lib\unittest\suite.py", line 84, in __call__
 return self.run(*args, **kwds)
 File "C:\vstinner\python\master\lib\unittest\suite.py", line 122, in run
 test(result)
 File "C:\vstinner\python\master\lib\unittest\suite.py", line 84, in __call__
 return self.run(*args, **kwds)
 File "C:\vstinner\python\master\lib\unittest\suite.py", line 122, in run
 test(result)
 File "C:\vstinner\python\master\lib\unittest\case.py", line 652, in __call__
 return self.run(*args, **kwds)
 File "C:\vstinner\python\master\lib\unittest\case.py", line 592, in run
 self._callTestMethod(testMethod)
 File "C:\vstinner\python\master\lib\unittest\case.py", line 549, in _callTestMethod
 method()
 File "C:\vstinner\python\master\lib\test\test_logging.py", line 5229, in test_namer_rotator_inheritance
 rh.emit(self.next_rec())
Message: '2'
Arguments: None
== Tests result: SUCCESS ==
1 test OK.
Total duration: 1.1 sec
Tests result: SUCCESS

Ah, I only see this output on Windows. On Linux, I don't see it.

What does os.rename do on Linux? Does it just overwrite existing files by default?

I wrote PR 25684 to fix this issue.
> What does os.rename do on Linux? Does it just overwrite existing files by default?
os.rename() calls rename():
https://man7.org/linux/man-pages/man2/rename.2.html
 rename() renames a file, moving it between directories if
 required. Any other hard links to the file (as created using
 link(2)) are unaffected. Open file descriptors for oldpath are
 also unaffected.
 If newpath already exists, it will be atomically replaced, so
 that there is no point at which another process attempting to
 access newpath will find it missing. However, there will
 probably be a window in which both oldpath and newpath refer to
 the file being renamed.
On Windows, os.rename() is implemented with MoveFileExW(src, dst, 0).
Maybe the test should use os.replace() instead of os.rename()? On Windows, os.replace() is implemented with with MoveFileExW(src, dst, MOVEFILE_REPLACE_EXISTING).
HandlerWithNamerAndRotator.rotator() of test_logging calls os.rename() when the file already exists:
if os.path.exists(source):
 os.rename(source, dest + ".rotated")
And the test fails with "Cannot create a file when that file already exists"... well yes, we just tested that it exists.

New changeset fe52eb62191e640e720d184a9a1a04e965b8a062 by Victor Stinner in branch 'master':
bpo-43961: Fix test_logging.test_namer_rotator_inheritance() (GH-25684)
https://github.com/python/cpython/commit/fe52eb62191e640e720d184a9a1a04e965b8a062

New changeset 629ef0fb9cad6ac340d3be884af7b47fb393ae99 by Miss Islington (bot) in branch '3.9':
bpo-43961: Fix test_logging.test_namer_rotator_inheritance() (GH-25684) (GH-25688)
https://github.com/python/cpython/commit/629ef0fb9cad6ac340d3be884af7b47fb393ae99