gh-112345: `typing.Protocol`: Let failed subclasscheck show non-method members (#112344)

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

1 files changed, 6 insertions, 1 deletions

diff --git a/Lib/typing.py b/Lib/typing.py
index a96c708..872aca8 100644
--- a/Lib/typing.py
+++ b/Lib/typing.py
@@ -1828,8 +1828,13 @@ class _ProtocolMeta(ABCMeta):
                 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: {str(non_method_attrs)[1:-1]}."
                 )
             if not getattr(cls, '_is_runtime_protocol', False):
                 raise TypeError(