gh-104935: typing: Fix interactions between `@runtime_checkable` and `Generic` (#104939)

---------

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

1 files changed, 3 insertions, 3 deletions

diff --git a/Lib/typing.py b/Lib/typing.py
index b32ff0c..85d129b 100644
--- a/Lib/typing.py
+++ b/Lib/typing.py
@@ -1894,7 +1894,7 @@ class Protocol(Generic, metaclass=_ProtocolMeta):
                     annotations = getattr(base, '__annotations__', {})
                     if (isinstance(annotations, collections.abc.Mapping) and
                             attr in annotations and
-                            issubclass(other, Generic) and other._is_protocol):
+                            issubclass(other, Generic) and getattr(other, '_is_protocol', False)):
                         break
                 else:
                     return NotImplemented
@@ -1912,7 +1912,7 @@ class Protocol(Generic, metaclass=_ProtocolMeta):
             if not (base in (object, Generic) or
                     base.__module__ in _PROTO_ALLOWLIST and
                     base.__name__ in _PROTO_ALLOWLIST[base.__module__] or
-                    issubclass(base, Generic) and base._is_protocol):
+                    issubclass(base, Generic) and getattr(base, '_is_protocol', False)):
                 raise TypeError('Protocols can only inherit from other'
                                 ' protocols, got %r' % base)
         if cls.__init__ is Protocol.__init__:
@@ -2059,7 +2059,7 @@ def runtime_checkable(cls):
     Warning: this will check only the presence of the required methods,
     not their type signatures!
     """
-    if not issubclass(cls, Generic) or not cls._is_protocol:
+    if not issubclass(cls, Generic) or not getattr(cls, '_is_protocol', False):
         raise TypeError('@runtime_checkable can be only applied to protocol classes,'
                         ' got %r' % cls)
     cls._is_runtime_protocol = True