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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Allow __slots__ on classes with Py_TPFLAGS_ITEMS_AT_END","articleBody":"# Bug report\r\n\r\nIf 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:\r\n`TypeError: nonempty __slots__ not supported for subtype of '...'`\r\n\r\nAssume that we have a `PyVarObject` `foo.FooBase` class (implementation below). Trying to inherit from the class and requesting weakref support doesn't work.\r\n\r\n```py\r\nimport foo\r\nclass WithWeakrefAndDict(foo.FooBase):\r\n    __slots__ = ('__weakref__', '__dict__')\r\n```\r\n\r\n\u003cdetails\u003e\r\n\u003csummary\u003e`foo` module implementation\u003c/summary\u003e\r\n\r\n```c\r\n#define PY_SSIZE_T_CLEAN\r\n#include \u003cPython.h\u003e\r\n\r\nstatic const Py_ssize_t N_EXTRA = 4;\r\n\r\ntypedef struct {\r\n    PyObject_VAR_HEAD\r\n} FooBase;\r\n\r\nstatic PyObject **\r\nFooBase_get_storage(PyObject *self)\r\n{\r\n    char *addr = (char *)self;\r\n    return (PyObject **)(addr + Py_TYPE(self)-\u003etp_basicsize);\r\n}\r\n\r\nstatic int\r\nFooBase_traverse(PyObject *self, visitproc visit, void *arg)\r\n{\r\n    PyObject **storage = FooBase_get_storage(self);\r\n    for (int i = 0; i \u003c N_EXTRA; i++) {\r\n        Py_VISIT(storage[i]);\r\n    }\r\n    Py_VISIT(Py_TYPE(self));\r\n    return 0;\r\n}\r\n\r\nstatic int\r\nFooBase_clear(PyObject *self)\r\n{\r\n    PyObject **storage = FooBase_get_storage(self);\r\n    for (int i = 0; i \u003c N_EXTRA; i++) {\r\n        Py_CLEAR(storage[i]);\r\n    }\r\n    return 0;\r\n}\r\n\r\nstatic void\r\nFooBase_dealloc(PyObject *self)\r\n{\r\n    PyTypeObject *tp = Py_TYPE(self);\r\n    PyObject_GC_UnTrack(self);\r\n    FooBase_clear(self);\r\n    PyObject_GC_Del(self);\r\n    if (tp-\u003etp_flags \u0026 Py_TPFLAGS_HEAPTYPE)\r\n        Py_DECREF(tp);\r\n}\r\n\r\nstatic PyObject *\r\nFooBase_new(PyTypeObject *type, PyObject *args, PyObject *kwargs)\r\n{\r\n    PyVarObject *obj = PyObject_GC_NewVar(PyVarObject, type, N_EXTRA);\r\n    if (obj == NULL)\r\n        return NULL;\r\n    PyObject **storage = FooBase_get_storage((PyObject *)obj);\r\n    for (int i = 0; i \u003c Py_SIZE(obj); i++)\r\n        storage[i] = NULL;\r\n\r\n    PyObject_GC_Track(obj);\r\n    return (PyObject *)obj;\r\n}\r\n\r\nstatic PyObject *\r\nFooBase_get_extra(PyObject *self, PyObject *args)\r\n{\r\n    Py_ssize_t idx;\r\n    if (!PyArg_ParseTuple(args, \"n\", \u0026idx))\r\n        return NULL;\r\n    if (idx \u003c 0 || idx \u003e= N_EXTRA) {\r\n        PyErr_Format(PyExc_ValueError, \"idx must be \u003e= 0 and \u003c %zd\", N_EXTRA);\r\n        return NULL;\r\n    }\r\n\r\n    PyObject **storage = FooBase_get_storage(self);\r\n    PyObject *value = storage[idx];\r\n    if (!value) {\r\n        Py_RETURN_NONE;\r\n    } else {\r\n        Py_INCREF(value);\r\n        return value;\r\n    }\r\n}\r\n\r\nstatic PyObject *\r\nFooBase_set_extra(PyObject *self, PyObject *args)\r\n{\r\n    Py_ssize_t idx;\r\n    PyObject *value;\r\n    if (!PyArg_ParseTuple(args, \"nO\", \u0026idx, \u0026value))\r\n        return NULL;\r\n    if (idx \u003c 0 || idx \u003e= N_EXTRA) {\r\n        PyErr_Format(PyExc_ValueError, \"idx must be \u003e= 0 and \u003c %zd\", N_EXTRA);\r\n        return NULL;\r\n    }\r\n\r\n    PyObject **storage = FooBase_get_storage(self);\r\n    Py_CLEAR(storage[idx]);\r\n    Py_INCREF(value);\r\n    storage[idx] = value;\r\n\r\n    Py_RETURN_NONE;\r\n}\r\n\r\nstatic PyMethodDef FooBase_methods[] = {\r\n    {\"get_extra\", FooBase_get_extra, METH_VARARGS, NULL},\r\n    {\"set_extra\", FooBase_set_extra, METH_VARARGS, NULL},\r\n    {NULL}\r\n};\r\n\r\nstatic PyTypeObject FooBase_Type = {\r\n    PyVarObject_HEAD_INIT(NULL, 0)\r\n    \"foo.FooBase\",\r\n    sizeof(FooBase),\r\n    .tp_dealloc = (destructor)FooBase_dealloc,\r\n    .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE | Py_TPFLAGS_HAVE_GC,\r\n    .tp_traverse = FooBase_traverse,\r\n    .tp_clear = FooBase_clear,\r\n    .tp_methods = FooBase_methods,\r\n    .tp_new = FooBase_new,\r\n    .tp_itemsize = sizeof(PyObject *)\r\n};\r\n\r\nstatic struct PyModuleDef foo_module = {\r\n    PyModuleDef_HEAD_INIT,\r\n    \"foo\",\r\n    NULL,\r\n    -1,\r\n};\r\n\r\nPyMODINIT_FUNC\r\nPyInit_foo(void)\r\n{\r\n    PyObject *m = PyModule_Create(\u0026foo_module);\r\n    if (!m)\r\n        return NULL;\r\n    if (PyType_Ready(\u0026FooBase_Type) \u003c 0)\r\n        return NULL;\r\n    Py_INCREF(\u0026FooBase_Type);\r\n    PyModule_AddObject(m, \"FooBase\", (PyObject *)\u0026FooBase_Type);\r\n    return m;\r\n}\r\n```\r\n\u003c/details\u003e\r\n\r\n# Your environment\r\n\r\n\u003c!-- Include as many relevant details as possible about the environment you experienced the bug in --\u003e\r\n\r\n- CPython versions tested on: 3.11.3\r\n- Operating system and architecture: Linux, x86_64\r\n\r\nCC: @encukou \r\n\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-141636\n* gh-141764\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/jbradaric","@type":"Person","name":"jbradaric"},"datePublished":"2023-04-24T07:06:25.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":10},"url":"https://github.com/103740/cpython/issues/103740"}