gh-103740: Support any slots in the type subclasses

Allow defining any __slots__ for a class derived from the type class or
other "variable-length" built-in type with the Py_TPFLAGS_ITEMS_AT_END flag.

Author: serhiy-storchaka

Doc/reference/datamodel.rst

@@ -2632,8 +2632,10 @@ Notes on using *__slots__*:
 
 * :exc:`TypeError` will be raised if *__slots__* other than *__dict__* and
   *__weakref__* are defined for a class derived from a
-  :c:member:`"variable-length" built-in type <PyTypeObject.tp_itemsize>` such as
-  :class:`int`, :class:`bytes`, and :class:`tuple`.
+  :c:member:`"variable-length" built-in type <PyTypeObject.tp_itemsize>`,
+  unless the C type is defined with :c:macro:`Py_TPFLAGS_ITEMS_AT_END`.
+  For example, they cannot be defined on subclasses of
+  :class:`int`, :class:`bytes`, or :class:`tuple`.
 
 * Any non-string :term:`iterable` may be assigned to *__slots__*.
 
@@ -2658,6 +2660,9 @@ Notes on using *__slots__*:
 
 .. versionchanged:: next
    Allowed defining the *__dict__* and *__weakref__* *__slots__* for any class.
+   Allowed defining any *__slots__* for a class derived from :class:`type` or
+   other :c:member:`"variable-length" built-in type <PyTypeObject.tp_itemsize>`
+   with the :c:macro:`Py_TPFLAGS_ITEMS_AT_END` flag.
 
 
 .. _class-customization:

Doc/whatsnew/3.15.rst

@@ -399,6 +399,12 @@ Other language changes
   (Contributed by Serhiy Storchaka in :gh:`41779`.)
 
 
+* Allowed defining any *__slots__* for a class derived from :class:`type` or
+  other :c:member:`"variable-length" built-in type <PyTypeObject.tp_itemsize>`
+  with the :c:macro:`Py_TPFLAGS_ITEMS_AT_END` flag.
+  (Contributed by Serhiy Storchaka in :gh:`103740`.)
+
+
 New modules
 ===========
 

Lib/test/test_descr.py

@@ -1320,6 +1320,32 @@ class X(object):
         with self.assertRaisesRegex(AttributeError, "'X' object has no attribute 'a'"):
             X().a
 
+    def test_slots_before_items(self):
+        class C(type):
+            __slots__ = ['a']
+        x = C('A', (int,), {})
+        self.assertNotHasAttr(x, "a")
+        x.a = 1
+        x.b = 2
+        self.assertEqual(x.a, 1)
+        self.assertEqual(x.b, 2)
+        self.assertNotIn('a', x.__dict__)
+        self.assertIn('b', x.__dict__)
+        del x.a
+        self.assertNotHasAttr(x, "a")
+
+    @unittest.skipIf(_testcapi is None, 'need the _testcapi module')
+    def test_slots_before_items2(self):
+        class D(_testcapi.HeapCCollection):
+            __slots__ = ['a']
+        x = D(1, 2, 3)
+        self.assertNotHasAttr(x, "a")
+        x.a = 42
+        self.assertEqual(x.a, 42)
+        del x.a
+        self.assertNotHasAttr(x, "a")
+        self.assertEqual(list(x), [1, 2, 3])
+
     def test_slots_special(self):
         # Testing __dict__ and __weakref__ in __slots__...
         class D(object):
@@ -1384,14 +1410,6 @@ class W(_testcapi.HeapCCollection):
             a.foo = 42
         self.assertIs(weakref.ref(a)(), a)
 
-        with self.assertRaises(TypeError):
-            class X(_testcapi.HeapCCollection):
-                __slots__ = ['x']
-
-        with self.assertRaises(TypeError):
-            class X(_testcapi.HeapCCollection):
-                __slots__ = ['__dict__', 'x']
-
     @support.subTests(('base', 'arg'), [
         (tuple, (1, 2, 3)),
         (int, 9876543210**2),

Misc/NEWS.d/next/Core_and_Builtins/2025-11-16-21-14-48.gh-issue-103740.rXIj5h.rst

@@ -0,0 +1,5 @@
+Allowed defining the *__dict__* and *__weakref__* :ref:`__slots__ <slots>`
+for any class. Allowed defining any *__slots__* for a class derived from
+:class:`type` or other :c:member:`"variable-length" built-in type
+<PyTypeObject.tp_itemsize>` with the :c:macro:`Py_TPFLAGS_ITEMS_AT_END`
+flag.

Objects/typeobject.c

@@ -4641,7 +4641,9 @@ type_new_descriptors(const type_new_ctx *ctx, PyTypeObject *type, PyObject *dict
     if (et->ht_slots != NULL) {
         PyMemberDef *mp = _PyHeapType_GET_MEMBERS(et);
         Py_ssize_t nslot = PyTuple_GET_SIZE(et->ht_slots);
-        if (ctx->base->tp_itemsize != 0) {
+        int after_items = (ctx->base->tp_itemsize != 0 &&
+                           !(ctx->base->tp_flags & Py_TPFLAGS_ITEMS_AT_END));
+        if (after_items) {
             PyErr_Format(PyExc_TypeError,
                          "arbitrary __slots__ not supported for subtype of '%s'",
                          ctx->base->tp_name);