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.

New changeset 9fe42b49c79c453d905d0395150ba0607fbab18b by Serhiy Storchaka (Zackery Spytz) in branch 'master':
bpo-37393: Fix deprecation warnings in test_ntpath. (GH-14357)
https://github.com/python/cpython/commit/9fe42b49c79c453d905d0395150ba0607fbab18b

New changeset 20372d65243d5573fba17d026681a3b24d95c284 by Miss Islington (bot) in branch '3.8':
bpo-37393: Fix deprecation warnings in test_ntpath. (GH-14357)
https://github.com/python/cpython/commit/20372d65243d5573fba17d026681a3b24d95c284

New changeset 695d7ad710de17b33870d34e0fff04a31f0f3e2b by Miss Islington (bot) in branch '3.7':
bpo-37393: Fix deprecation warnings in test_ntpath. (GH-14357)
https://github.com/python/cpython/commit/695d7ad710de17b33870d34e0fff04a31f0f3e2b

Thank you Zackery!

We should also remove that deprecation warning, as PEP 528 (or was it PEP 529) undeprecated bytes paths.