gh-144873: Remove Overzealous isinstance Optimization

Lib/test/test_types.py

@@ -909,6 +909,40 @@ def __subclasscheck__(cls, sub):
         self.assertIsSubclass(int, x)
         self.assertRaises(ZeroDivisionError, issubclass, list, x)
 
+    def test_custom_instancecheck(self):
+        class CustomIsInstanceMeta(type):
+            def __instancecheck__(cls, instance):
+                return type(instance) is int
+
+        class CustomIsInstance(metaclass=CustomIsInstanceMeta):
+            ...
+
+        class CustomIsInstanceSubclass(CustomIsInstance):
+            ...
+
+        self.assertTrue(isinstance(5, CustomIsInstance))
+        self.assertFalse(isinstance(CustomIsInstance(), CustomIsInstance))
+        self.assertFalse(isinstance(CustomIsInstanceSubclass(), CustomIsInstance))
+
+    def test_custom_subclasscheck(self):
+        class CustomIsSubclassMeta(type):
+            def __subclasscheck__(cls, subcls):
+                return subcls is int
+
+        class CustomIsSubclass(metaclass=CustomIsSubclassMeta):
+            ...
+
+        class CustomIsSubclassSubclass(CustomIsSubclass):
+            ...
+
+        self.assertTrue(issubclass(int, CustomIsSubclass))
+        self.assertFalse(isinstance(4, CustomIsSubclass))
+        self.assertFalse(issubclass(CustomIsSubclass, CustomIsSubclass))
+        self.assertTrue(isinstance(CustomIsSubclass(), CustomIsSubclass))
+        self.assertFalse(issubclass(CustomIsSubclassSubclass, CustomIsSubclass))
+        self.assertTrue(isinstance(CustomIsSubclassSubclass(), CustomIsSubclass))
+
+
     def test_or_type_operator_with_TypeVar(self):
         TV = typing.TypeVar('T')
         self.assertEqual(TV | str, typing.Union[TV, str])

Objects/abstract.c

@@ -2638,11 +2638,6 @@ object_isinstance(PyObject *inst, PyObject *cls)
 static int
 object_recursive_isinstance(PyThreadState *tstate, PyObject *inst, PyObject *cls)
 {
-    /* Quick test for an exact match */
-    if (Py_IS_TYPE(inst, (PyTypeObject *)cls)) {
-        return 1;
-    }
-
     /* We know what type's __instancecheck__ does. */
     if (PyType_CheckExact(cls)) {
         return object_isinstance(inst, cls);