2.2.6 Weak Reference Support

##
2.2.6 Weak Reference Support

One of the goals of Python's weak-reference implementation is to allow any type to participate in the weak reference mechanism without incurring the overhead on those objects which do not benefit by weak referencing (such as numbers).

For an object to be weakly referencable, the extension must include a `PyObject*` field in the instance structure for the use of the weak reference mechanism; it must be initialized to `NULL` by the object's constructor. It must also set the `tp_weaklistoffset` field of the corresponding type object to the offset of the field. For example, the instance type is defined with the following structure:

    typedef struct {
        PyObject_HEAD
        PyClassObject *in_class;       /* The class object */
        PyObject      *in_dict;        /* A dictionary */
        PyObject      *in_weakreflist; /* List of weak references */
    } PyInstanceObject;

The statically-declared type object for instances is defined this way:

    PyTypeObject PyInstance_Type = {
        PyObject_HEAD_INIT(&PyType_Type)
        0,
        "module.instance",

        /* Lots of stuff omitted for brevity... */

        Py_TPFLAGS_DEFAULT,                         /* tp_flags */
        0,                                          /* tp_doc */
        0,                                          /* tp_traverse */
        0,                                          /* tp_clear */
        0,                                          /* tp_richcompare */
        offsetof(PyInstanceObject, in_weakreflist), /* tp_weaklistoffset */
    };

The type constructor is responsible for initializing the weak reference list to `NULL`:

    static PyObject *
    instance_new() {
        /* Other initialization stuff omitted for brevity */

        self->in_weakreflist = NULL;

        return (PyObject *) self;
    }

The only further addition is that the destructor needs to call the weak reference manager to clear any weak references. This should be done before any other parts of the destruction have occurred, but is only required if the weak reference list is non-`NULL`:

    static void
    instance_dealloc(PyInstanceObject *inst)
    {
        /* Allocate temporaries if needed, but do not begin
           destruction just yet.
         */

        if (inst->in_weakreflist != NULL)
            PyObject_ClearWeakRefs((PyObject *) inst);

        /* Proceed with object destruction normally. */
    }