gh-130870: Preserve GenericAlias subclasses in typing.get_type_hints()

Author: Viicos

Lib/test/test_typing.py

@@ -7114,6 +7114,24 @@ def add_right(self, node: 'Node[T]' = None):
         right_hints = get_type_hints(t.add_right, globals(), locals())
         self.assertEqual(right_hints['node'], Node[T])
 
+    def test_get_type_hints_preserve_generic_alias_subclasses(self):
+        # https://github.com/python/cpython/issues/130870
+        # A real world example of this is `collections.abc.Callable`. When parameterized,
+        # the result is a subclass of `types.GenericAlias`.
+        class MyAlias(types.GenericAlias):
+            pass
+
+        class MyClass:
+            def __class_getitem__(cls, args):
+                return MyAlias(cls, args)
+
+        # Using a forward reference is important, otherwise it works as expected.
+        # `y` tests that the `GenericAlias` subclass is preserved when stripping `Annotated`.
+        def func(x: MyClass['int'], y: MyClass[Annotated[int, ...]]): ...
+
+        assert isinstance(get_type_hints(func)['x'], MyAlias)
+        assert isinstance(get_type_hints(func)['y'], MyAlias)
+
 
 class GetUtilitiesTestCase(TestCase):
     def test_get_origin(self):

Lib/typing.py

@@ -407,6 +407,17 @@ def inner(*args, **kwds):
     return decorator
 
 
+def _rebuild_generic_alias(alias: GenericAlias, args: tuple[object, ...]) -> GenericAlias:
+    is_unpacked = alias.__unpacked__
+    if _should_unflatten_callable_args(alias, args):
+        t = alias.__origin__[(args[:-1], args[-1])]
+    else:
+        t = alias.__origin__[args]
+    if is_unpacked:
+        t = Unpack[t]
+    return t
+
+
 def _deprecation_warning_for_no_type_params_passed(funcname: str) -> None:
     import warnings
 
@@ -454,25 +465,20 @@ def _eval_type(t, globalns, localns, type_params=_sentinel, *, recursive_guard=f
                 _make_forward_ref(arg) if isinstance(arg, str) else arg
                 for arg in t.__args__
             )
-            is_unpacked = t.__unpacked__
-            if _should_unflatten_callable_args(t, args):
-                t = t.__origin__[(args[:-1], args[-1])]
-            else:
-                t = t.__origin__[args]
-            if is_unpacked:
-                t = Unpack[t]
+        else:
+            args = t.__args__
 
         ev_args = tuple(
             _eval_type(
                 a, globalns, localns, type_params, recursive_guard=recursive_guard,
                 format=format, owner=owner,
             )
-            for a in t.__args__
+            for a in args
         )
         if ev_args == t.__args__:
             return t
         if isinstance(t, GenericAlias):
-            return GenericAlias(t.__origin__, ev_args)
+            return _rebuild_generic_alias(t, ev_args)
         if isinstance(t, Union):
             return functools.reduce(operator.or_, ev_args)
         else:
@@ -2404,7 +2410,7 @@ def _strip_annotations(t):
         stripped_args = tuple(_strip_annotations(a) for a in t.__args__)
         if stripped_args == t.__args__:
             return t
-        return GenericAlias(t.__origin__, stripped_args)
+        return _rebuild_generic_alias(t, stripped_args)
     if isinstance(t, Union):
         stripped_args = tuple(_strip_annotations(a) for a in t.__args__)
         if stripped_args == t.__args__: