C:\CPython\cpython>python.bat -Werror -m test test_ntpath
Running Debug|x64 interpreter...
Run tests sequentially
0:00:00 load avg: 0.00 [1/1] test_ntpath
test test_ntpath failed -- Traceback (most recent call last):
  File "C:\CPython\cpython\\lib\ntpath.py", line 518, in abspath
    return normpath(_getfullpathname(path))
OSError: [WinError 123] The filename, directory name, or volume label syntax is
incorrect: b''

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "C:\CPython\cpython\lib\test\test_ntpath.py", line 285, in test_abspath
    tester('ntpath.abspath("")', cwd_dir)
  File "C:\CPython\cpython\lib\test\test_ntpath.py", line 41, in tester
    gotResult = eval(fn)
  File "<string>", line 1, in <module>
  File "C:\CPython\cpython\\lib\ntpath.py", line 520, in abspath
    return _abspath_fallback(path)
  File "C:\CPython\cpython\\lib\ntpath.py", line 501, in _abspath_fallback
    cwd = os.getcwdb()
DeprecationWarning: The Windows bytes API has been deprecated, use Unicode filenames
instead

test_ntpath failed

== Tests result: FAILURE ==

1 test failed:
    test_ntpath

Total duration: 1 sec 297 ms
Tests result: FAILURE


It seems that eval() is called an extra time without a warning filter in the tester() function.