2.2.6 Supporting the Cycle Collector

This example shows only enough of the implementation of an extension type to show how the garbage collector support needs to be added. It shows the definition of the object structure, the tp_traverse, tp_clear and tp_dealloc implementations, the type structure, and a constructor -- the module initialization needed to export the constructor to Python is not shown as there are no special considerations there for the collector. To make this interesting, assume that the module exposes ways for the container field of the object to be modified. Note that since no checks are made on the type of the object used to initialize container, we have to assume that it may be a container.

#include "Python.h"

typedef struct {
    PyObject_HEAD
    PyObject *container;
} MyObject;

static int
my_traverse(MyObject *self, visitproc visit, void *arg)
{
    if (self->container != NULL)
        return visit(self->container, arg);
    else
        return 0;
}

static int
my_clear(MyObject *self)
{
    Py_XDECREF(self->container);
    self->container = NULL;

    return 0;
}

static void
my_dealloc(MyObject *self)
{
    PyObject_GC_UnTrack((PyObject *) self);
    Py_XDECREF(self->container);
    PyObject_GC_Del(self);
}

static PyTypeObject
MyObject_Type = {
    PyObject_HEAD_INIT(NULL)
    0,
    "MyObject",
    sizeof(MyObject),
    0,
    (destructor)my_dealloc,     /* tp_dealloc */
    0,                          /* tp_print */
    0,                          /* tp_getattr */
    0,                          /* tp_setattr */
    0,                          /* tp_compare */
    0,                          /* tp_repr */
    0,                          /* tp_as_number */
    0,                          /* tp_as_sequence */
    0,                          /* tp_as_mapping */
    0,                          /* tp_hash */
    0,                          /* tp_call */
    0,                          /* tp_str */
    0,                          /* tp_getattro */
    0,                          /* tp_setattro */
    0,                          /* tp_as_buffer */
    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,
    0,                          /* tp_doc */
    (traverseproc)my_traverse,  /* tp_traverse */
    (inquiry)my_clear,          /* tp_clear */
    0,                          /* tp_richcompare */
    0,                          /* tp_weaklistoffset */
};

/* This constructor should be made accessible from Python. */
static PyObject *
new_object(PyObject *unused, PyObject *args)
{
    PyObject *container = NULL;
    MyObject *result = NULL;

    if (PyArg_ParseTuple(args, "|O:new_object", &container)) {
        result = PyObject_GC_New(MyObject, &MyObject_Type);
        if (result != NULL) {
            result->container = container;
            PyObject_GC_Track(result);
        }
    }
    return (PyObject *) result;
}