gh-133653: Fix subclassing HelpFormatter

Lib/argparse.py

@@ -2723,9 +2723,17 @@ def format_help(self):
         return formatter.format_help()
 
     def _get_formatter(self):
-        if isinstance(self.formatter_class, type) and issubclass(
-            self.formatter_class, HelpFormatter
-        ):
+        import inspect
+        if len(
+            [v.kind for (k, v) in
+             inspect.signature(self.formatter_class).parameters.items()
+             if k in ('prefix_chars', 'color')
+                and v.kind in (
+                        inspect._ParameterKind.POSITIONAL_OR_KEYWORD,
+                        inspect._ParameterKind.KEYWORD_ONLY,
+                )
+             ]
+        ) == 2:
             return self.formatter_class(
                 prog=self.prog,
                 prefix_chars=self.prefix_chars,

Lib/test/test_argparse.py

@@ -5176,6 +5176,30 @@ class TestHelpTupleMetavarPositional(HelpTestCase):
     version = ''
 
 
+class TestHelpFormatter(HelpTestCase):
+    """Test the HelpFormatter"""
+
+    # Test subclassing the help formatter
+    class MyFormatter(argparse.HelpFormatter):
+        def __init__(self, prog) -> None:
+            super().__init__(prog)
+
+    parser_signature = Sig(
+        prog="PROG",
+        formatter_class=MyFormatter,
+        description="Test with subclassing the help formatter",
+    )
+    usage = '''\
+        usage: PROG [-h]
+        '''
+    help = usage + '''\
+
+        Test with subclassing the help formatter
+
+        options:
+          -h, --help  show this help message and exit
+        '''
+
 class TestHelpRawText(HelpTestCase):
     """Test the RawTextHelpFormatter"""
 

Misc/NEWS.d/next/Library/2025-05-08-16-28-05.gh-issue-133653.7cxHXN.rst

@@ -0,0 +1 @@
+Fix subclassing :meth:`!argparse.HelpFormatter`. Patch by Hugo van Kemenade.