gh-102433: Add tests for how classes with properties interact with `isinstance()` checks on `typing.runtime_checkable` protocols (#102449)

Co-authored-by: Carl Meyer <carl@oddbird.net>

Lib/test/test_typing.py

@@ -2530,6 +2530,94 @@ class ProtocolTests(BaseTestCase):
         with self.assertRaises(TypeError):
             isinstance(C(), BadPG)
 
+    def test_protocols_isinstance_properties_and_descriptors(self):
+        class C:
+            @property
+            def attr(self):
+                return 42
+
+        class CustomDescriptor:
+            def __get__(self, obj, objtype=None):
+                return 42
+
+        class D:
+            attr = CustomDescriptor()
+
+        # Check that properties set on superclasses
+        # are still found by the isinstance() logic
+        class E(C): ...
+        class F(D): ...
+
+        class Empty: ...
+
+        T = TypeVar('T')
+
+        @runtime_checkable
+        class P(Protocol):
+            @property
+            def attr(self): ...
+
+        @runtime_checkable
+        class P1(Protocol):
+            attr: int
+
+        @runtime_checkable
+        class PG(Protocol[T]):
+            @property
+            def attr(self): ...
+
+        @runtime_checkable
+        class PG1(Protocol[T]):
+            attr: T
+
+        for protocol_class in P, P1, PG, PG1:
+            for klass in C, D, E, F:
+                with self.subTest(
+                    klass=klass.__name__,
+                    protocol_class=protocol_class.__name__
+                ):
+                    self.assertIsInstance(klass(), protocol_class)
+
+            with self.subTest(klass="Empty", protocol_class=protocol_class.__name__):
+                self.assertNotIsInstance(Empty(), protocol_class)
+
+        class BadP(Protocol):
+            @property
+            def attr(self): ...
+
+        class BadP1(Protocol):
+            attr: int
+
+        class BadPG(Protocol[T]):
+            @property
+            def attr(self): ...
+
+        class BadPG1(Protocol[T]):
+            attr: T
+
+        for obj in PG[T], PG[C], PG1[T], PG1[C], BadP, BadP1, BadPG, BadPG1:
+            for klass in C, D, E, F, Empty:
+                with self.subTest(klass=klass.__name__, obj=obj):
+                    with self.assertRaises(TypeError):
+                        isinstance(klass(), obj)
+
+    def test_protocols_isinstance_not_fooled_by_custom_dir(self):
+        @runtime_checkable
+        class HasX(Protocol):
+            x: int
+
+        class CustomDirWithX:
+            x = 10
+            def __dir__(self):
+                return []
+
+        class CustomDirWithoutX:
+            def __dir__(self):
+                return ["x"]
+
+        self.assertIsInstance(CustomDirWithX(), HasX)
+        self.assertNotIsInstance(CustomDirWithoutX(), HasX)
+
     def test_protocols_isinstance_py36(self):
         class APoint:
             def __init__(self, x, y, label):