Share __dict__ and __weakref__ descriptors

python · encukou · Jul 20, 2025 · Jul 22, 2025 · Jul 22, 2025 · Jul 22, 2025
commit d0d2f73566699e2c0114fcd1466e023b53e01b77
diff --git a/Include/internal/pycore_interp_structs.h b/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 {

diff --git a/Include/internal/pycore_typeobject.h b/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)

@@ -4036,29 +4036,15 @@ subtype_getweakref(PyObject *obj, void *context)
     return Py_NewRef(result);
 }
 
-/* Three variants on the subtype_getsets list. */
-
-static char subtype_getset_dict_name[] = "__dict__";
-static char subtype_getset_weakref_name[] = "__weakref__";
-
-static PyGetSetDef subtype_getsets_full[] = {
-    {subtype_getset_dict_name, subtype_dict, subtype_setdict,
-     PyDoc_STR("dictionary for instance variables")},
-    {subtype_getset_weakref_name, subtype_getweakref, NULL,
-     PyDoc_STR("list of weak references to the object")},
-    {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_dict_only[] = {
-    {subtype_getset_dict_name, subtype_dict, subtype_setdict,
-     PyDoc_STR("dictionary for instance variables")},
-    {0}
-};
-
-static PyGetSetDef subtype_getsets_weakref_only[] = {
-    {subtype_getset_weakref_name, 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 +4580,36 @@ 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 +4647,38 @@ 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 +4783,7 @@ 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 +6667,14 @@ _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)
 {
@@ -8332,16 +8365,7 @@ type_add_getset(PyTypeObject *type)
 
     PyObject *dict = lookup_tp_dict(type);
     for (; gsp->name != NULL; gsp++) {
-        PyTypeObject *descr_type = type;
-        // Hack: dict and weakref descriptors are created for `object`,
-        // rather than this specific type.
-        // We identify their PyGetSetDef by pointer equality on name.
-        if (gsp->name == subtype_getset_dict_name
-            || gsp->name == subtype_getset_weakref_name)
-        {
-            descr_type = &PyBaseObject_Type;
-        }
-        PyObject *descr = PyDescr_NewGetSet(descr_type, gsp);
+        PyObject *descr = PyDescr_NewGetSet(type, gsp);
         if (descr == NULL) {
             return -1;
         }

@@ -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);