gh-61011: Fix inheritance of nested mutually exclusive groups in argparse

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: Dougal Sutherland <[email protected]>

Author: serhiy-storchaka

Lib/argparse.py

@@ -1521,7 +1521,11 @@ def _add_container_actions(self, container):
         # 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

@@ -2929,6 +2929,35 @@ def test_groups_parents(self):
     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
+Dougal 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.