gh-135228: When @dataclass(slots=True) replaces a dataclass, make the original class collectible (take 2)

Lib/dataclasses.py

@@ -1338,6 +1338,9 @@ def _add_slots(cls, is_frozen, weakref_slot, defined_fields):
                 or _update_func_cell_for__class__(member.fdel, cls, newcls)):
                 break
 
+    # gh-135228: Make sure the original class can be garbage collected.
+    sys._clear_type_descriptors(cls)
+
     return newcls
 
 

Lib/test/test_dataclasses/__init__.py

@@ -3804,6 +3804,41 @@ class WithCorrectSuper(CorrectSuper):
         # that we create internally.
         self.assertEqual(CorrectSuper.args, ["default", "default"])
 
+    def test_original_class_is_gced(self):
+        # gh-135228: Make sure when we replace the class with slots=True, the original class
+        # gets garbage collected.
+        def make_simple():
+            @dataclass(slots=True)
+            class SlotsTest:
+                pass
+
+            return SlotsTest
+
+        def make_with_annotations():
+            @dataclass(slots=True)
+            class SlotsTest:
+                x: int
+
+            return SlotsTest
+
+        def make_with_annotations_and_method():
+            @dataclass(slots=True)
+            class SlotsTest:
+                x: int
+
+                def method(self) -> int:
+                    return self.x
+
+            return SlotsTest
+
+        for make in (make_simple, make_with_annotations, make_with_annotations_and_method):
+            with self.subTest(make=make):
+                C = make()
+                support.gc_collect()
+                candidates = [cls for cls in object.__subclasses__() if cls.__name__ == 'SlotsTest'
+                              and cls.__firstlineno__ == make.__code__.co_firstlineno + 1]
+                self.assertEqual(candidates, [C])
+
 
 class TestDescriptors(unittest.TestCase):
     def test_set_name(self):

Misc/NEWS.d/next/Library/2025-07-20-16-56-55.gh-issue-135228.n_XIao.rst

@@ -0,0 +1,4 @@
+When :mod:`dataclasses` replaces a class with a slotted dataclass, the
+original class is now garbage collected again. Earlier changes in Python
+3.14 caused this class to remain in existence together with the replacement
+class synthesized by :mod:`dataclasses`.

Python/sysmodule.c

@@ -2641,6 +2641,51 @@ 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);
+    if (!_PyType_HasFeature(typeobj, Py_TPFLAGS_HEAPTYPE)) {
+        PyErr_SetString(PyExc_TypeError, "argument must be a heap type");
+        return NULL;
+    }
+    PyObject *dict = _PyType_GetDict(typeobj);
+    PyObject *dunder_dict = NULL;
+    if (PyDict_PopString(dict, "__dict__", &dunder_dict) < 0) {
+        return NULL;
+    }
+    PyObject *dunder_weakref = NULL;
+    if (PyDict_PopString(dict, "__weakref__", &dunder_weakref) < 0) {
+        PyType_Modified(typeobj);
+        Py_XDECREF(dunder_dict);
+        return NULL;
+    }
+    PyType_Modified(typeobj);
+    // We try to hold onto a reference to these until after we call
+    // PyType_Modified(), in case their deallocation triggers somer user code
+    // that tries to do something to the type.
+    Py_XDECREF(dunder_dict);
+    Py_XDECREF(dunder_weakref);
+    Py_RETURN_NONE;
+}
+
+
 /*[clinic input]
 sys._is_gil_enabled -> bool
 
@@ -2837,6 +2882,7 @@ 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