Use a new private sys function instead

Author: JelleZijlstra

Lib/dataclasses.py

@@ -1340,10 +1340,7 @@ def _add_slots(cls, is_frozen, weakref_slot, defined_fields):
 
     # gh-135228: Make sure the original class can be garbage collected.
     # Bypass mapping proxy to allow __dict__ to be removed
-    old_cls_dict = cls.__dict__ | _deproxier
-    old_cls_dict.pop('__dict__', None)
-    if "__weakref__" in cls.__dict__:
-        del cls.__weakref__
+    sys._clear_type_descriptors(cls)
 
     return newcls
 
@@ -1739,11 +1736,3 @@ def _replace(self, /, **changes):
     # changes that aren't fields, this will correctly raise a
     # TypeError.
     return self.__class__(**changes)
-
-
-# Hack to the get the underlying dict out of a mappingproxy
-# Use it with: cls.__dict__ | _deproxier
-class _Deproxier:
-    def __ror__(self, other):
-        return other
-_deproxier = _Deproxier()

Python/clinic/sysmodule.c.h

@@ -2641,6 +2641,38 @@ sys__baserepl_impl(PyObject *module)
     Py_RETURN_NONE;
 }
 
+/*[clinic input]
+sys._clear_type_descriptors
+
+    type: object
+    /
+
+Private function for clearing certain descriptors from a type's dictionary.
+
+See gh-135228 for context.
+[clinic start generated code]*/
+
+static PyObject *
+sys__clear_type_descriptors(PyObject *module, PyObject *type)
+/*[clinic end generated code: output=7d5cefcf861909e0 input=5fdc23500d477de6]*/
+{
+    if (!PyType_Check(type)) {
+        PyErr_SetString(PyExc_TypeError, "argument must be a type");
+        return NULL;
+    }
+    PyTypeObject *typeobj = (PyTypeObject *)(type);
+    PyObject *dict = _PyType_GetDict(typeobj);
+    if (PyDict_PopString(dict, "__dict__", NULL) < 0) {
+        return NULL;
+    }
+    if (PyDict_PopString(dict, "__weakref__", NULL) < 0) {
+        return NULL;
+    }
+    PyType_Modified(typeobj);
+    Py_RETURN_NONE;
+}
+
+
 /*[clinic input]
 sys._is_gil_enabled -> bool
 
@@ -2837,6 +2869,7 @@ static PyMethodDef sys_methods[] = {
     SYS__STATS_DUMP_METHODDEF
 #endif
     SYS__GET_CPU_COUNT_CONFIG_METHODDEF
+    SYS__CLEAR_TYPE_DESCRIPTORS_METHODDEF
     SYS__IS_GIL_ENABLED_METHODDEF
     SYS__DUMP_TRACELETS_METHODDEF
     {NULL, NULL}  // sentinel