gh-157127: Reword the argparse mutually exclusive group error message (#157134)

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 following arguments is required: --foo, --bar
 
    Note that currently mutually exclusive argument groups do not support the
    *title* and *description* arguments of

Lib/argparse.py

@@ -2499,8 +2499,8 @@ def consume_positionals(start_index):
                     names = [_get_action_name(action)
                              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))
+                    msg = _('one of the following arguments is required: %s')
+                    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 following arguments is required: --foo, --spam',
                 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 following arguments is required: --foo, --spam, badger',
                 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 following arguments is required: --bar, --baz',
                                self.parser.parse_args, [])
 
     def test_conflicting_mutually_exclusive_args_optional_with_metavar(self):

Lib/test/translationdata/argparse/msgids.txt

@@ -18,7 +18,7 @@ invalid %(type)s value: %(value)r
 invalid choice: %(value)r (choose from %(choices)s)
 invalid choice: %(value)r, maybe you meant %(closest)r? (choose from %(choices)s)
 not allowed with argument %s
-one of the arguments %s is required
+one of the following arguments is required: %s
 option '%(option)s' is deprecated
 options
 positional arguments

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

@@ -0,0 +1,3 @@
+Reword the :mod:`argparse` error message reported for a missing required
+mutually exclusive group to ``one of the following arguments is required:
+--foo, --bar``, so that the argument names are separated by commas.