gh-157127: Separate names with commas in argparse mutually exclusive group error

Author: lazerg

Doc/library/argparse.rst

@@ -2099,7 +2099,7 @@ Mutual exclusion
      >>> group.add_argument('--bar', action='store_false')
      >>> parser.parse_args([])
      usage: PROG [-h] (--foo | --bar)
-     PROG: error: one of the arguments --foo --bar is required
+     PROG: error: one of the arguments --foo, --bar is required
 
    Note that currently mutually exclusive argument groups do not support the
    *title* and *description* arguments of

Lib/argparse.py

@@ -2500,7 +2500,7 @@ def consume_positionals(start_index):
                              for action in group._group_actions
                              if action.help is not SUPPRESS]
                     msg = _('one of the arguments %s is required')
-                    raise ArgumentError(None, msg % ' '.join(names))
+                    raise ArgumentError(None, msg % ', '.join(names))
 
         # return the updated namespace and the extra arguments
         return namespace, extras

Lib/test/test_argparse.py

@@ -6866,7 +6866,7 @@ def test_required_exclusive(self):
         args = parser.parse_intermixed_args('1 --foo 2'.split())
         self.assertEqual(NS(badger=['1', '2'], foo=True, spam=None), args)
         self.assertRaisesRegex(argparse.ArgumentError,
-                'one of the arguments --foo --spam is required',
+                'one of the arguments --foo, --spam is required',
                 parser.parse_intermixed_args, '1 2'.split())
         self.assertEqual(group.required, True)
 
@@ -6882,7 +6882,7 @@ def test_required_exclusive_with_positional(self):
         args = parser.parse_intermixed_args(['a', 'b'])
         self.assertEqual(NS(foo=False, spam=None, badger=['a', 'b']), args)
         self.assertRaisesRegex(argparse.ArgumentError,
-                'one of the arguments --foo --spam badger is required',
+                'one of the arguments --foo, --spam, badger is required',
                 parser.parse_intermixed_args, [])
         self.assertRaisesRegex(argparse.ArgumentError,
                 'argument badger: not allowed with argument --foo',
@@ -7258,7 +7258,7 @@ def test_required_mutually_exclusive_args(self):
         group.add_argument('--bar')
         group.add_argument('--baz')
         self.assertRaisesRegex(argparse.ArgumentError,
-                               'one of the arguments --bar --baz is required',
+                               'one of the arguments --bar, --baz is required',
                                self.parser.parse_args, [])
 
     def test_conflicting_mutually_exclusive_args_optional_with_metavar(self):

Misc/NEWS.d/next/Library/2026-09-08-00-31-56.gh-issue-157127.gduPd0.rst

@@ -0,0 +1,2 @@
+Separate the argument names with commas in the :mod:`argparse` error message
+reported for a missing required mutually exclusive group.