gh-74690: typing._ProtocolMeta.__instancecheck__: Exit early for protocols that only have callable members

Lib/test/test_typing.py

@@ -2577,6 +2577,18 @@ def meth(x): ...
         class PG(Protocol[T]):
             def meth(x): ...
 
+        @runtime_checkable
+        class WeirdProto(Protocol):
+            meth = str.maketrans
+
+        class CustomCallable:
+            def __call__(self, *args, **kwargs):
+                pass
+
+        @runtime_checkable
+        class WeirderProto(Protocol):
+            meth = CustomCallable()
+
         class BadP(Protocol):
             def meth(x): ...
 
@@ -2588,6 +2600,8 @@ def meth(x): ...
 
         self.assertIsInstance(C(), P)
         self.assertIsInstance(C(), PG)
+        self.assertIsInstance(C(), WeirdProto)
+        self.assertIsInstance(C(), WeirderProto)
         with self.assertRaises(TypeError):
             isinstance(C(), PG[T])
         with self.assertRaises(TypeError):

Lib/typing.py

@@ -2032,7 +2032,11 @@ def __instancecheck__(cls, instance):
         if super().__instancecheck__(instance):
             return True
 
-        if is_protocol_cls:
+        # Skip the below loop for protocols where `cls.__callable_proto_members_only__ == True`.
+        # For these protocols, this just duplicates checks that have already been done
+        # in the `_proto_hook` function,
+        # which is called as part of the super().__instancecheck__ method above.
+        if is_protocol_cls and not cls.__callable_proto_members_only__:
             getattr_static = _lazy_load_getattr_static()
             for attr in cls.__protocol_attrs__:
                 try: