gh-104555: Runtime-checkable protocols: Don't let previous calls to isinstance() influence whether issubclass() raises an exception

Lib/test/test_typing.py

@@ -2654,6 +2654,78 @@ class D(PNonCall): ...
         with self.assertRaises(TypeError):
             issubclass(D, PNonCall)
 
+    def test_no_weird_caching_with_issubclass_after_isinstance(self):
+        @runtime_checkable
+        class Spam(Protocol):
+            x: int
+
+        class Eggs:
+            def __init__(self) -> None:
+                self.x = 42
+
+        # gh-104555: ABCMeta might cache the result of this isinstance check
+        # if we called super().__instancecheck__ in the wrong place
+        # in _ProtocolMeta.__instancecheck__...
+        self.assertIsInstance(Eggs(), Spam)
+
+        # ...and if it did, then TypeError wouldn't be raised here!
+        with self.assertRaises(TypeError):
+            issubclass(Eggs, Spam)
+
+    def test_no_weird_caching_with_issubclass_after_isinstance2(self):
+        @runtime_checkable
+        class Spam(Protocol):
+            x: int
+
+        class Eggs: ...
+
+        # gh-104555: ABCMeta might cache the result of this isinstance check
+        # if we called super().__instancecheck__ in the wrong place
+        # in _ProtocolMeta.__instancecheck__...
+        self.assertNotIsInstance(Eggs(), Spam)
+
+        # ...and if it did, then TypeError wouldn't be raised here!
+        with self.assertRaises(TypeError):
+            issubclass(Eggs, Spam)
+
+    def test_no_weird_caching_with_issubclass_after_isinstance3(self):
+        @runtime_checkable
+        class Spam(Protocol):
+            x: int
+
+        class Eggs:
+            def __getattr__(self, attr):
+                if attr == "x":
+                    return 42
+                raise AttributeError(attr)
+
+        # gh-104555: ABCMeta might cache the result of this isinstance check
+        # if we called super().__instancecheck__ in the wrong place
+        # in _ProtocolMeta.__instancecheck__...
+        self.assertNotIsInstance(Eggs(), Spam)
+
+        # ...and if it did, then TypeError wouldn't be raised here!
+        with self.assertRaises(TypeError):
+            issubclass(Eggs, Spam)
+
+    def test_no_weird_caching_with_issubclass_after_isinstance_pep695(self):
+        @runtime_checkable
+        class Spam[T](Protocol):
+            x: T
+
+        class Eggs[T]:
+            def __init__(self, x: T) -> None:
+                self.x = x
+
+        # gh-104555: ABCMeta might cache the result of this isinstance check
+        # if we called super().__instancecheck__ in the wrong place
+        # in _ProtocolMeta.__instancecheck__...
+        self.assertIsInstance(Eggs(42), Spam)
+
+        # ...and if it did, then TypeError wouldn't be raised here!
+        with self.assertRaises(TypeError):
+            issubclass(Eggs, Spam)
+
     def test_protocols_isinstance(self):
         T = TypeVar('T')
 

Lib/typing.py

@@ -1775,8 +1775,8 @@ def _pickle_pskwargs(pskwargs):
 
 
 class _ProtocolMeta(ABCMeta):
-    # This metaclass is really unfortunate and exists only because of
-    # the lack of __instancehook__.
+    # This metaclass is somewhat unfortunate,
+    # but is necessary for several reasons...
     def __init__(cls, *args, **kwargs):
         super().__init__(*args, **kwargs)
         cls.__protocol_attrs__ = _get_protocol_attrs(cls)
@@ -1786,6 +1786,17 @@ def __init__(cls, *args, **kwargs):
             callable(getattr(cls, attr, None)) for attr in cls.__protocol_attrs__
         )
 
+    def __subclasscheck__(cls, other):
+        if (
+            getattr(cls, '_is_protocol', False)
+            and not cls.__callable_proto_members_only__
+            and not _allow_reckless_class_checks(depth=2)
+        ):
+            raise TypeError(
+                "Protocols with non-method members don't support issubclass()"
+            )
+        return super().__subclasscheck__(other)
+
     def __instancecheck__(cls, instance):
         # We need this method for situations where attributes are
         # assigned in __init__.
@@ -1869,11 +1880,6 @@ def _proto_hook(other):
                 raise TypeError("Instance and class checks can only be used with"
                                 " @runtime_checkable protocols")
 
-            if not cls.__callable_proto_members_only__ :
-                if _allow_reckless_class_checks():
-                    return NotImplemented
-                raise TypeError("Protocols with non-method members"
-                                " don't support issubclass()")
             if not isinstance(other, type):
                 # Same error message as for issubclass(1, int).
                 raise TypeError('issubclass() arg 1 must be a class')