python · JelleZijlstra · Jan 20, 2024 · Jan 20, 2024 · Jan 20, 2024 · Jan 20, 2024
diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -1,3 +1,14 @@
+# Unreleased
+
+- Speedup `issubclass()` checks against simple runtime-checkable protocols by
+  around 6% (backporting https://github.com/python/cpython/pull/112717, by Alex
+  Waygood).
+- Fix a regression in the implementation of protocols where `typing.Protocol`
+  classes that were not marked as `@runtime_checkable` would be unnecessarily
+  introspected, potentially causing exceptions to be raised if the protocol had
+  problematic members. Patch by Alex Waygood, backporting
+  https://github.com/python/cpython/pull/113401.
+
 # Release 4.9.0 (December 9, 2023)
 
 This feature release adds `typing_extensions.ReadOnly`, as specified

diff --git a/src/test_typing_extensions.py b/src/test_typing_extensions.py
@@ -2817,8 +2817,8 @@ def meth(self): pass  # noqa: B027
 
         self.assertNotIn("__protocol_attrs__", vars(NonP))
         self.assertNotIn("__protocol_attrs__", vars(NonPR))
-        self.assertNotIn("__callable_proto_members_only__", vars(NonP))
-        self.assertNotIn("__callable_proto_members_only__", vars(NonPR))
+        self.assertNotIn("__non_callable_proto_members__", vars(NonP))
+        self.assertNotIn("__non_callable_proto_members__", vars(NonPR))
 
         acceptable_extra_attrs = {
             '_is_protocol', '_is_runtime_protocol', '__parameters__',
@@ -2891,11 +2891,26 @@ def __subclasshook__(cls, other):
     @skip_if_py312b1
     def test_issubclass_fails_correctly(self):
         @runtime_checkable
-        class P(Protocol):
+        class NonCallableMembers(Protocol):
             x = 1
+
+        class NotRuntimeCheckable(Protocol):
+            def callable_member(self) -> int: ...
+
+        @runtime_checkable
+        class RuntimeCheckable(Protocol):
+            def callable_member(self) -> int: ...
+
         class C: pass
-        with self.assertRaisesRegex(TypeError, r"issubclass\(\) arg 1 must be a class"):
-            issubclass(C(), P)
+
+        # These three all exercise different code paths,
+        # but should result in the same error message:
+        for protocol in NonCallableMembers, NotRuntimeCheckable, RuntimeCheckable:
+            with self.subTest(proto_name=protocol.__name__):
+                with self.assertRaisesRegex(
+                    TypeError, r"issubclass\(\) arg 1 must be a class"
+                ):
+                    issubclass(C(), protocol)
 
     def test_defining_generic_protocols(self):
         T = TypeVar('T')
@@ -3456,6 +3471,7 @@ def method(self) -> None: ...
 
     @skip_if_early_py313_alpha
     def test_protocol_issubclass_error_message(self):
+        @runtime_checkable
         class Vec2D(Protocol):
             x: float
             y: float
@@ -3471,6 +3487,39 @@ def square_norm(self) -> float:
         with self.assertRaisesRegex(TypeError, re.escape(expected_error_message)):
             issubclass(int, Vec2D)
 
+    def test_nonruntime_protocol_interaction_with_evil_classproperty(self):
+        class classproperty:
+            def __get__(self, instance, type):
+                raise RuntimeError("NO")
+
+        class Commentable(Protocol):
+            evil = classproperty()
+
+        # recognised as a protocol attr,
+        # but not actually accessed by the protocol metaclass
+        # (which would raise RuntimeError) for non-runtime protocols.
+        # See gh-113320
+        self.assertEqual(get_protocol_members(Commentable), {"evil"})
+
+    def test_runtime_protocol_interaction_with_evil_classproperty(self):
+        class CustomError(Exception): pass
+
+        class classproperty:
+            def __get__(self, instance, type):
+                raise CustomError
+
+        with self.assertRaises(TypeError) as cm:
+            @runtime_checkable
+            class Commentable(Protocol):
+                evil = classproperty()
+
+        exc = cm.exception
+        self.assertEqual(
+            exc.args[0],
+            "Failed to determine whether protocol member 'evil' is a method member"
+        )
+        self.assertIs(type(exc.__cause__), CustomError)
+
 
