Tests

python · AlexWaygood · Apr 2, 2023 · Mar 24, 2023 · Mar 24, 2023 · Mar 25, 2023
commit ef713954946720ba28d194f73985667798712688
@@ -2555,6 +2555,74 @@ def meth(x): ...
         with self.assertRaises(TypeError):
             isinstance(C(), BadPG)
 
+    def test_protocols_isinstance_attribute_access_with_side_effects(self):
+        class C:
+            @property
+            def attr(self):
+                raise AttributeError('no')
+
+        class CustomDescriptor:
+            def __get__(self, obj, objtype=None):
+                raise RuntimeError("NO")
+
+        class D:
+            attr = CustomDescriptor()
+
+        # Check that properties set on superclasses
+        # are still found by the isinstance() logic
+        class E(C): ...
+        class F(D): ...
+
+        class WhyWouldYouDoThis:
+            def __getattr__(self, name):
+                raise RuntimeError("wut")
+
+        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="WhyWouldYouDoThis",
+                protocol_class=protocol_class.__name__
+            ):
+                self.assertNotIsInstance(WhyWouldYouDoThis(), protocol_class)
+
+    def test_protocols_isinstance___slots__(self):
+        # As per the consensus in https://github.com/python/typing/issues/1367,
+        # this is desirable behaviour
+        @runtime_checkable
+        class HasX(Protocol):
+            x: int
+
+        class HasNothingButSlots:
+            __slots__ = ("x",)
+
+        self.assertIsInstance(HasNothingButSlots(), HasX)
+
     def test_protocols_isinstance_properties_and_descriptors(self):
         class C:
             @property