gh-153772: Make abc isinstance() tolerate instances without __class__

Lib/_py_abc.py

@@ -92,7 +92,12 @@ def _abc_caches_clear(cls):
     def __instancecheck__(cls, instance):
         """Override for isinstance(instance, cls)."""
         # Inline the cache checking
-        subclass = instance.__class__
+        try:
+            subclass = instance.__class__
+        except AttributeError:
+            # Fall back to the type when the instance has no __class__,
+            # matching the behaviour of the built-in isinstance() (gh-153772).
+            subclass = type(instance)
         if subclass in cls._abc_cache:
             return True
         subtype = type(instance)

Lib/test/test_abc.py

@@ -380,6 +380,25 @@ class C(str): pass
             self.assertIsSubclass(C, A)
             self.assertIsSubclass(C, (A,))
 
+        def test_instancecheck_no_class(self):
+            # gh-153772: __instancecheck__ must fall back to type(instance)
+            # when the instance has no __class__, matching isinstance().
+            class NoClass:
+                def __getattribute__(self, name):
+                    if name == "__class__":
+                        raise AttributeError(name)
+                    return super().__getattribute__(name)
+
+            class A(metaclass=abc_ABCMeta):
+                pass
+
+            obj = NoClass()
+            # Must return False rather than propagating the AttributeError.
+            self.assertNotIsInstance(obj, A)
+            # Registering the actual type makes the fallback report a match.
+            A.register(NoClass)
+            self.assertIsInstance(obj, A)
+
         def test_registration_edge_cases(self):
             class A(metaclass=abc_ABCMeta):
                 pass

Misc/NEWS.d/next/Library/2026-07-19-17-05-00.gh-issue-153772.cobjTK.rst

@@ -0,0 +1,5 @@
+:func:`isinstance` checks against :mod:`collections.abc` classes such as
+:class:`~collections.abc.Mapping` no longer raise :exc:`AttributeError`
+when the instance has no ``__class__``.  The abstract base class machinery
+now falls back to the object's type in that case, matching the behaviour of
+the built-in :func:`isinstance`.

Modules/_abc.c

@@ -629,11 +629,15 @@ _abc__abc_instancecheck_impl(PyObject *module, PyObject *self,
         return NULL;
     }
 
-    subclass = PyObject_GetAttr(instance, &_Py_ID(__class__));
-    if (subclass == NULL) {
+    if (PyObject_GetOptionalAttr(instance, &_Py_ID(__class__), &subclass) < 0) {
         Py_DECREF(impl);
         return NULL;
     }
+    if (subclass == NULL) {
+        /* Fall back to the type when the instance has no __class__, matching
+           the behaviour of the built-in isinstance() (gh-153772). */
+        subclass = Py_NewRef((PyObject *)Py_TYPE(instance));
+    }
     /* Inline the cache checking. */
     int incache = _in_weak_set(impl, &impl->_abc_cache, subclass);
     if (incache < 0) {