gh-86463: Fix default prog in subparsers if usage is used in the main parser (GH-125891)

The usage parameter of argparse.ArgumentParser no longer
affects the default value of the prog parameter in subparsers.

Previously the full custom usage of the main parser was used as
the prog prefix in subparsers.

Author: serhiy-storchaka

Doc/library/argparse.rst

@@ -192,6 +192,12 @@ 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, you may also want to
+consider passing  the ``prog`` argument to :meth:`~ArgumentParser.add_subparsers`
+or the ``prog`` and the ``usage`` arguments to
+:meth:`~_SubParsersAction.add_parser`, to ensure consistent command prefixes and 
+usage information across subparsers.
+
 
 .. _description:
 
@@ -1810,6 +1816,10 @@ Sub-commands
    .. versionchanged:: 3.7
       New *required* keyword-only parameter.
 
+   .. versionchanged:: 3.14
+      Subparser's *prog* is no longer affected by a custom usage message in
+      the main parser.
+
 
 FileType objects
 ^^^^^^^^^^^^^^^^

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

@@ -2409,16 +2409,17 @@ 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(
@@ -2455,7 +2456,8 @@ 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)
@@ -2477,6 +2479,47 @@ 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.