gh-41779: Allow defining any __slots__ for a class derived from tuple

Doc/reference/datamodel.rst

@@ -2633,7 +2633,7 @@ 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`.
+  :class:`int`, :class:`bytes`, and :class:`type`, except :class:`tuple`.
 
 * Any non-string :term:`iterable` may be assigned to *__slots__*.
 
@@ -2658,6 +2658,7 @@ 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:`tuple`.
 
 
 .. _class-customization:

Doc/whatsnew/3.15.rst

@@ -398,6 +398,10 @@ Other language changes
   for any class.
   (Contributed by Serhiy Storchaka in :gh:`41779`.)
 
+* Allowed defining any :ref:`__slots__ <slots>` for a class derived from
+  :class:`tuple` (including classes created by :func:`collections.namedtuple`).
+  (Contributed by Serhiy Storchaka in :gh:`41779`.)
+
 
 New modules
 ===========

Include/descrobject.h

@@ -84,6 +84,7 @@ struct PyMemberDef {
 #define Py_AUDIT_READ          2 // Added in 3.10, harmless no-op before that
 #define _Py_WRITE_RESTRICTED   4 // Deprecated, no-op. Do not reuse the value.
 #define Py_RELATIVE_OFFSET     8
+#define _Py_AFTER_ITEMS        16
 
 PyAPI_FUNC(PyObject *) PyMember_GetOne(const char *, PyMemberDef *);
 PyAPI_FUNC(int) PyMember_SetOne(char *, PyMemberDef *, PyObject *);

Lib/test/test_descr.py

@@ -1320,6 +1320,18 @@ class X(object):
         with self.assertRaisesRegex(AttributeError, "'X' object has no attribute 'a'"):
             X().a
 
+    def test_slots_after_items(self):
+        class C(tuple):
+            __slots__ = ['a']
+        x = C((1, 2, 3))
+        self.assertNotHasAttr(x, "__dict__")
+        self.assertNotHasAttr(x, "a")
+        x.a = 42
+        self.assertEqual(x.a, 42)
+        del x.a
+        self.assertNotHasAttr(x, "a")
+        self.assertEqual(x, (1, 2, 3))
+
     def test_slots_special(self):
         # Testing __dict__ and __weakref__ in __slots__...
         class D(object):
@@ -1422,6 +1434,9 @@ class W(base):
         self.assertIs(weakref.ref(a)(), a)
         self.assertEqual(a, base(arg))
 
+    @support.subTests('base', [int, bytes] +
+                      ([_testcapi.HeapCCollection] if _testcapi else []))
+    def test_unsupported_slots(self, base):
         with self.assertRaises(TypeError):
             class X(base):
                 __slots__ = ['x']

Misc/NEWS.d/next/Core_and_Builtins/2025-11-19-20-42-21.gh-issue-41779.Psz9Vo.rst

@@ -0,0 +1,3 @@
+Allowed defining any :ref:`__slots__ <slots>` for a class derived from
+:class:`tuple` (including classes created by
+:func:`collections.namedtuple`).

Objects/typeobject.c

@@ -2568,6 +2568,8 @@ _PyHeapType_GET_MEMBERS(PyHeapTypeObject* type)
     return PyObject_GetItemData((PyObject *)type);
 }
 
+void *_PyMember_GetOffset(PyObject *obj, PyMemberDef *l);
+
 static int
 traverse_slots(PyTypeObject *type, PyObject *self, visitproc visit, void *arg)
 {
@@ -2578,7 +2580,7 @@ traverse_slots(PyTypeObject *type, PyObject *self, visitproc visit, void *arg)
     mp = _PyHeapType_GET_MEMBERS((PyHeapTypeObject *)type);
     for (i = 0; i < n; i++, mp++) {
         if (mp->type == Py_T_OBJECT_EX) {
-            char *addr = (char *)self + mp->offset;
+            void *addr = _PyMember_GetOffset(self, mp);
             PyObject *obj = *(PyObject **)addr;
             if (obj != NULL) {
                 int err = visit(obj, arg);
@@ -2653,7 +2655,7 @@ clear_slots(PyTypeObject *type, PyObject *self)
     mp = _PyHeapType_GET_MEMBERS((PyHeapTypeObject *)type);
     for (i = 0; i < n; i++, mp++) {
         if (mp->type == Py_T_OBJECT_EX && !(mp->flags & Py_READONLY)) {
-            char *addr = (char *)self + mp->offset;
+            void *addr = _PyMember_GetOffset(self, mp);
             PyObject *obj = *(PyObject **)addr;
             if (obj != NULL) {
                 *(PyObject **)addr = NULL;
@@ -4641,7 +4643,11 @@ 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 (ctx->base->tp_itemsize != 0 &&
+            !(ctx->base->tp_flags & Py_TPFLAGS_TUPLE_SUBCLASS))
+        {
             PyErr_Format(PyExc_TypeError,
                          "arbitrary __slots__ not supported for subtype of '%s'",
                          ctx->base->tp_name);
@@ -4655,6 +4661,9 @@ type_new_descriptors(const type_new_ctx *ctx, PyTypeObject *type, PyObject *dict
             }
             mp->type = Py_T_OBJECT_EX;
             mp->offset = slotoffset;
+            if (after_items) {
+                mp->flags |= _Py_AFTER_ITEMS;
+            }
 
