gh-135228: Create __dict__ and __weakref__ descriptors for object (GH-136966)

This partially reverts #137047, keeping the tests for GC collectability of the
original class that dataclass adds `__slots__` to.
The reference leaks solved there are instead solved by having the `__dict__` &
`__weakref__` descriptors not tied to (and referencing) their class.

Instead, they're shared between all classes that need them (within
an interpreter).
The `__objclass__` ol the descriptors is set to `object`, since these
descriptors work with *any* object. (The appropriate checks were already
made in the get/set code, so the `__objclass__` check was redundant.)

The repr of these descriptors (and any others whose `__objclass__` is `object`)
now doesn't mention the objclass.

This change required adjustment of introspection code that checks
`__objclass__` to determine an object's “own” (i.e. not inherited) `__dict__`.
Third-party code that does similar introspection of the internals will also
need adjusting.


Co-authored-by: Jelle Zijlstra <[email protected]>

Author: encukou

Doc/whatsnew/3.15.rst

@@ -209,6 +209,13 @@ Other language changes
   as keyword arguments at construction time.
   (Contributed by Serhiy Storchaka, Oleg Iarygin, and Yoav Nir in :gh:`74185`.)
 
+* The :attr:`~object.__dict__` and :attr:`!__weakref__` descriptors now use a
+  single descriptor instance per interpreter, shared across all types that
+  need them.
+  This speeds up class creation, and helps avoid reference cycles.
+  (Contributed by Petr Viktorin in :gh:`135228`.)
+
+
 New modules
 ===========
 

Include/internal/pycore_interp_structs.h

@@ -691,6 +691,13 @@ struct _Py_interp_cached_objects {
     PyTypeObject *paramspecargs_type;
     PyTypeObject *paramspeckwargs_type;
     PyTypeObject *constevaluator_type;
+
+    /* Descriptors for __dict__ and __weakref__ */
+#ifdef Py_GIL_DISABLED
+    PyMutex descriptor_mutex;
+#endif
+    PyObject *dict_descriptor;
+    PyObject *weakref_descriptor;
 };
 
 struct _Py_interp_static_objects {

Include/internal/pycore_typeobject.h

@@ -40,6 +40,7 @@ extern void _PyTypes_FiniTypes(PyInterpreterState *);
 extern void _PyTypes_FiniExtTypes(PyInterpreterState *interp);
 extern void _PyTypes_Fini(PyInterpreterState *);
 extern void _PyTypes_AfterFork(void);
+extern void _PyTypes_FiniCachedDescriptors(PyInterpreterState *);
 
 static inline PyObject **
 _PyStaticType_GET_WEAKREFS_LISTPTR(managed_static_type_state *state)

Lib/dataclasses.py

@@ -1283,10 +1283,6 @@ def _add_slots(cls, is_frozen, weakref_slot, defined_fields):
     if '__slots__' in cls.__dict__:
         raise TypeError(f'{cls.__name__} already specifies __slots__')
 
-    # gh-102069: Remove existing __weakref__ descriptor.
-    # gh-135228: Make sure the original class can be garbage collected.
-    sys._clear_type_descriptors(cls)
-
     # Create a new dict for our new class.
     cls_dict = dict(cls.__dict__)
     field_names = tuple(f.name for f in fields(cls))
@@ -1304,6 +1300,11 @@ def _add_slots(cls, is_frozen, weakref_slot, defined_fields):
         #  available in _MARKER.
         cls_dict.pop(field_name, None)
 
+    # Remove __dict__ and `__weakref__` descriptors.
+    # They'll be added back if applicable.
+    cls_dict.pop('__dict__', None)
+    cls_dict.pop('__weakref__', None)  # gh-102069
+
     # And finally create the class.
     qualname = getattr(cls, '__qualname__', None)
     newcls = type(cls)(cls.__name__, cls.__bases__, cls_dict)

Lib/inspect.py

@@ -1698,7 +1698,8 @@ def _shadowed_dict_from_weakref_mro_tuple(*weakref_mro):
             class_dict = dunder_dict['__dict__']
             if not (type(class_dict) is types.GetSetDescriptorType and
                     class_dict.__name__ == "__dict__" and
-                    class_dict.__objclass__ is entry):
+                    (class_dict.__objclass__ is object or
+                     class_dict.__objclass__ is entry)):
                 return class_dict
     return _sentinel
 

Lib/test/test_descr.py

@@ -6013,5 +6013,69 @@ class A(metaclass=M):
                 pass
 
 
+class TestGenericDescriptors(unittest.TestCase):
+    def test___dict__(self):
+        class CustomClass:
+            pass
+        class SlotClass:
+            __slots__ = ['foo']
+        class SlotSubClass(SlotClass):
+            pass
+        class IntSubclass(int):
+            pass
+
+        dict_descriptor = CustomClass.__dict__['__dict__']
+        self.assertEqual(dict_descriptor.__objclass__, object)
+
+        for cls in CustomClass, SlotSubClass, IntSubclass:
+            with self.subTest(cls=cls):
+                self.assertIs(cls.__dict__['__dict__'], dict_descriptor)
+                instance = cls()
+                instance.attr = 123
+                self.assertEqual(
+                    dict_descriptor.__get__(instance, cls),
+                    {'attr': 123},
+                )
+        with self.assertRaises(AttributeError):
+            print(dict_descriptor.__get__(True, bool))
+        with self.assertRaises(AttributeError):
+            print(dict_descriptor.__get__(SlotClass(), SlotClass))
+
+        # delegation to type.__dict__
+        self.assertIsInstance(
+            dict_descriptor.__get__(type, type),
+            types.MappingProxyType,
+        )
+
+    def test___weakref__(self):
+        class CustomClass:
+            pass
+        class SlotClass:
+            __slots__ = ['foo']
+        class SlotSubClass(SlotClass):
+            pass
+        class IntSubclass(int):
+            pass
+
+        weakref_descriptor = CustomClass.__dict__['__weakref__']
+        self.assertEqual(weakref_descriptor.__objclass__, object)
+
+        for cls in CustomClass, SlotSubClass:
+            with self.subTest(cls=cls):
+                self.assertIs(cls.__dict__['__weakref__'], weakref_descriptor)
+                instance = cls()
+                instance.attr = 123
+                self.assertEqual(
+                    weakref_descriptor.__get__(instance, cls),
+                    None,
+                )
+        with self.assertRaises(AttributeError):
+            weakref_descriptor.__get__(True, bool)
+        with self.assertRaises(AttributeError):
+            weakref_descriptor.__get__(SlotClass(), SlotClass)
+        with self.assertRaises(AttributeError):
+            weakref_descriptor.__get__(IntSubclass(), IntSubclass)
+
+
 if __name__ == "__main__":
     unittest.main()

Misc/NEWS.d/next/Core_and_Builtins/2025-08-05-10-22-15.gh-issue-136966.J5lrE0.rst

@@ -0,0 +1,4 @@
+The :attr:`object.__dict__` and :attr:`!__weakref__` descriptors now use a
+single descriptor instance per interpreter, shared across all types that
+need them.
+This speeds up class creation, and helps avoid reference cycles.

Objects/descrobject.c

@@ -39,41 +39,41 @@ descr_name(PyDescrObject *descr)
 }
 
 static PyObject *
-descr_repr(PyDescrObject *descr, const char *format)
+descr_repr(PyDescrObject *descr, const char *kind)
 {
     PyObject *name = NULL;
     if (descr->d_name != NULL && PyUnicode_Check(descr->d_name))
         name = descr->d_name;
 
-    return PyUnicode_FromFormat(format, name, "?", descr->d_type->tp_name);
+    if (descr->d_type == &PyBaseObject_Type) {
+        return PyUnicode_FromFormat("<%s '%V'>", kind, name, "?");
+    }
+    return PyUnicode_FromFormat("<%s '%V' of '%s' objects>",
+                                kind, name, "?", descr->d_type->tp_name);
 }
 
 static PyObject *
 method_repr(PyObject *descr)
 {
-    return descr_repr((PyDescrObject *)descr,
-                      "<method '%V' of '%s' objects>");
+    return descr_repr((PyDescrObject *)descr, "method");
 }
 
 static PyObject *
 member_repr(PyObject *descr)
 {
-    return descr_repr((PyDescrObject *)descr,
-                      "<member '%V' of '%s' objects>");
+    return descr_repr((PyDescrObject *)descr, "member");
 }
 
 static PyObject *
 getset_repr(PyObject *descr)
 {
-    return descr_repr((PyDescrObject *)descr,
-                      "<attribute '%V' of '%s' objects>");
+    return descr_repr((PyDescrObject *)descr, "attribute");
 }
 
 static PyObject *
 wrapperdescr_repr(PyObject *descr)
 {
-    return descr_repr((PyDescrObject *)descr,
-                      "<slot wrapper '%V' of '%s' objects>");
+    return descr_repr((PyDescrObject *)descr, "slot wrapper");
 }
 
 static int

Objects/typeobject.c

@@ -4039,26 +4039,15 @@ subtype_getweakref(PyObject *obj, void *context)
     return Py_NewRef(result);
 }
 
-/* Three variants on the subtype_getsets list. */
-
-static PyGetSetDef subtype_getsets_full[] = {
-    {"__dict__", subtype_dict, subtype_setdict,
-     PyDoc_STR("dictionary for instance variables")},
-    {"__weakref__", subtype_getweakref, NULL,
-     PyDoc_STR("list of weak references to the object")},
-    {0}
-};
-
-static PyGetSetDef subtype_getsets_dict_only[] = {
-    {"__dict__", subtype_dict, subtype_setdict,
-     PyDoc_STR("dictionary for instance variables")},
-    {0}
+/* getset definitions for common descriptors */
+static PyGetSetDef subtype_getset_dict = {
+    "__dict__", subtype_dict, subtype_setdict,
+    PyDoc_STR("dictionary for instance variables"),
 };
 
-static PyGetSetDef subtype_getsets_weakref_only[] = {
-    {"__weakref__", subtype_getweakref, NULL,
-     PyDoc_STR("list of weak references to the object")},
-    {0}
+static PyGetSetDef subtype_getset_weakref = {
+    "__weakref__", subtype_getweakref, NULL,
+    PyDoc_STR("list of weak references to the object"),
 };
 
 static int
@@ -4594,10 +4583,36 @@ type_new_classmethod(PyObject *dict, PyObject *attr)
     return 0;
 }
 
+/* Add __dict__ or __weakref__ descriptor */
+static int
+type_add_common_descriptor(PyInterpreterState *interp,
+                           PyObject **cache,
+                           PyGetSetDef *getset_def,
+                           PyObject *dict)
+{
+#ifdef Py_GIL_DISABLED
+    PyMutex_Lock(&interp->cached_objects.descriptor_mutex);
+#endif
+    PyObject *descr = *cache;
+    if (!descr) {
+        descr = PyDescr_NewGetSet(&PyBaseObject_Type, getset_def);
+        *cache = descr;
+    }
+#ifdef Py_GIL_DISABLED
+    PyMutex_Unlock(&interp->cached_objects.descriptor_mutex);
+#endif
+    if (!descr) {
+        return -1;
+    }
+    if (PyDict_SetDefaultRef(dict, PyDescr_NAME(descr), descr, NULL) < 0) {
+        return -1;
+    }
+    return 0;
+}
 
 /* Add descriptors for custom slots from __slots__, or for __dict__ */
 static int
-type_new_descriptors(const type_new_ctx *ctx, PyTypeObject *type)
+type_new_descriptors(const type_new_ctx *ctx, PyTypeObject *type, PyObject *dict)
 {
     PyHeapTypeObject *et = (PyHeapTypeObject *)type;
     Py_ssize_t slotoffset = ctx->base->tp_basicsize;
@@ -4635,25 +4650,38 @@ type_new_descriptors(const type_new_ctx *ctx, PyTypeObject *type)
     type->tp_basicsize = slotoffset;
     type->tp_itemsize = ctx->base->tp_itemsize;
     type->tp_members = _PyHeapType_GET_MEMBERS(et);
+
+    PyInterpreterState *interp = _PyInterpreterState_GET();
+
+    if (type->tp_dictoffset) {
+        if (type_add_common_descriptor(
+            interp,
+            &interp->cached_objects.dict_descriptor,
+            &subtype_getset_dict,
+            dict) < 0)
+        {
+            return -1;
+        }
+    }
+    if (type->tp_weaklistoffset) {
+        if (type_add_common_descriptor(
+            interp,
+            &interp->cached_objects.weakref_descriptor,
+            &subtype_getset_weakref,
+            dict) < 0)
+        {
+            return -1;
+        }
+    }
+
     return 0;
 }
 
 
 static void
 type_new_set_slots(const type_new_ctx *ctx, PyTypeObject *type)
 {
-    if (type->tp_weaklistoffset && type->tp_dictoffset) {
-        type->tp_getset = subtype_getsets_full;
-    }
-    else if (type->tp_weaklistoffset && !type->tp_dictoffset) {
-        type->tp_getset = subtype_getsets_weakref_only;
-    }
-    else if (!type->tp_weaklistoffset && type->tp_dictoffset) {
-        type->tp_getset = subtype_getsets_dict_only;
-    }
-    else {
-        type->tp_getset = NULL;
-    }
+    type->tp_getset = NULL;
 
     /* Special case some slots */
     if (type->tp_dictoffset != 0 || ctx->nslot > 0) {
@@ -4758,7 +4786,7 @@ type_new_set_attrs(const type_new_ctx *ctx, PyTypeObject *type)
         return -1;
     }
 
-    if (type_new_descriptors(ctx, type) < 0) {
+    if (type_new_descriptors(ctx, type, dict) < 0) {
         return -1;
     }
 
@@ -6642,6 +6670,14 @@ _PyStaticType_FiniBuiltin(PyInterpreterState *interp, PyTypeObject *type)
 }
 
 
+void
+_PyTypes_FiniCachedDescriptors(PyInterpreterState *interp)
+{
+    Py_CLEAR(interp->cached_objects.dict_descriptor);
+    Py_CLEAR(interp->cached_objects.weakref_descriptor);
+}
+
+
 static void
 type_dealloc(PyObject *self)
 {