The call to 'ngettext' in 'ArgumentParser._match_argument' (Lib/argparse.py) uses a non-integer value, which causes it to fail with a nonsensical exception. Non-integer values were deprecated in bpo-28692.

This happens because the 'default' error message is computed unconditionally, even when the value of 'action.nargs' is 'None' and another message will be ultimately selected.

This issue is similar to bpo-35785 which was not reproduced by other people at the moment and was eventually closed. I could not create a short reproducer but could match several of the points described in that issue. A little debugging turned up that gettext was loading a catalog from the 'natural' PyPI package which uses 'Plural-Forms' too.

  File "/usr/local/Cellar/python/3.7.5/Frameworks/Python.framework/Versions/3.7/lib/python3.7/argparse.py", line 1755, in parse_args
    args, argv = self.parse_known_args(args, namespace)
  File "/usr/local/Cellar/python/3.7.5/Frameworks/Python.framework/Versions/3.7/lib/python3.7/argparse.py", line 1787, in parse_known_args
    namespace, args = self._parse_known_args(args, namespace)
  File "/usr/local/Cellar/python/3.7.5/Frameworks/Python.framework/Versions/3.7/lib/python3.7/argparse.py", line 1993, in _parse_known_args
    start_index = consume_optional(start_index)
  File "/usr/local/Cellar/python/3.7.5/Frameworks/Python.framework/Versions/3.7/lib/python3.7/argparse.py", line 1923, in consume_optional
    arg_count = match_argument(action, selected_patterns)
  File "/usr/local/Cellar/python/3.7.5/Frameworks/Python.framework/Versions/3.7/lib/python3.7/argparse.py", line 2086, in _match_argument
    action.nargs) % action.nargs
  File "/usr/local/Cellar/python/3.7.5/Frameworks/Python.framework/Versions/3.7/lib/python3.7/gettext.py", line 631, in ngettext
    return dngettext(_current_domain, msgid1, msgid2, n)
  File "/usr/local/Cellar/python/3.7.5/Frameworks/Python.framework/Versions/3.7/lib/python3.7/gettext.py", line 610, in dngettext
    return t.ngettext(msgid1, msgid2, n)
  File "/usr/local/Cellar/python/3.7.5/Frameworks/Python.framework/Versions/3.7/lib/python3.7/gettext.py", line 462, in ngettext
    tmsg = self._catalog[(msgid1, self.plural(n))]
  File "<string>", line 4, in func
  File "/usr/local/Cellar/python/3.7.5/Frameworks/Python.framework/Versions/3.7/lib/python3.7/gettext.py", line 168, in _as_int
    (n.__class__.__name__,)) from None
TypeError: Plural value must be an integer, got NoneType

New changeset be5c79e0338005d675a64ba6e5b137e850d556d1 by Tal Einat (Federico Bond) in branch 'master':
bpo-38821: Fix crash in argparse when using gettext (GH-17192)
https://github.com/python/cpython/commit/be5c79e0338005d675a64ba6e5b137e850d556d1

Thanks for the report and the PR with a fix, Federico!

As discussed on in the PR comments, the PR did not include tests since there are currently no tests for argparse i18n. Please create a new issue for that, referencing this one.

New changeset ecb2afc1bc1c2f3f9f99b09dd866100c3c7dcab7 by Miss Islington (bot) in branch '3.7':
bpo-38821: Fix crash in argparse when using gettext (GH-17192)
https://github.com/python/cpython/commit/ecb2afc1bc1c2f3f9f99b09dd866100c3c7dcab7

New changeset 836f137f7ae0799de937e5281cb1da2bfdb8a69d by Miss Islington (bot) in branch '3.8':
bpo-38821: Fix crash in argparse when using gettext (GH-17192)
https://github.com/python/cpython/commit/836f137f7ae0799de937e5281cb1da2bfdb8a69d