GH-64532: Include parent's required optional arguments in subparser usage (#142355)

Lib/argparse.py

@@ -2004,14 +2004,16 @@ def add_subparsers(self, **kwargs):
             self._subparsers = self._positionals
 
         # prog defaults to the usage message of this parser, skipping
-        # optional arguments and with no "usage:" prefix
+        # non-required optional arguments and with no "usage:" prefix
         if kwargs.get('prog') is None:
             # Create formatter without color to avoid storing ANSI codes in prog
             formatter = self.formatter_class(prog=self.prog)
             formatter._set_color(False)
             positionals = self._get_positional_actions()
+            required_optionals = [action for action in self._get_optional_actions()
+                                  if action.required]
             groups = self._mutually_exclusive_groups
-            formatter.add_usage(None, positionals, groups, '')
+            formatter.add_usage(None, required_optionals + positionals, groups, '')
             kwargs['prog'] = formatter.format_help().strip()
 
         # create the parsers action and add it to the positionals list

Lib/test/test_argparse.py

@@ -2770,6 +2770,16 @@ def test_optional_subparsers(self):
         ret = parser.parse_args(())
         self.assertIsNone(ret.command)
 
+    def test_subparser_help_with_parent_required_optional(self):
+        parser = ErrorRaisingArgumentParser(prog='PROG')
+        parser.add_argument('--foo', required=True)
+        parser.add_argument('--bar')
+        subparsers = parser.add_subparsers()
+        parser_sub = subparsers.add_parser('sub')
+        parser_sub.add_argument('arg')
+        self.assertEqual(parser_sub.format_usage(),
+                         'usage: PROG --foo FOO sub [-h] arg\n')
+
     def test_help(self):
         self.assertEqual(self.parser.format_usage(),
                          'usage: PROG [-h] [--foo] bar {1,2,3} ...\n')

Misc/NEWS.d/next/Library/2025-12-06-16-45-34.gh-issue-64532.4OXZpF.rst

@@ -0,0 +1 @@
+Subparser help now includes required optional arguments from the parent parser in the usage, making it clearer what arguments are needed to run a subcommand. Patch by Savannah Ostrowski.