From 027f629dd5711be1348e5461177b3de7ed0dc44e Mon Sep 17 00:00:00 2001
From: Jelle Zijlstra <jelle.zijlstra@gmail.com>
Date: Sun, 20 Jul 2025 16:57:40 -0700
Subject: [PATCH 1/3] gh-135228: When @dataclass(slots=True) replaces a
 dataclass, make the original class collectible

An interesting hack, but more localized in scope than #135230.

This may be a breaking change if people intentionally keep the original class around
when using `@dataclass(slots=True)`, and then use `__dict__` or `__weakref__` on the
original class.
---
 Lib/dataclasses.py                            | 13 +++++++
 Lib/test/test_dataclasses/__init__.py         | 35 +++++++++++++++++++
 ...-07-20-16-56-55.gh-issue-135228.n_XIao.rst |  4 +++
 3 files changed, 52 insertions(+)
 create mode 100644 Misc/NEWS.d/next/Library/2025-07-20-16-56-55.gh-issue-135228.n_XIao.rst

diff --git a/Lib/dataclasses.py b/Lib/dataclasses.py
index 83ea623dce6281..9a1d5071e4157c 100644
--- a/Lib/dataclasses.py
+++ b/Lib/dataclasses.py
@@ -1338,6 +1338,11 @@ 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.
+    old_cls_dict = cls.__dict__ | _deproxier
+    old_cls_dict.pop('__weakref__', None)
+    old_cls_dict.pop('__dict__', None)
+
     return newcls
 
 
@@ -1732,3 +1737,11 @@ 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()
diff --git a/Lib/test/test_dataclasses/__init__.py b/Lib/test/test_dataclasses/__init__.py
index e98a8f284cec9f..6bf5e5b3e5554b 100644
--- a/Lib/test/test_dataclasses/__init__.py
+++ b/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):
diff --git a/Misc/NEWS.d/next/Library/2025-07-20-16-56-55.gh-issue-135228.n_XIao.rst b/Misc/NEWS.d/next/Library/2025-07-20-16-56-55.gh-issue-135228.n_XIao.rst
new file mode 100644
index 00000000000000..ee8962c6f46e75
--- /dev/null
+++ b/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`.

From e0d61f7a8655e7a3f1fcc035e3c1b9fc4af4d9b4 Mon Sep 17 00:00:00 2001
From: Jelle Zijlstra <jelle.zijlstra@gmail.com>
Date: Mon, 21 Jul 2025 19:20:36 -0700
Subject: [PATCH 2/3] Apply suggestions from code review

Co-authored-by: Alyssa Coghlan <ncoghlan@gmail.com>
---
 Lib/dataclasses.py | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/Lib/dataclasses.py b/Lib/dataclasses.py
index 9a1d5071e4157c..d8bb6976aa7095 100644
--- a/Lib/dataclasses.py
+++ b/Lib/dataclasses.py
@@ -1339,9 +1339,10 @@ def _add_slots(cls, is_frozen, weakref_slot, defined_fields):
                 break
 
     # 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('__weakref__', None)
     old_cls_dict.pop('__dict__', None)
+    del cls.__weakref__
 
     return newcls
 

From d5450023143fc6cb5229b37ddf616e222d2d99e4 Mon Sep 17 00:00:00 2001
From: Jelle Zijlstra <jelle.zijlstra@gmail.com>
Date: Mon, 21 Jul 2025 19:29:43 -0700
Subject: [PATCH 3/3] Update Lib/dataclasses.py

---
 Lib/dataclasses.py | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/Lib/dataclasses.py b/Lib/dataclasses.py
index d8bb6976aa7095..22b78bb2fbe6ed 100644
--- a/Lib/dataclasses.py
+++ b/Lib/dataclasses.py
@@ -1342,7 +1342,8 @@ def _add_slots(cls, is_frozen, weakref_slot, defined_fields):
     # Bypass mapping proxy to allow __dict__ to be removed
     old_cls_dict = cls.__dict__ | _deproxier
     old_cls_dict.pop('__dict__', None)
-    del cls.__weakref__
+    if "__weakref__" in cls.__dict__:
+        del cls.__weakref__
 
     return newcls