gh-65865: Raise early errors for invalid help strings in argparse (GH-124899)

Lib/argparse.py

@@ -588,17 +588,20 @@ class HelpFormatter(object):
         return result
 
     def _expand_help(self, action):
+        help_string = self._get_help_string(action)
+        if '%' not in help_string:
+            return help_string
         params = dict(vars(action), prog=self._prog)
         for name in list(params):
-            if params[name] is SUPPRESS:
+            value = params[name]
+            if value is SUPPRESS:
                 del params[name]
-        for name in list(params):
-            if hasattr(params[name], '__name__'):
-                params[name] = params[name].__name__
+            elif hasattr(value, '__name__'):
+                params[name] = value.__name__
         if params.get('choices') is not None:
             choices_str = ', '.join([str(c) for c in params['choices']])
             params['choices'] = choices_str
-        return self._get_help_string(action) % params
+        return help_string % params
 
     def _iter_indented_subactions(self, action):
         try:
@@ -1180,9 +1183,13 @@ class _SubParsersAction(Action):
             help = kwargs.pop('help')
             choice_action = self._ChoicesPseudoAction(name, aliases, help)
             self._choices_actions.append(choice_action)
+        else:
+            choice_action = None
 
         # create the parser and add it to the map
         parser = self._parser_class(**kwargs)
+        if choice_action is not None:
+            parser._check_help(choice_action)
         self._name_parser_map[name] = parser
 
         # make parser available under aliases also
@@ -1449,11 +1456,12 @@ class _ActionsContainer(object):
 
         # raise an error if the metavar does not match the type
         if hasattr(self, "_get_formatter"):
+            formatter = self._get_formatter()
             try:
-                self._get_formatter()._format_args(action, None)
+                formatter._format_args(action, None)
             except TypeError:
                 raise ValueError("length of metavar tuple does not match nargs")
-
+        self._check_help(action)
         return self._add_action(action)
 
     def add_argument_group(self, *args, **kwargs):
@@ -1635,6 +1643,14 @@ class _ActionsContainer(object):
             if not action.option_strings:
                 action.container._remove_action(action)
 
+    def _check_help(self, action):
+        if action.help and hasattr(self, "_get_formatter"):
+            formatter = self._get_formatter()
+            try:
+                formatter._expand_help(action)
+            except (ValueError, TypeError, KeyError) as exc:
+                raise ValueError('badly formed help string') from exc
+
 
 class _ArgumentGroup(_ActionsContainer):
 
@@ -1852,6 +1868,7 @@ class ArgumentParser(_AttributeHolder, _ActionsContainer):
         # create the parsers action and add it to the positionals list
         parsers_class = self._pop_action_class(kwargs, 'parsers')
         action = parsers_class(option_strings=[], **kwargs)
+        self._check_help(action)
         self._subparsers._add_action(action)
 
         # return the created parsers action

Lib/test/test_argparse.py

@@ -2623,6 +2623,29 @@ class TestAddSubparsers(TestCase):
               --foo       foo help
             '''))
 
+    def assert_bad_help(self, context_type, func, *args, **kwargs):
+        with self.assertRaisesRegex(ValueError, 'badly formed help string') as cm:
+            func(*args, **kwargs)
+        self.assertIsInstance(cm.exception.__context__, context_type)
+
+    def test_invalid_subparsers_help(self):
+        parser = ErrorRaisingArgumentParser(prog='PROG')
+        self.assert_bad_help(ValueError, parser.add_subparsers, help='%Y-%m-%d')
+        parser = ErrorRaisingArgumentParser(prog='PROG')
+        self.assert_bad_help(KeyError, parser.add_subparsers, help='%(spam)s')
+        parser = ErrorRaisingArgumentParser(prog='PROG')
+        self.assert_bad_help(TypeError, parser.add_subparsers, help='%(prog)d')
+
+    def test_invalid_subparser_help(self):
+        parser = ErrorRaisingArgumentParser(prog='PROG')
+        subparsers = parser.add_subparsers()
+        self.assert_bad_help(ValueError, subparsers.add_parser, '1',
+                             help='%Y-%m-%d')
+        self.assert_bad_help(KeyError, subparsers.add_parser, '1',
+                             help='%(spam)s')
+        self.assert_bad_help(TypeError, subparsers.add_parser, '1',
+                             help='%(prog)d')
+
     def test_subparser_title_help(self):
         parser = ErrorRaisingArgumentParser(prog='PROG',
                                             description='main description')
@@ -5375,6 +5398,14 @@ class TestInvalidArgumentConstructors(TestCase):
         self.assertValueError('--foo', action="store-true",
                               errmsg='unknown action')
 
+    def test_invalid_help(self):
+        self.assertValueError('--foo', help='%Y-%m-%d',
+                              errmsg='badly formed help string')
+        self.assertValueError('--foo', help='%(spam)s',
+                              errmsg='badly formed help string')
+        self.assertValueError('--foo', help='%(prog)d',
+                              errmsg='badly formed help string')
+
     def test_multiple_dest(self):
         parser = argparse.ArgumentParser()
         parser.add_argument(dest='foo')

Misc/NEWS.d/next/Library/2024-10-02-16-35-07.gh-issue-65865.S2D4wq.rst

@@ -0,0 +1,3 @@
+:mod:`argparse` now raises early error for invalid ``help`` arguments to
+:meth:`~argparse.ArgumentParser.add_argument`,
+:meth:`~argparse.ArgumentParser.add_subparsers` and :meth:`!add_parser`.