GH-87041: Fix incorrect indentation in argparse help (GH-124230)

In case of usage a long command along with max_help_position more than
the length of the command, the command's help was incorrectly started
on the new line.

Co-authored-by: Pavel Ditenbir <pavel.ditenbir@gmail.com>

Author: savannahostrowski

Lib/argparse.py

@@ -261,13 +261,12 @@ def add_argument(self, action):
 
             # find all invocations
             get_invocation = self._format_action_invocation
-            invocations = [get_invocation(action)]
+            invocation_lengths = [len(get_invocation(action)) + self._current_indent]
             for subaction in self._iter_indented_subactions(action):
-                invocations.append(get_invocation(subaction))
+                invocation_lengths.append(len(get_invocation(subaction)) + self._current_indent)
 
             # update the maximum item length
-            invocation_length = max(map(len, invocations))
-            action_length = invocation_length + self._current_indent
+            action_length = max(invocation_lengths)
             self._action_max_length = max(self._action_max_length,
                                           action_length)
 

Lib/test/test_argparse.py

@@ -4960,6 +4960,46 @@ def custom_type(string):
     version = ''
 
 
+class TestHelpUsageLongSubparserCommand(TestCase):
+    """Test that subparser commands are formatted correctly in help"""
+    maxDiff = None
+
+    def test_parent_help(self):
+        def custom_formatter(prog):
+            return argparse.RawTextHelpFormatter(prog, max_help_position=50)
+
+        parent_parser = argparse.ArgumentParser(
+                prog='PROG',
+                formatter_class=custom_formatter
+        )
+
+        cmd_subparsers = parent_parser.add_subparsers(title="commands",
+                                                      metavar='CMD',
+                                                      help='command to use')
+        cmd_subparsers.add_parser("add",
+                                  help="add something")
+
+        cmd_subparsers.add_parser("remove",
+                                  help="remove something")
+
+        cmd_subparsers.add_parser("a-very-long-command",
+                                  help="command that does something")
+
+        parser_help = parent_parser.format_help()
+        self.assertEqual(parser_help, textwrap.dedent('''\
+            usage: PROG [-h] CMD ...
+
+            options:
+              -h, --help             show this help message and exit
+
+            commands:
+              CMD                    command to use
+                add                  add something
+                remove               remove something
+                a-very-long-command  command that does something
+        '''))
+
+
 # =====================================
 # Optional/Positional constructor tests
 # =====================================

Misc/NEWS.d/next/Library/2024-09-19-03-46-59.gh-issue-87041.9Ox7Bv.rst

@@ -0,0 +1 @@
+Fix a bug in :mod:`argparse` where lengthy subparser argument help is incorrectly indented.