[Python-Dev] Weak references: dereference notification

Thu Nov 10 08:15:00 CET 2005

Gustavo J. A. M. Carneiro wrote:
> OK, but what if it is a subclass of a builtin type, with instance
> variables? What if the PyObject is GC'ed but the ObjC object remains
> alive, and later you get a new reference to it? Do you create a new
> PyObject wrapper for it? What happened to the instance variables?

Normally, wrappers don't have state. But if you do have state, this
is how it could work:
1. Make two Python objects, PyState and PyWrapper (actually,
 PyState doesn't need to be a Python object)
 PyState holds the instance variables, and PyWrapper just
 holds a pointer to a GObject.
2. When a Python reference to a GObject is created for the
 first time, create both a PyState and a PyWrapper. Have
 the GObject point to the PyState, and the PyWrapper to
 the GObject. Have the PyState weakly reference the
 PyWrapper.
3. When the refcount to the PyWrapper drops to zero, discard it.
4. When somebody asks for the data in the PyWrapper,
 go to the GObject, then to the PyState, and return the
 data from there.
5. When somebody wants a reference to a GObject which already
 has a PyState, check the weak reference to find out
 whether there is a PyWrapper already. If yes, return it;
 if not, create a new one (and weakly reference it).
6. When the GObject is discarded, drop the PyState as well.
This has the following properties:
1. There are no cyclic references for wrapping GObjects.
2. Weakly-referencing wrappers is supported; if there
 are no strong Python references to the wrapper,
 the wrapper goes away, and, potentially, the GObject
 as well.
3. The PyState object lives as long as the GObject.
4. Using "is" for GObjects/PyWrappers "works": there is
 at most one PyWrapper per GObject at any time.
5. id() of a GObject may change over time, if the wrapper
 becomes unreferenced and then recreated.
Regards,
Martin