gh-86463: Fix default prog in subparsers if usage is used in the main parser

Doc/library/argparse.rst

@@ -192,6 +192,11 @@ arguments it contains. The default message can be overridden with the
 The ``%(prog)s`` format specifier is available to fill in the program name in
 your usage messages.
 
+When a custom usage message is specified for the main parser, consider
+also passing the ``prog`` argument to :meth:`~ArgumentParser.add_subparsers`
+or the ``prog`` and the ``usage`` arguments to
+:meth:`~_SubParsersAction.add_parser`.
+
 
 .. _description:
 

Lib/argparse.py

@@ -1889,7 +1889,7 @@ def add_subparsers(self, **kwargs):
             formatter = self._get_formatter()
             positionals = self._get_positional_actions()
             groups = self._mutually_exclusive_groups
-            formatter.add_usage(self.usage, positionals, groups, '')
+            formatter.add_usage(None, positionals, groups, '')
             kwargs['prog'] = formatter.format_help().strip()
 
         # create the parsers action and add it to the positionals list

Lib/test/test_argparse.py

@@ -2388,16 +2388,17 @@ def assertArgumentParserError(self, *args, **kwargs):
         self.assertRaises(ArgumentParserError, *args, **kwargs)
 
     def _get_parser(self, subparser_help=False, prefix_chars=None,
-                    aliases=False):
+                    aliases=False, usage=None):
         # create a parser with a subparsers argument
         if prefix_chars:
             parser = ErrorRaisingArgumentParser(
-                prog='PROG', description='main description', prefix_chars=prefix_chars)
+                prog='PROG', description='main description', usage=usage,
+                prefix_chars=prefix_chars)
             parser.add_argument(
                 prefix_chars[0] * 2 + 'foo', action='store_true', help='foo help')
         else:
             parser = ErrorRaisingArgumentParser(
-                prog='PROG', description='main description')
+                prog='PROG', description='main description', usage=usage)
             parser.add_argument(
                 '--foo', action='store_true', help='foo help')
         parser.add_argument(
@@ -2434,7 +2435,8 @@ def _get_parser(self, subparser_help=False, prefix_chars=None,
         parser2.add_argument('z', type=complex, nargs='*', help='z help')
 
         # add third sub-parser
-        parser3_kwargs = dict(description='3 description')
+        parser3_kwargs = dict(description='3 description',
+                              usage='PROG --foo bar 3 t ...')
         if subparser_help:
             parser3_kwargs['help'] = '3 help'
         parser3 = subparsers.add_parser('3', **parser3_kwargs)
@@ -2456,6 +2458,47 @@ def test_parse_args_failures(self):
             args = args_str.split()
             self.assertArgumentParserError(self.parser.parse_args, args)
 
+    def test_parse_args_failures_details(self):
+        for args_str, usage_str, error_str in [
+            ('',
+             'usage: PROG [-h] [--foo] bar {1,2,3} ...',
+             'PROG: error: the following arguments are required: bar'),
+            ('0.5 1 -y',
+             'usage: PROG bar 1 [-h] [-w W] {a,b,c}',
+             'PROG bar 1: error: the following arguments are required: x'),
+            ('0.5 3',
+             'usage: PROG --foo bar 3 t ...',
+             'PROG bar 3: error: the following arguments are required: t'),
+        ]:
+            with self.subTest(args_str):
+                args = args_str.split()
+                with self.assertRaises(ArgumentParserError) as cm:
+                    self.parser.parse_args(args)
+                self.assertEqual(cm.exception.args[0], 'SystemExit')
+                self.assertEqual(cm.exception.args[2], f'{usage_str}\n{error_str}\n')
+
+    def test_parse_args_failures_details_custom_usage(self):
+        parser = self._get_parser(usage='PROG [--foo] bar 1 [-w W] {a,b,c}\n'
+                                 '       PROG --foo bar 3 t ...')
+        for args_str, usage_str, error_str in [
+            ('',
+             'usage: PROG [--foo] bar 1 [-w W] {a,b,c}\n'
+             '       PROG --foo bar 3 t ...',
+             'PROG: error: the following arguments are required: bar'),
+            ('0.5 1 -y',
+             'usage: PROG bar 1 [-h] [-w W] {a,b,c}',
+             'PROG bar 1: error: the following arguments are required: x'),
+            ('0.5 3',
+             'usage: PROG --foo bar 3 t ...',
+             'PROG bar 3: error: the following arguments are required: t'),
+        ]:
+            with self.subTest(args_str):
+                args = args_str.split()
+                with self.assertRaises(ArgumentParserError) as cm:
+                    parser.parse_args(args)
+                self.assertEqual(cm.exception.args[0], 'SystemExit')
+                self.assertEqual(cm.exception.args[2], f'{usage_str}\n{error_str}\n')
+
     def test_parse_args(self):
         # check some non-failure cases:
         self.assertEqual(

Misc/NEWS.d/next/Library/2024-10-23-20-05-54.gh-issue-86463.jvFTI_.rst

@@ -0,0 +1,2 @@
+The ``usage`` parameter of :class:`argparse.ArgumentParser` no longer
+affects the default value of the ``prog`` parameter in subparsers.