Backport CPython PR 26067

srittau · Apr 12, 2023 · Apr 9, 2023 · Apr 9, 2023 · Apr 9, 2023 · Apr 9, 2023
commit 587d84341242c1abdb6ca69e2b94d8a54786916a
diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -3,6 +3,10 @@
 - Add `typing_extensions.Buffer`, a marker class for buffer types, as proposed
   by PEP 688. Equivalent to `collections.abc.Buffer` in Python 3.12. Patch by
   Jelle Zijlstra.
+- Backport [CPython PR 26067](https://github.com/python/cpython/pull/26067)
+  (originally by Yurii Karabas), ensuring that `isinstance()` calls on
+  protocols raise `TypeError` when the protocol is not decorated with
+  `@runtime_checkable`. Patch by Alex Waygood.
 
 # Release 4.5.0 (February 14, 2023)
 

diff --git a/src/test_typing_extensions.py b/src/test_typing_extensions.py
@@ -1918,6 +1918,13 @@ class CustomProtocol(TestCase, Protocol):
         class CustomContextManager(typing.ContextManager, Protocol):
             pass
 
+    def test_non_runtime_protocol_isinstance_check(self):
+        class P(Protocol):
+            x: int
+
+        with self.assertRaisesRegex(TypeError, "@runtime_checkable"):
+            isinstance(1, P)
+
     def test_no_init_same_for_different_protocol_implementations(self):
         class CustomProtocolWithoutInitA(Protocol):
             pass

diff --git a/src/typing_extensions.py b/src/typing_extensions.py
@@ -468,11 +468,17 @@ def _caller(depth=2):
         return None
 
 
-# 3.8+
-if hasattr(typing, 'Protocol'):
+# A bug in runtime-checkable protocols was fixed in 3.10+,
+# but we backport it to all versions
+if sys.version_info >= (3, 10):
     Protocol = typing.Protocol
-# 3.7
 else:
+    def _allow_reckless_class_checks(depth=4):
+        """Allow instance and class checks for special stdlib modules.
+        The abc and functools modules indiscriminately call isinstance() and
+        issubclass() on the whole MRO of a user class, which may contain protocols.
+        """
+        return _caller(depth) in {'abc', 'functools', None}
 
     def _no_init(self, *args, **kwargs):
         if type(self)._is_protocol:
@@ -484,11 +490,19 @@ class _ProtocolMeta(abc.ABCMeta):  # noqa: B024
         def __instancecheck__(cls, instance):
             # We need this method for situations where attributes are
             # assigned in __init__.
-            if ((not getattr(cls, '_is_protocol', False) or
+            is_protocol_cls = getattr(cls, "_is_protocol", False)
+            if (
+                is_protocol_cls and
+                not getattr(cls, '_is_runtime_protocol', False) and
+                not _allow_reckless_class_checks(depth=2)
+            ):
+                raise TypeError("Instance and class checks can only be used with"
+                                " @runtime_checkable protocols")
+            if ((not is_protocol_cls or
                  _is_callable_members_only(cls)) and
                     issubclass(instance.__class__, cls)):
                 return True
-            if cls._is_protocol:
+            if is_protocol_cls:
                 if all(hasattr(instance, attr) and
                        (not callable(getattr(cls, attr, None)) or
                         getattr(instance, attr) is not None)
@@ -581,12 +595,12 @@ def _proto_hook(other):
                 if not cls.__dict__.get('_is_protocol', None):
                     return NotImplemented
                 if not getattr(cls, '_is_runtime_protocol', False):
-                    if _caller(depth=3) in {'abc', 'functools'}:
+                    if _allow_reckless_class_checks():
                         return NotImplemented
                     raise TypeError("Instance and class checks can only be used with"
                                     " @runtime protocols")
                 if not _is_callable_members_only(cls):
-                    if _caller(depth=3) in {'abc', 'functools'}:
+                    if _allow_reckless_class_checks():
                         return NotImplemented
                     raise TypeError("Protocols with non-method members"
                                     " don't support issubclass()")