Add missing dunder attributes for TypeAliasType instances (#470)

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

CHANGELOG.md

@@ -9,6 +9,9 @@
   with `@typing_extensions.deprecated`. Patch by Sebastian Rittau.
 - Fix bug where `TypeAliasType` instances could be subscripted even
   where they were not generic. Patch by [Daraan](https://github.com/Daraan).
+- Fix bug where a subscripted `TypeAliasType` instance did not have all
+  attributes of the original `TypeAliasType` instance on older Python versions.
+  Patch by [Daraan](https://github.com/Daraan) and Alex Waygood.
 
 # Release 4.12.2 (June 7, 2024)
 

src/test_typing_extensions.py

@@ -7247,6 +7247,29 @@ def test_getitem(self):
         self.assertEqual(get_args(fully_subscripted), (Iterable[float],))
         self.assertIs(get_origin(fully_subscripted), ListOrSetT)
 
+    def test_alias_attributes(self):
+        T = TypeVar('T')
+        T2 = TypeVar('T2')
+        ListOrSetT = TypeAliasType("ListOrSetT", Union[List[T], Set[T]], type_params=(T,))
+
+        subscripted = ListOrSetT[int]
+        self.assertEqual(subscripted.__module__, ListOrSetT.__module__)
+        self.assertEqual(subscripted.__name__, "ListOrSetT")
+        self.assertEqual(subscripted.__value__, Union[List[T], Set[T]])
+        self.assertEqual(subscripted.__type_params__, (T,))
+
+        still_generic = ListOrSetT[Iterable[T2]]
+        self.assertEqual(still_generic.__module__, ListOrSetT.__module__)
+        self.assertEqual(still_generic.__name__, "ListOrSetT")
+        self.assertEqual(still_generic.__value__, Union[List[T], Set[T]])
+        self.assertEqual(still_generic.__type_params__, (T,))
+
+        fully_subscripted = still_generic[float]
+        self.assertEqual(fully_subscripted.__module__, ListOrSetT.__module__)
+        self.assertEqual(fully_subscripted.__name__, "ListOrSetT")
+        self.assertEqual(fully_subscripted.__value__, Union[List[T], Set[T]])
+        self.assertEqual(fully_subscripted.__type_params__, (T,))
+
     def test_subscription_without_type_params(self):
         Simple = TypeAliasType("Simple", int)
         with self.assertRaises(TypeError, msg="Only generic type aliases are subscriptable"):
@@ -7260,7 +7283,6 @@ def test_subscription_without_type_params(self):
         with self.assertRaises(TypeError, msg="Only generic type aliases are subscriptable"):
             MissingTypeParamsErr[int]
 
-
     def test_pickle(self):
         global Alias
         Alias = TypeAliasType("Alias", int)

src/typing_extensions.py

@@ -3452,6 +3452,37 @@ def _is_unionable(obj):
             TypeAliasType,
         ))
 
+    if sys.version_info < (3, 10):
+        # Copied and pasted from https://github.com/python/cpython/blob/986a4e1b6fcae7fe7a1d0a26aea446107dd58dd2/Objects/genericaliasobject.c#L568-L582,
+        # so that we emulate the behaviour of `types.GenericAlias`
+        # on the latest versions of CPython
+        _ATTRIBUTE_DELEGATION_EXCLUSIONS = frozenset({
+            "__class__",
+            "__bases__",
+            "__origin__",
+            "__args__",
+            "__unpacked__",
+            "__parameters__",
+            "__typing_unpacked_tuple_args__",
+            "__mro_entries__",
+            "__reduce_ex__",
+            "__reduce__",
+            "__copy__",
+            "__deepcopy__",
+        })
+
+        class _TypeAliasGenericAlias(typing._GenericAlias, _root=True):
+            def __getattr__(self, attr):
+                if attr in _ATTRIBUTE_DELEGATION_EXCLUSIONS:
+                    return object.__getattr__(self, attr)
+                return getattr(self.__origin__, attr)
+
+            if sys.version_info < (3, 9):
+                def __getitem__(self, item):
+                    result = super().__getitem__(item)
+                    result.__class__ = type(self)
+                    return result
+
     class TypeAliasType:
         """Create named, parameterized type aliases.
 
@@ -3529,13 +3560,16 @@ def __getitem__(self, parameters):
                 raise TypeError("Only generic type aliases are subscriptable")
             if not isinstance(parameters, tuple):
                 parameters = (parameters,)
-            parameters = [
+            # Using 3.9 here will create problems with Concatenate
+            if sys.version_info >= (3, 10):
+                return _types.GenericAlias(self, parameters)
+            parameters = tuple(
                 typing._type_check(
                     item, f'Subscripting {self.__name__} requires a type.'
                 )
                 for item in parameters
-            ]
-            return typing._GenericAlias(self, tuple(parameters))
+            )
+            return _TypeAliasGenericAlias(self, parameters)
 
         def __reduce__(self):
             return self.__name__