pythongh-96310: Fix a traceback when all options in a mutually exclus…

…ive group are suppressed

Reproducer depends on terminal size - the traceback occurs when there's
an option long enough so the usage line doesn't fit the terminal width.
Option order is also important for reproducibility.

Excluding empty groups (with all options suppressed) from inserts
fixes the problem.

Lib/argparse.py

@@ -405,6 +405,8 @@ def _format_actions_usage(self, actions, groups):
             else:
                 end = start + len(group._group_actions)
                 if actions[start:end] == group._group_actions:
+                    if all((action.help is SUPPRESS for action in group._group_actions)):
+                        continue
                     for action in group._group_actions:
                         group_actions.add(action)
                     if not group.required:

Lib/test/test_argparse.py

@@ -2695,6 +2695,25 @@ def test_help(self):
               '''
         self.assertEqual(parser.format_help(), textwrap.dedent(expected))
 
+    def test_help_subparser_all_mutually_exclusive_group_members_suppressed(self):
+        self.maxDiff = None
+        parser = ErrorRaisingArgumentParser(prog='PROG')
+        commands = parser.add_subparsers(title="commands", dest="command")
+        cmd_foo = commands.add_parser("foo")
+        group = cmd_foo.add_mutually_exclusive_group()
+        group.add_argument('--verbose', action='store_true', help=argparse.SUPPRESS)
+        group.add_argument('--quiet', action='store_true', help=argparse.SUPPRESS)
+        cmd_foo.add_argument("--longlonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglong")
+        expected = '''\
+            usage: PROG foo [-h]
+                            [--longlonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglong LONGLONGLONGLONGLONGLONGLONGLONGLONGLONGLONGLONGLONGLONGLONGLONGLONGLONGLONGLONGLONGLONGLONGLONGLONGLONGLONGLONGLONGLONGLONGLONG]
+
+            options:
+              -h, --help            show this help message and exit
+              --longlonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglong LONGLONGLONGLONGLONGLONGLONGLONGLONGLONGLONGLONGLONGLONGLONGLONGLONGLONGLONGLONGLONGLONGLONGLONGLONGLONGLONGLONGLONGLONGLONGLONG
+              '''
+        self.assertEqual(cmd_foo.format_help(), textwrap.dedent(expected))
+
     def test_empty_group(self):
         # See issue 26952
         parser = argparse.ArgumentParser()