Benedikt's review

Author: StanFromIreland

Lib/test/test_typing.py

@@ -4731,10 +4731,10 @@ def __init_subclass__(cls) -> None:
         class Sub(Base, Generic[T]):
             pass
 
-        with self.assertRaisesRegex(TypeError,
-                                  r"type object '.*Sub' has no attribute "
-                                    r"'__parameters__'; Maybe you forgot to call "
-                                    r"super\(\).__init_subclass__\(\)\?"):
+        msg = (r"type object '.*Sub' has no attribute " +
+              re.escape("'__parameters__'. Maybe you forgot to call "
+                        "super().__init_subclass__()?"))
+        with self.assertRaisesRegex(TypeError, msg):
             Sub[int]
 
     def test_generic_subclass_checks(self):

Lib/typing.py

@@ -1162,7 +1162,7 @@ def _generic_class_getitem(cls, args):
             parameters = cls.__parameters__
         except AttributeError:
             raise TypeError(f"type object '{cls.__qualname__}' has no attribute "
-                            f"'__parameters__'; Maybe you forgot to call "
+                            f"'__parameters__'. Maybe you forgot to call "
                             f"super().__init_subclass__()?") from None
         for param in parameters:
             prepare = getattr(param, '__typing_prepare_subst__', None)

Misc/NEWS.d/next/Library/2025-10-11-10-02-56.gh-issue-139905.UyJIR_.rst

@@ -1,2 +1,3 @@
-Add suggestion to inscrutable error about ``cls.__parameters__`` not
-existing, because ``super().__init_subclass__()`` was not called.
+Add suggestion to error message for :class:`typing.Generic` subclasses when
+``cls.__parameters__`` is missing due to a parent class failing to call
+``super().__init_subclass__()`` in its ``__init_subclass__``.