bpo-38821: Fix crash in argparse when using gettext (GH-17192)

(cherry picked from commit be5c79e)

Co-authored-by: Federico Bond <[email protected]>

Author: miss-islington

Lib/argparse.py

@@ -2080,10 +2080,11 @@ def _match_argument(self, action, arg_strings_pattern):
                 OPTIONAL: _('expected at most one argument'),
                 ONE_OR_MORE: _('expected at least one argument'),
             }
-            default = ngettext('expected %s argument',
+            msg = nargs_errors.get(action.nargs)
+            if msg is None:
+                msg = ngettext('expected %s argument',
                                'expected %s arguments',
                                action.nargs) % action.nargs
-            msg = nargs_errors.get(action.nargs, default)
             raise ArgumentError(action, msg)
 
         # return the number of arguments matched

Misc/NEWS.d/next/Library/2019-11-16-23-26-25.bpo-38821.-albNN.rst

@@ -0,0 +1 @@
+Fix unhandled exceptions in :mod:`argparse` when internationalizing error messages for arguments with ``nargs`` set to special (non-integer) values.  Patch by Federico Bond.