gh-112345: Let failed protocol subclasscheck show non-method members

Lib/test/test_typing.py

@@ -4091,6 +4091,22 @@ def method(self) -> None: ...
         self.assertIsInstance(Foo(), ProtocolWithMixedMembers)
         self.assertNotIsInstance(42, ProtocolWithMixedMembers)
 
+    def test_protocol_issubclass_error_message(self):
+        class Vec2D(Protocol):
+            x: float
+            y: float
+
+            def square_norm(self) -> float:
+                return self.x ** 2 + self.y ** 2
+
+        self.assertEqual(Vec2D.__protocol_attrs__, {'x', 'y', 'square_norm'})
+        expected_error_message = (
+            "Protocols with non-method members don't support issubclass()."
+            " Non-method members: ['x', 'y']."
+        )
+        with self.assertRaisesRegex(TypeError, re.escape(expected_error_message)):
+            issubclass(int, Vec2D)
+
 
 class GenericTests(BaseTestCase):
 

Lib/typing.py

@@ -1828,8 +1828,13 @@ def __subclasscheck__(cls, other):
                 not cls.__callable_proto_members_only__
                 and cls.__dict__.get("__subclasshook__") is _proto_hook
             ):
+                non_method_attrs = sorted(
+                    attr for attr in cls.__protocol_attrs__
+                    if not callable(getattr(cls, attr, None))
+                )
                 raise TypeError(
-                    "Protocols with non-method members don't support issubclass()"
+                    "Protocols with non-method members don't support issubclass()."
+                    f" Non-method members: {non_method_attrs}."
                 )
             if not getattr(cls, '_is_runtime_protocol', False):
                 raise TypeError(

Misc/NEWS.d/next/Library/2023-11-23-17-25-27.gh-issue-112345.FFApHx.rst

@@ -0,0 +1,2 @@
+Improve error message when trying to call :func:`issubclass` against a :class:`typing.Protocol` that has non-method members.
+Patch by Randolf Scholz.