[Python-Dev] Weak references: dereference notification
Gustavo J. A. M. Carneiro
gjc at inescporto.pt
Thu Nov 10 13:04:23 CET 2005
Qui, 2005-11-10 às 08:15 +0100, "Martin v. Löwis" escreveu:
> 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
> 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.
This was my first approach, actually, in patch 4.1 in . Only your
property 2 above drove me to try a different approach -- the weakrefs
may become invalid while the GObject may still be alive. That's a bit
"surprising". Of course, if I could override weakref.ref() for GObject
wrapper types, even that could be worked around... ;-)
Gustavo J. A. M. Carneiro
<gjc at inescporto.pt> <gustavo at users.sourceforge.net>
The universe is always one step beyond logic.
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