gh-113008: Correct argparse usage output for required, mutually exclusive groups

Lib/argparse.py

@@ -398,13 +398,17 @@ def _format_actions_usage(self, actions, groups):
                 raise ValueError(f'empty group {group}')
 
             try:
-                start = actions.index(group._group_actions[0])
+                start = _sys.maxsize
+                for item in group._group_actions:
+                    index = actions.index(item)
+                    if index < start:
+                        start = index
             except ValueError:
                 continue
             else:
                 group_action_count = len(group._group_actions)
                 end = start + group_action_count
-                if actions[start:end] == group._group_actions:
+                if set(actions[start:end]) == set(group._group_actions):
 
                     suppressed_actions_count = 0
                     for action in group._group_actions:

Lib/test/test_argparse.py

@@ -2836,6 +2836,29 @@ def test_help(self):
               '''
         self.assertEqual(parser.format_help(), textwrap.dedent(expected))
 
+    def test_optional_order(self):
+        parser = ErrorRaisingArgumentParser(prog="PROG")
+        group = parser.add_mutually_exclusive_group(required=True)
+        group.add_argument("-", dest="FOO")
+        group.add_argument("bar", nargs="?")
+        expected = '''\
+            usage: PROG [-h] (- FOO | bar)
+
+            positional arguments:
+              bar
+
+            options:
+              -h, --help  show this help message and exit
+              - FOO
+              '''
+        self.assertEqual(parser.format_help(), textwrap.dedent(expected))
+
+        parser = ErrorRaisingArgumentParser(prog="PROG")
+        group = parser.add_mutually_exclusive_group(required=True)
+        group.add_argument("bar", nargs="?")
+        group.add_argument("-", dest="FOO")
+        self.assertEqual(parser.format_help(), textwrap.dedent(expected))
+
     def test_empty_group(self):
         # See issue 26952
         parser = argparse.ArgumentParser()

Misc/NEWS.d/next/Library/2023-12-14-13-43-27.gh-issue-113008.jWYn8T.rst

@@ -0,0 +1 @@
+Correct argparse usage output for required, mutually exclusive groups containing a positional argument