1 files changed, 28 insertions, 18 deletions
diff --git a/Lib/typing.py b/Lib/typing.py
index d7d7935..d278b4e 100644
--- a/Lib/typing.py
+++ b/Lib/typing.py
@@ -1670,7 +1670,7 @@ class _TypingEllipsis:
 _TYPING_INTERNALS = frozenset({
     '__parameters__', '__orig_bases__',  '__orig_class__',
     '_is_protocol', '_is_runtime_protocol', '__protocol_attrs__',
-    '__callable_proto_members_only__', '__type_params__',
+    '__non_callable_proto_members__', '__type_params__',
 })
 
 _SPECIAL_NAMES = frozenset({
@@ -1833,11 +1833,6 @@ class _ProtocolMeta(ABCMeta):
         super().__init__(*args, **kwargs)
         if getattr(cls, "_is_protocol", False):
             cls.__protocol_attrs__ = _get_protocol_attrs(cls)
-            # PEP 544 prohibits using issubclass()
-            # with protocols that have non-method members.
-            cls.__callable_proto_members_only__ = all(
-                callable(getattr(cls, attr, None)) for attr in cls.__protocol_attrs__
-            )
 
     def __subclasscheck__(cls, other):
         if cls is Protocol:
@@ -1846,25 +1841,23 @@ class _ProtocolMeta(ABCMeta):
             getattr(cls, '_is_protocol', False)
             and not _allow_reckless_class_checks()
         ):
+            if not getattr(cls, '_is_runtime_protocol', False):
+                _type_check_issubclass_arg_1(other)
+                raise TypeError(
+                    "Instance and class checks can only be used with "
+                    "@runtime_checkable protocols"
+                )
             if (
-                not cls.__callable_proto_members_only__
+                # this attribute is set by @runtime_checkable:
+                cls.__non_callable_proto_members__
                 and cls.__dict__.get("__subclasshook__") is _proto_hook
             ):
                 _type_check_issubclass_arg_1(other)
-                non_method_attrs = sorted(
-                    attr for attr in cls.__protocol_attrs__
-                    if not callable(getattr(cls, attr, None))
-                )
+                non_method_attrs = sorted(cls.__non_callable_proto_members__)
                 raise TypeError(
                     "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):
-                _type_check_issubclass_arg_1(other)
-                raise TypeError(
-                    "Instance and class checks can only be used with "
-                    "@runtime_checkable protocols"
-                )
         return _abc_subclasscheck(cls, other)
 
     def __instancecheck__(cls, instance):
@@ -1892,7 +1885,8 @@ class _ProtocolMeta(ABCMeta):
                 val = getattr_static(instance, attr)
             except AttributeError:
                 break
-            if val is None and callable(getattr(cls, attr, None)):
+            # this attribute is set by @runtime_checkable:
+            if val is None and attr not in cls.__non_callable_proto_members__:
                 break
         else:
             return True
@@ -2114,6 +2108,22 @@ def runtime_checkable(cls):
         raise TypeError('@runtime_checkable can be only applied to protocol classes,'
                         ' got %r' % cls)
     cls._is_runtime_protocol = True
+    # PEP 544 prohibits using issubclass()
+    # with protocols that have non-method members.
+    # See gh-113320 for why we compute this attribute here,
+    # rather than in `_ProtocolMeta.__init__`
+    cls.__non_callable_proto_members__ = set()
+    for attr in cls.__protocol_attrs__:
+        try:
+            is_callable = callable(getattr(cls, attr, None))
+        except Exception as e:
+            raise TypeError(
+                f"Failed to determine whether protocol member {attr!r} "
+                "is a method member"
+            ) from e
+        else:
+            if not is_callable:
+                cls.__non_callable_proto_members__.add(attr)
     return cls