 class Point2DGeneric(Generic[T], TypedDict):
     a: T
@@ -5263,7 +5312,7 @@ def test_typing_extensions_defers_when_possible(self):
                 'SupportsRound', 'Unpack',
             }
         if sys.version_info < (3, 13):
-            exclude |= {'NamedTuple', 'Protocol'}
+            exclude |= {'NamedTuple', 'Protocol', 'runtime_checkable'}
         if not hasattr(typing, 'ReadOnly'):
             exclude |= {'TypedDict', 'is_typeddict'}
         for item in typing_extensions.__all__:

diff --git a/src/typing_extensions.py b/src/typing_extensions.py
@@ -473,7 +473,7 @@ def clear_overloads():
     "_is_runtime_protocol", "__dict__", "__slots__", "__parameters__",
     "__orig_bases__", "__module__", "_MutableMapping__marker", "__doc__",
     "__subclasshook__", "__orig_class__", "__init__", "__new__",
-    "__protocol_attrs__", "__callable_proto_members_only__",
+    "__protocol_attrs__", "__non_callable_proto_members__",
     "__match_args__",
 }
 
@@ -521,6 +521,22 @@ def _no_init(self, *args, **kwargs):
         if type(self)._is_protocol:
             raise TypeError('Protocols cannot be instantiated')
 
+    def _type_check_issubclass_arg_1(arg):
+        """Raise TypeError if `arg` is not an instance of `type`
+        in `issubclass(arg, <protocol>)`.
+
+        In most cases, this is verified by type.__subclasscheck__.
+        Checking it again unnecessarily would slow down issubclass() checks,
+        so, we don't perform this check unless we absolutely have to.
+
+        For various error paths, however,
+        we want to ensure that *this* error message is shown to the user
+        where relevant, rather than a typing.py-specific error message.
+        """
+        if not isinstance(arg, type):
+            # Same error message as for issubclass(1, int).
+            raise TypeError('issubclass() arg 1 must be a class')
+
     # Inheriting from typing._ProtocolMeta isn't actually desirable,
     # but is necessary to allow typing.Protocol and typing_extensions.Protocol
     # to mix without getting TypeErrors about "metaclass conflict"
@@ -551,11 +567,6 @@ def __init__(cls, *args, **kwargs):
             abc.ABCMeta.__init__(cls, *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:
@@ -564,26 +575,23 @@ def __subclasscheck__(cls, other):
                 getattr(cls, '_is_protocol', False)
                 and not _allow_reckless_class_checks()
             ):
-                if not isinstance(other, type):
-                    # Same error message as for issubclass(1, int).
-                    raise TypeError('issubclass() arg 1 must be a class')
+                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
                 ):
-                    non_method_attrs = sorted(
-                        attr for attr in cls.__protocol_attrs__
-                        if not callable(getattr(cls, attr, None))
-                    )
+                    _type_check_issubclass_arg_1(other)
+                    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):
-                    raise TypeError(
-                        "Instance and class checks can only be used with "
-                        "@runtime_checkable protocols"
-                    )
             return abc.ABCMeta.__subclasscheck__(cls, other)
 
         def __instancecheck__(cls, instance):
@@ -610,7 +618,8 @@ def __instancecheck__(cls, instance):
                     val = inspect.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
@@ -678,8 +687,58 @@ def __init_subclass__(cls, *args, **kwargs):
                 cls.__init__ = _no_init
 
 
+if sys.version_info >= (3, 13):
+    runtime_checkable = typing.runtime_checkable
+else:
+    def runtime_checkable(cls):
+        """Mark a protocol class as a runtime protocol.
+
+        Such protocol can be used with isinstance() and issubclass().
+        Raise TypeError if applied to a non-protocol class.
+        This allows a simple-minded structural check very similar to
+        one trick ponies in collections.abc such as Iterable.
+
+        For example::
+
+            @runtime_checkable
+            class Closable(Protocol):
+                def close(self): ...
+
+            assert isinstance(open('/some/file'), Closable)
+
+        Warning: this will check only the presence of the required methods,
+        not their type signatures!
+        """
+        if not issubclass(cls, typing.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
+
+        # Only execute the following block if it's a typing_extensions.Protocol class.
+        # typing.Protocol classes don't need it.
+        if isinstance(cls, _ProtocolMeta):
+            # 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
+
+
 # The "runtime" alias exists for backwards compatibility.
-runtime = runtime_checkable = typing.runtime_checkable
+runtime = runtime_checkable
 
 
 # Our version of runtime-checkable protocols is faster on Python 3.8-3.11