pythongh-86463: Fix default prog in subparsers if usage is used in th…

…e parent parser

The placeholder '...' is now used as the prog prefix in subparsers
if a custom usage is specified in the parent parser and prog is not
specified in the subparser.

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

Doc/library/argparse.rst

@@ -343,6 +343,9 @@ The default message can be overridden with the ``usage=`` keyword argument::
 The ``%(prog)s`` format specifier is available to fill in the program name in
 your usage messages.
 
+When you specify custom usage, always specify also prog_ and usage_
+arguments to :meth:`subparsers <_SubParsersAction.add_parser>`.
+
 
 .. _description:
 

Lib/argparse.py

@@ -1813,11 +1813,14 @@ def add_subparsers(self, **kwargs):
         # prog defaults to the usage message of this parser, skipping
         # optional arguments and with no "usage:" prefix
         if kwargs.get('prog') is None:
-            formatter = self._get_formatter()
-            positionals = self._get_positional_actions()
-            groups = self._mutually_exclusive_groups
-            formatter.add_usage(self.usage, positionals, groups, '')
-            kwargs['prog'] = formatter.format_help().strip()
+            if self.usage is None:
+                formatter = self._get_formatter()
+                positionals = self._get_positional_actions()
+                groups = self._mutually_exclusive_groups
+                formatter.add_usage(self.usage, positionals, groups, '')
+                kwargs['prog'] = formatter.format_help().strip()
+            else:
+                kwargs['prog'] = '...'
 
         # create the parsers action and add it to the positionals list
         parsers_class = self._pop_action_class(kwargs, 'parsers')

Lib/test/test_argparse.py

@@ -2203,16 +2203,17 @@ 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(
@@ -2249,7 +2250,8 @@ 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)
@@ -2271,6 +2273,47 @@ 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: ... 1 [-h] [-w W] {a,b,c}',
+             '... 1: error: the following arguments are required: x'),
+            ('0.5 3',
+             'usage: PROG --foo bar 3 t ...',
+             '... 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/2024-09-25-21-27-37.gh-issue-86463.lcBmsO.rst

@@ -0,0 +1,3 @@
+:mod:`argparse` now uses the placeholder ``'...'`` as the ``prog`` prefix in
+subparsers if a custom ``usage`` is specified in the parent parser and
+``prog`` is not specified in the subparser.