Fix isinstance check for Generic classes

CHANGELOG.md

@@ -1,3 +1,7 @@
+# Unreleased
+
+- Fix failing `type object ... has no attribute '_is_protocol'` on Python 3.7
+
 # Release 4.6.1 (May 23, 2023)
 
 - Change deprecated `@runtime` to formal API `@runtime_checkable` in the error

src/test_typing_extensions.py

@@ -4320,6 +4320,24 @@ class Y(Generic[T], NamedTuple):
         self.assertEqual(Y.__orig_bases__, (Generic[T], NamedTuple))
         self.assertEqual(Y.__mro__, (Y, Generic, tuple, object))
 
+        class Z(Y[int]):
+            ...
+        self.assertEqual(Z.__bases__, (Y,))
+        self.assertEqual(Z.__orig_bases__, (Y[int],))
+        self.assertEqual(Z.__mro__, (Z, Y, Generic, tuple, object))
+
+        @runtime_checkable
+        class Proto(Protocol):
+            @property
+            def x(self):
+                ...
+
+        z = Z(3)
+        self.assertIs(type(z), Z)
+        self.assertIsInstance(z, Z)
+        self.assertIsInstance(z, Proto)
+        self.assertEqual(z.x, 3)
+
         for G in X, Y:
             with self.subTest(type=G):
                 self.assertEqual(G.__parameters__, (T,))
@@ -4330,6 +4348,8 @@ class Y(Generic[T], NamedTuple):
 
                 a = A(3)
                 self.assertIs(type(a), G)
+                self.assertIsInstance(a, G)
+                self.assertIsInstance(a, Proto)
                 self.assertEqual(a.x, 3)
 
                 things = "arguments" if sys.version_info >= (3, 11) else "parameters"

src/typing_extensions.py

@@ -9,7 +9,6 @@
 import typing
 import warnings
 
-
 __all__ = [
     # Super-special typing primitives.
     'Any',
@@ -659,7 +658,9 @@ def _proto_hook(cls, other):
                     isinstance(annotations, collections.abc.Mapping)
                     and attr in annotations
                     and issubclass(other, (typing.Generic, _ProtocolMeta))
-                    and other._is_protocol
+                    # All subclasses of Generic have an _is_proto attribute on 3.8+
+                    # But not on 3.7
+                    and getattr(other, "_is_protocol", False)
                 ):
                     break
             else: