gh-135228: Create __dict__ and __weakref__ descriptors for object

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)
 {

Python/pylifecycle.c

@@ -1906,6 +1906,7 @@ finalize_interp_clear(PyThreadState *tstate)
  _PyXI_Fini(tstate->interp);
  _PyExc_ClearExceptionGroupType(tstate->interp);
  _Py_clear_generic_types(tstate->interp);
+ _PyTypes_FiniCachedDescriptors(tstate->interp);
 
  /* Clear interpreter state and all thread states */
  _PyInterpreterState_Clear(tstate);