gh-61011: Fix inheritance of nested mutually exclusive groups in argparse (GH-125210)

Previously, all nested mutually exclusive groups lost their connection
to the group containing them and were displayed as belonging directly
to the parser.

Co-authored-by: Danica J. Sutherland <djsutherland@users.noreply.github.com>

Lib/argparse.py

@@ -1521,7 +1521,11 @@ class _ActionsContainer(object):
         # NOTE: if add_mutually_exclusive_group ever gains title= and
         # description= then this code will need to be expanded as above
         for group in container._mutually_exclusive_groups:
-            mutex_group = self.add_mutually_exclusive_group(
+            if group._container is container:
+                cont = self
+            else:
+                cont = title_group_map[group._container.title]
+            mutex_group = cont.add_mutually_exclusive_group(
                 required=group.required)
 
             # map the actions to their new mutex group

Lib/test/test_argparse.py

@@ -2942,6 +2942,35 @@ class TestParentParsers(TestCase):
     def test_wrong_type_parents(self):
         self.assertRaises(TypeError, ErrorRaisingArgumentParser, parents=[1])
 
+    def test_mutex_groups_parents(self):
+        parent = ErrorRaisingArgumentParser(add_help=False)
+        g = parent.add_argument_group(title='g', description='gd')
+        g.add_argument('-w')
+        g.add_argument('-x')
+        m = g.add_mutually_exclusive_group()
+        m.add_argument('-y')
+        m.add_argument('-z')
+        parser = ErrorRaisingArgumentParser(prog='PROG', parents=[parent])
+
+        self.assertRaises(ArgumentParserError, parser.parse_args,
+            ['-y', 'Y', '-z', 'Z'])
+
+        parser_help = parser.format_help()
+        self.assertEqual(parser_help, textwrap.dedent('''\
+            usage: PROG [-h] [-w W] [-x X] [-y Y | -z Z]
+
+            options:
+              -h, --help  show this help message and exit
+
+            g:
+              gd
+
+              -w W
+              -x X
+              -y Y
+              -z Z
+        '''))
+
 # ==============================
 # Mutually exclusive group tests
 # ==============================

Misc/ACKS

@@ -1814,6 +1814,7 @@ Reuben Sumner
 Eryk Sun
 Sanjay Sundaresan
 Marek Šuppa
+Danica J. Sutherland
 Hisao Suzuki
 Kalle Svensson
 Andrew Svetlov

Misc/NEWS.d/next/Library/2024-10-09-21-42-43.gh-issue-61011.pQXZb1.rst

@@ -0,0 +1,4 @@
+Fix inheritance of nested mutually exclusive groups from parent parser in
+:class:`argparse.ArgumentParser`. Previously, all nested mutually exclusive
+groups lost their connection to the group containing them and were displayed
+as belonging directly to the parser.