gh-105144: Runtime-checkable protocols: move all 'sanity checks' to `_ProtocolMeta.__subclasscheck__` (#105152)

1 files changed, 15 insertions, 19 deletions

diff --git a/Lib/typing.py b/Lib/typing.py
index 8c87479..f589be7 100644
--- a/Lib/typing.py
+++ b/Lib/typing.py
@@ -1727,7 +1727,7 @@ def _caller(depth=1, default='__main__'):
         pass
     return None
 
-def _allow_reckless_class_checks(depth=3):
+def _allow_reckless_class_checks(depth=2):
     """Allow instance and class checks for special stdlib modules.
 
     The abc and functools modules indiscriminately call isinstance() and
@@ -1782,14 +1782,22 @@ class _ProtocolMeta(ABCMeta):
             )
 
     def __subclasscheck__(cls, other):
+        if not isinstance(other, type):
+            # Same error message as for issubclass(1, int).
+            raise TypeError('issubclass() arg 1 must be a class')
         if (
             getattr(cls, '_is_protocol', False)
-            and not cls.__callable_proto_members_only__
-            and not _allow_reckless_class_checks(depth=2)
+            and not _allow_reckless_class_checks()
         ):
-            raise TypeError(
-                "Protocols with non-method members don't support issubclass()"
-            )
+            if not cls.__callable_proto_members_only__:
+                raise TypeError(
+                    "Protocols with non-method members don't support issubclass()"
+                )
+            if not getattr(cls, '_is_runtime_protocol', False):
+                raise TypeError(
+                    "Instance and class checks can only be used with "
+                    "@runtime_checkable protocols"
+                )
         return super().__subclasscheck__(other)
 
     def __instancecheck__(cls, instance):
@@ -1801,7 +1809,7 @@ class _ProtocolMeta(ABCMeta):
 
         if (
             not getattr(cls, '_is_runtime_protocol', False) and
-            not _allow_reckless_class_checks(depth=2)
+            not _allow_reckless_class_checks()
         ):
             raise TypeError("Instance and class checks can only be used with"
                             " @runtime_checkable protocols")
@@ -1869,18 +1877,6 @@ class Protocol(Generic, metaclass=_ProtocolMeta):
             if not cls.__dict__.get('_is_protocol', False):
                 return NotImplemented
 
-            # First, perform various sanity checks.
-            if not getattr(cls, '_is_runtime_protocol', False):
-                if _allow_reckless_class_checks():
-                    return NotImplemented
-                raise TypeError("Instance and class checks can only be used with"
-                                " @runtime_checkable protocols")
-
-            if not isinstance(other, type):
-                # Same error message as for issubclass(1, int).
-                raise TypeError('issubclass() arg 1 must be a class')
-
-            # Second, perform the actual structural compatibility check.
             for attr in cls.__protocol_attrs__:
                 for base in other.__mro__:
                     # Check if the members appears in the class dictionary...