Allow __slots__ on classes with Py_TPFLAGS_ITEMS_AT_END

[jbradaric]

Bug report

If a base class is a PyVarObject, classes inheriting from that base cannot add __dict__ or __weakref__ through __slots__ declaration on the class. If __slots__ is not empty, the following exception is raised:
TypeError: nonempty __slots__ not supported for subtype of '...'

Assume that we have a PyVarObject foo.FooBase class (implementation below). Trying to inherit from the class and requesting weakref support doesn't work.

import foo
class WithWeakrefAndDict(foo.FooBase):
    __slots__ = ('__weakref__', '__dict__')

`foo` module implementation

#define PY_SSIZE_T_CLEAN
#include <Python.h>

static const Py_ssize_t N_EXTRA = 4;

typedef struct {
    PyObject_VAR_HEAD
} FooBase;

static PyObject **
FooBase_get_storage(PyObject *self)
{
    char *addr = (char *)self;
    return (PyObject **)(addr + Py_TYPE(self)->tp_basicsize);
}

static int
FooBase_traverse(PyObject *self, visitproc visit, void *arg)
{
    PyObject **storage = FooBase_get_storage(self);
    for (int i = 0; i < N_EXTRA; i++) {
        Py_VISIT(storage[i]);
    }
    Py_VISIT(Py_TYPE(self));
    return 0;
}

static int
FooBase_clear(PyObject *self)
{
    PyObject **storage = FooBase_get_storage(self);
    for (int i = 0; i < N_EXTRA; i++) {
        Py_CLEAR(storage[i]);
    }
    return 0;
}

static void
FooBase_dealloc(PyObject *self)
{
    PyTypeObject *tp = Py_TYPE(self);
    PyObject_GC_UnTrack(self);
    FooBase_clear(self);
    PyObject_GC_Del(self);
    if (tp->tp_flags & Py_TPFLAGS_HEAPTYPE)
        Py_DECREF(tp);
}

static PyObject *
FooBase_new(PyTypeObject *type, PyObject *args, PyObject *kwargs)
{
    PyVarObject *obj = PyObject_GC_NewVar(PyVarObject, type, N_EXTRA);
    if (obj == NULL)
        return NULL;
    PyObject **storage = FooBase_get_storage((PyObject *)obj);
    for (int i = 0; i < Py_SIZE(obj); i++)
        storage[i] = NULL;

    PyObject_GC_Track(obj);
    return (PyObject *)obj;
}

static PyObject *
FooBase_get_extra(PyObject *self, PyObject *args)
{
    Py_ssize_t idx;
    if (!PyArg_ParseTuple(args, "n", &idx))
        return NULL;
    if (idx < 0 || idx >= N_EXTRA) {
        PyErr_Format(PyExc_ValueError, "idx must be >= 0 and < %zd", N_EXTRA);
        return NULL;
    }

    PyObject **storage = FooBase_get_storage(self);
    PyObject *value = storage[idx];
    if (!value) {
        Py_RETURN_NONE;
    } else {
        Py_INCREF(value);
        return value;
    }
}

static PyObject *
FooBase_set_extra(PyObject *self, PyObject *args)
{
    Py_ssize_t idx;
    PyObject *value;
    if (!PyArg_ParseTuple(args, "nO", &idx, &value))
        return NULL;
    if (idx < 0 || idx >= N_EXTRA) {
        PyErr_Format(PyExc_ValueError, "idx must be >= 0 and < %zd", N_EXTRA);
        return NULL;
    }

    PyObject **storage = FooBase_get_storage(self);
    Py_CLEAR(storage[idx]);
    Py_INCREF(value);
    storage[idx] = value;

    Py_RETURN_NONE;
}

static PyMethodDef FooBase_methods[] = {
    {"get_extra", FooBase_get_extra, METH_VARARGS, NULL},
    {"set_extra", FooBase_set_extra, METH_VARARGS, NULL},
    {NULL}
};

static PyTypeObject FooBase_Type = {
    PyVarObject_HEAD_INIT(NULL, 0)
    "foo.FooBase",
    sizeof(FooBase),
    .tp_dealloc = (destructor)FooBase_dealloc,
    .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE | Py_TPFLAGS_HAVE_GC,
    .tp_traverse = FooBase_traverse,
    .tp_clear = FooBase_clear,
    .tp_methods = FooBase_methods,
    .tp_new = FooBase_new,
    .tp_itemsize = sizeof(PyObject *)
};

static struct PyModuleDef foo_module = {
    PyModuleDef_HEAD_INIT,
    "foo",
    NULL,
    -1,
};

PyMODINIT_FUNC
PyInit_foo(void)
{
    PyObject *m = PyModule_Create(&foo_module);
    if (!m)
        return NULL;
    if (PyType_Ready(&FooBase_Type) < 0)
        return NULL;
    Py_INCREF(&FooBase_Type);
    PyModule_AddObject(m, "FooBase", (PyObject *)&FooBase_Type);
    return m;
}

Your environment

• CPython versions tested on: 3.11.3
• Operating system and architecture: Linux, x86_64

CC: @encukou

[ronaldoussoren]

This is a duplicate of #61497. The isn't isn't so much that these particular slots aren't supported, but that no slots are supported at all var these objects.

[encukou]

Let's use this issue for enabling this with Py_TPFLAGS_ITEMS_AT_END specifically. I plan to look into it for 3.13.