Fix edge-case Protocol bug on Python 3.7 (#242)

CHANGELOG.md

@@ -39,6 +39,13 @@
     or `NT = NamedTuple("NT", None)` is now deprecated.
   - Creating a `TypedDict` with zero fields using the syntax `TD = TypedDict("TD")`
     or `TD = TypedDict("TD", None)` is now deprecated.
+- Fix bug on Python 3.7 where a protocol `X` that had a member `a` would not be
+  considered an implicit subclass of an unrelated protocol `Y` that only has a
+  member `a`. Where the members of `X` are a superset of the members of `Y`,
+  `X` should always be considered a subclass of `Y` iff `Y` is a
+  runtime-checkable protocol that only has callable members. Patch by Alex
+  Waygood (backporting CPython PR
+  https://github.com/python/cpython/pull/105835).
 
 # Release 4.6.3 (June 1, 2023)
 

src/test_typing_extensions.py

@@ -1987,6 +1987,80 @@ def x(self): ...
         with self.assertRaisesRegex(TypeError, only_classes_allowed):
             issubclass(1, BadPG)
 
+    def test_implicit_issubclass_between_two_protocols(self):
+        @runtime_checkable
+        class CallableMembersProto(Protocol):
+            def meth(self): ...
+
+        # All the below protocols should be considered "subclasses"
+        # of CallableMembersProto at runtime,
+        # even though none of them explicitly subclass CallableMembersProto
+
+        class IdenticalProto(Protocol):
+            def meth(self): ...
+
+        class SupersetProto(Protocol):
+            def meth(self): ...
+            def meth2(self): ...
+
+        class NonCallableMembersProto(Protocol):
+            meth: Callable[[], None]
+
+        class NonCallableMembersSupersetProto(Protocol):
+            meth: Callable[[], None]
+            meth2: Callable[[str, int], bool]
+
+        class MixedMembersProto1(Protocol):
+            meth: Callable[[], None]
+            def meth2(self): ...
+
+        class MixedMembersProto2(Protocol):
+            def meth(self): ...
+            meth2: Callable[[str, int], bool]
+
+        for proto in (
+            IdenticalProto, SupersetProto, NonCallableMembersProto,
+            NonCallableMembersSupersetProto, MixedMembersProto1, MixedMembersProto2
+        ):
+            with self.subTest(proto=proto.__name__):
+                self.assertIsSubclass(proto, CallableMembersProto)
+
+        # These two shouldn't be considered subclasses of CallableMembersProto, however,
+        # since they don't have the `meth` protocol member
+
+        class EmptyProtocol(Protocol): ...
+        class UnrelatedProtocol(Protocol):
+            def wut(self): ...
+
+        self.assertNotIsSubclass(EmptyProtocol, CallableMembersProto)
+        self.assertNotIsSubclass(UnrelatedProtocol, CallableMembersProto)
+
+        # These aren't protocols at all (despite having annotations),
+        # so they should only be considered subclasses of CallableMembersProto
+        # if they *actually have an attribute* matching the `meth` member
+        # (just having an annotation is insufficient)
+
+        class AnnotatedButNotAProtocol:
+            meth: Callable[[], None]
+
+        class NotAProtocolButAnImplicitSubclass:
+            def meth(self): pass
+
+        class NotAProtocolButAnImplicitSubclass2:
+            meth: Callable[[], None]
+            def meth(self): pass
+
+        class NotAProtocolButAnImplicitSubclass3:
+            meth: Callable[[], None]
+            meth2: Callable[[int, str], bool]
+            def meth(self): pass
+            def meth(self, x, y): return True
+
+        self.assertNotIsSubclass(AnnotatedButNotAProtocol, CallableMembersProto)
+        self.assertIsSubclass(NotAProtocolButAnImplicitSubclass, CallableMembersProto)
+        self.assertIsSubclass(NotAProtocolButAnImplicitSubclass2, CallableMembersProto)
+        self.assertIsSubclass(NotAProtocolButAnImplicitSubclass3, CallableMembersProto)
+
     @skip_if_py312b1
     def test_issubclass_and_isinstance_on_Protocol_itself(self):
         class C:

src/typing_extensions.py

@@ -604,23 +604,10 @@ def _no_init(self, *args, **kwargs):
         # to mix without getting TypeErrors about "metaclass conflict"
         _typing_Protocol = typing.Protocol
         _ProtocolMetaBase = type(_typing_Protocol)
-
-        def _is_protocol(cls):
-            return (
-                isinstance(cls, type)
-                and issubclass(cls, typing.Generic)
-                and getattr(cls, "_is_protocol", False)
-            )
     else:
         _typing_Protocol = _marker
         _ProtocolMetaBase = abc.ABCMeta
 
-        def _is_protocol(cls):
-            return (
-                isinstance(cls, _ProtocolMeta)
-                and getattr(cls, "_is_protocol", False)
-            )
-
     class _ProtocolMeta(_ProtocolMetaBase):
         # This metaclass is somewhat unfortunate,
         # but is necessary for several reasons...
@@ -634,9 +621,9 @@ def __new__(mcls, name, bases, namespace, **kwargs):
             elif {Protocol, _typing_Protocol} & set(bases):
                 for base in bases:
                     if not (
-                        base in {object, typing.Generic}
+                        base in {object, typing.Generic, Protocol, _typing_Protocol}
                         or base.__name__ in _PROTO_ALLOWLIST.get(base.__module__, [])
-                        or _is_protocol(base)
+                        or is_protocol(base)
                     ):
                         raise TypeError(
                             f"Protocols can only inherit from other protocols, "
@@ -740,8 +727,7 @@ def _proto_hook(cls, other):
                 if (
                     isinstance(annotations, collections.abc.Mapping)
                     and attr in annotations
-                    and issubclass(other, (typing.Generic, _ProtocolMeta))
-                    and getattr(other, "_is_protocol", False)
+                    and is_protocol(other)
                 ):
                     break
             else: