Better fix

Author: AlexWaygood

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__.
@@ -1798,17 +1809,10 @@ def __instancecheck__(cls, instance):
             raise TypeError("Instance and class checks can only be used with"
                             " @runtime_checkable protocols")
 
-        # gh-104555: Don't call super().__instancecheck__ here,
-        # ABCMeta.__instancecheck__ would erroneously use it to populate the cache,
-        # which would cause incorrect results for *issubclass()* calls
-        if type.__instancecheck__(cls, instance):
+        if super().__instancecheck__(instance):
             return True
 
         if is_protocol_cls:
-            # Fast path for protocols with only callable members
-            if cls.__callable_proto_members_only__ and issubclass(type(instance), cls):
-                return True
-
             getattr_static = _lazy_load_getattr_static()
             for attr in cls.__protocol_attrs__:
                 try:
@@ -1820,7 +1824,7 @@ def __instancecheck__(cls, instance):
             else:
                 return True
 
-        return super().__instancecheck__(instance)
+        return False
 
 
 class Protocol(Generic, metaclass=_ProtocolMeta):
@@ -1876,11 +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')