gh-132493: lazy evaluation of annotations in typing._proto_hook

Lib/test/test_typing.py

@@ -4554,6 +4554,28 @@ class Commentable(Protocol):
         )
         self.assertIs(type(exc.__cause__), CustomError)
 
+    def test_isinstance_with_deferred_evaluation_of_annotations(self):
+        @runtime_checkable
+        class P(Protocol):
+            x: undefined | int
+            def meth(self):
+                ...
+
+        class DeferredClass:
+            x: undefined
+
+        class DeferredClassImplementingP:
+            x: undefined | int
+
+            def __init__(self):
+                self.x = 0
+
+            def meth(self):
+                ...
+
+        self.assertFalse(isinstance(DeferredClass(), P))
+        self.assertTrue(isinstance(DeferredClassImplementingP(), P))
+
 
 class GenericTests(BaseTestCase):
 

Lib/typing.py

@@ -1801,7 +1801,7 @@ def _get_protocol_attrs(cls):
     for base in cls.__mro__[:-1]:  # without object
         if base.__name__ in {'Protocol', 'Generic'}:
             continue
-        annotations = getattr(base, '__annotations__', {})
+        annotations = _lazy_annotationlib.get_annotations(base, format=_lazy_annotationlib.Format.FORWARDREF)
         for attr in (*base.__dict__, *annotations):
             if not attr.startswith('_abc_') and attr not in EXCLUDED_ATTRIBUTES:
                 attrs.add(attr)
@@ -2018,10 +2018,10 @@ def _proto_hook(cls, other):
                 break
 
             # ...or in annotations, if it is a sub-protocol.
-            annotations = getattr(base, '__annotations__', {})
-            if (isinstance(annotations, collections.abc.Mapping) and
-                    attr in annotations and
-                    issubclass(other, Generic) and getattr(other, '_is_protocol', False)):
+            annotations = _lazy_annotationlib.get_annotations(base, format=_lazy_annotationlib.Format.FORWARDREF)
+            if (attr in annotations
+                and issubclass(other, Generic)
+                and getattr(other, '_is_protocol', False)):
                 break
         else:
             return NotImplemented

Misc/NEWS.d/next/Library/2025-04-15-08-39-14.gh-issue-132493.V0gLkU.rst

@@ -0,0 +1,4 @@
+:class:`typing.Protocol` now uses :func:`annotationlib.get_annotations` when
+checking whether or not an instance implements the protocol with
+:func:`isinstance`. This enables support for ``isinstance`` checks against
+classes with deferred annotations.