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Objects/cellobject.c
/* Cell object implementation */

#include "Python.h"

PyObject *
PyCell_New(PyObject *obj)
{
    PyCellObject *op;

    op = (PyCellObject *)PyObject_GC_New(PyCellObject, &PyCell_Type);
    if (op == NULL)
        return NULL;
    op->ob_ref = obj;
    Py_XINCREF(obj);

    _PyObject_GC_TRACK(op);
    return (PyObject *)op;
}

PyObject *
PyCell_Get(PyObject *op)
{
    if (!PyCell_Check(op)) {
        PyErr_BadInternalCall();
        return NULL;
    }
    Py_XINCREF(((PyCellObject*)op)->ob_ref);
    return PyCell_GET(op);
}

int
PyCell_Set(PyObject *op, PyObject *obj)
{
    PyObject* oldobj;
    if (!PyCell_Check(op)) {
        PyErr_BadInternalCall();
        return -1;
    }
    oldobj = PyCell_GET(op);
    Py_XINCREF(obj);
    PyCell_SET(op, obj);
    Py_XDECREF(oldobj);
    return 0;
}

static void
cell_dealloc(PyCellObject *op)
{
    _PyObject_GC_UNTRACK(op);
    Py_XDECREF(op->ob_ref);
    PyObject_GC_Del(op);
}

static int
cell_compare(PyCellObject *a, PyCellObject *b)
{
    /* Py3K warning for comparisons  */
    if (PyErr_WarnPy3k("cell comparisons not supported in 3.x",
                       1) < 0) {
        return -2;
    }

    if (a->ob_ref == NULL) {
        if (b->ob_ref == NULL)
            return 0;
        return -1;
    } else if (b->ob_ref == NULL)
        return 1;
    return PyObject_Compare(a->ob_ref, b->ob_ref);
}

static PyObject *
cell_repr(PyCellObject *op)
{
    if (op->ob_ref == NULL)
        return PyString_FromFormat("<cell at %p: empty>", op);

    return PyString_FromFormat("<cell at %p: %.80s object at %p>",
                               op, op->ob_ref->ob_type->tp_name,
                               op->ob_ref);
}

static int
cell_traverse(PyCellObject *op, visitproc visit, void *arg)
{
    Py_VISIT(op->ob_ref);
    return 0;
}

static int
cell_clear(PyCellObject *op)
{
    Py_CLEAR(op->ob_ref);
    return 0;
}

static PyObject *
cell_get_contents(PyCellObject *op, void *closure)
{
    if (op->ob_ref == NULL)
    {
        PyErr_SetString(PyExc_ValueError, "Cell is empty");
        return NULL;
    }
    Py_INCREF(op->ob_ref);
    return op->ob_ref;
}

static PyGetSetDef cell_getsetlist[] = {
    {"cell_contents", (getter)cell_get_contents, NULL},
    {NULL} /* sentinel */
};

PyTypeObject PyCell_Type = {
    PyVarObject_HEAD_INIT(&PyType_Type, 0)
    "cell",
    sizeof(PyCellObject),
    0,
    (destructor)cell_dealloc,               /* tp_dealloc */
    0,                                      /* tp_print */
    0,                                          /* tp_getattr */
    0,                                          /* tp_setattr */
    (cmpfunc)cell_compare,                      /* tp_compare */
    (reprfunc)cell_repr,                        /* tp_repr */
    0,                                          /* tp_as_number */
    0,                                          /* tp_as_sequence */
    0,                                          /* tp_as_mapping */
    0,                                          /* tp_hash */
    0,                                          /* tp_call */
    0,                                          /* tp_str */
    PyObject_GenericGetAttr,                    /* tp_getattro */
    0,                                          /* tp_setattro */
    0,                                          /* tp_as_buffer */
    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,/* tp_flags */
    0,                                          /* tp_doc */
    (traverseproc)cell_traverse,                /* tp_traverse */
    (inquiry)cell_clear,                        /* tp_clear */
    0,                                          /* tp_richcompare */
    0,                                          /* tp_weaklistoffset */
    0,                                          /* tp_iter */
    0,                                          /* tp_iternext */
    0,                                          /* tp_methods */
    0,                                          /* tp_members */
    cell_getsetlist,                            /* tp_getset */
};