             /* __dict__ and __weakref__ are already filtered out */
             assert(strcmp(mp->name, "__dict__") != 0);

Python/specialize.c

@@ -139,6 +139,7 @@ _PyCode_Quicken(_Py_CODEUNIT *instructions, Py_ssize_t size, int enable_counters
 #define SPEC_FAIL_ATTR_METACLASS_OVERRIDDEN 34
 #define SPEC_FAIL_ATTR_SPLIT_DICT 35
 #define SPEC_FAIL_ATTR_DESCR_NOT_DEFERRED 36
+#define SPEC_FAIL_ATTR_SLOT_AFTER_ITEMS 37
 
 /* Binary subscr and store subscr */
 
@@ -810,6 +811,10 @@ do_specialize_instance_load_attr(PyObject* owner, _Py_CODEUNIT* instr, PyObject*
                 SPECIALIZATION_FAIL(LOAD_ATTR, SPEC_FAIL_EXPECTED_ERROR);
                 return -1;
             }
+            if (dmem->flags & _Py_AFTER_ITEMS) {
+                SPECIALIZATION_FAIL(LOAD_ATTR, SPEC_FAIL_ATTR_SLOT_AFTER_ITEMS);
+                return -1;
+            }
             if (dmem->flags & Py_AUDIT_READ) {
                 SPECIALIZATION_FAIL(LOAD_ATTR, SPEC_FAIL_ATTR_AUDITED_SLOT);
                 return -1;
@@ -1004,6 +1009,10 @@ _Py_Specialize_StoreAttr(_PyStackRef owner_st, _Py_CODEUNIT *instr, PyObject *na
                 SPECIALIZATION_FAIL(STORE_ATTR, SPEC_FAIL_EXPECTED_ERROR);
                 goto fail;
             }
+            if (dmem->flags & _Py_AFTER_ITEMS) {
+                SPECIALIZATION_FAIL(LOAD_ATTR, SPEC_FAIL_ATTR_SLOT_AFTER_ITEMS);
+                goto fail;
+            }
             if (dmem->flags & Py_READONLY) {
                 SPECIALIZATION_FAIL(STORE_ATTR, SPEC_FAIL_ATTR_READ_ONLY);
                 goto fail;

Python/structmember.c

@@ -20,6 +20,17 @@ member_get_object(const char *addr, const char *obj_addr, PyMemberDef *l)
     return v;
 }
 
+void *
+_PyMember_GetOffset(PyObject *obj, PyMemberDef *mp)
+{
+    unsigned char *addr = (unsigned char *)obj + mp->offset;
+    if (mp->flags & _Py_AFTER_ITEMS) {
+        PyTypeObject *type = Py_TYPE(obj);
+        addr += _Py_SIZE_ROUND_UP(Py_SIZE(obj) * type->tp_itemsize, SIZEOF_VOID_P);
+    }
+    return addr;
+}
+
 PyObject *
 PyMember_GetOne(const char *obj_addr, PyMemberDef *l)
 {
@@ -31,7 +42,7 @@ PyMember_GetOne(const char *obj_addr, PyMemberDef *l)
         return NULL;
     }
 
-    const char* addr = obj_addr + l->offset;
+    const void *addr = _PyMember_GetOffset((PyObject *)obj_addr, l);
     switch (l->type) {
     case Py_T_BOOL:
         v = PyBool_FromLong(FT_ATOMIC_LOAD_CHAR_RELAXED(*(char*)addr));
@@ -80,7 +91,7 @@ PyMember_GetOne(const char *obj_addr, PyMemberDef *l)
         v = PyUnicode_FromString((char*)addr);
         break;
     case Py_T_CHAR: {
-        char char_val = FT_ATOMIC_LOAD_CHAR_RELAXED(*addr);
+        char char_val = FT_ATOMIC_LOAD_CHAR_RELAXED(*(char*)addr);
         v = PyUnicode_FromStringAndSize(&char_val, 1);
         break;
     }
@@ -151,10 +162,8 @@ PyMember_SetOne(char *addr, PyMemberDef *l, PyObject *v)
         return -1;
     }
 
-#ifdef Py_GIL_DISABLED
-    PyObject *obj = (PyObject *) addr;
-#endif
-    addr += l->offset;
+    PyObject *obj = (PyObject *)addr;
+    addr = _PyMember_GetOffset(obj, l);
 
     if ((l->flags & Py_READONLY))
     {