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Extending and Embedding the Python Interpreter |
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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. */
}
Release 2.5.2, documentation updated on 21th February, 2008.
See About this document... for information on suggesting changes.
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