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 inbpo-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 tobpo-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 NoneTypeError: Plural value must be an integer, got NoneType

New changesetbe5c79e0338005d675a64ba6e5b137e850d556d1 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 changesetecb2afc1bc1c2f3f9f99b09dd866100c3c7dcab7 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 changeset836f137f7ae0799de937e5281cb1da2bfdb8a69d 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