[Python-3000] PEP: Eliminate del (original) (raw)

Jean-Paul Calderone exarkun at divmod.com
Sat May 5 18:27:08 CEST 2007

On Sat, 05 May 2007 13:21:59 +0200, Giovanni Bajo <rasky at develer.com> wrote:

On 04/05/2007 20.35, Adam Olsen wrote:

Any attempt that keeps the entire contents of dict alive is doomed. It's likely to contain a cycle back to the original object, and avoiding that is the whole point of jumping through these hoops. Uh? If dict contains a cycle back to the original object, then the object is part of a cycle already, with or without getting an additional reference to the dict within the finalization callback.

If the dict contains a cycle back to the original object, then if you keep the dict alive in the weakref callback (which is what you are doing if the weakref callback references the dict - it does not weakly reference it), then you will keep the original object alive and the weakref callback will never run, because the original object will live forever.

Contrariwise, if the weakref callback has only a reference to the particular objects which it needs, then it doesn't matter if there is a cycle through some other objects which are in the dict, since the weakref callback will not keep the cycle alive: eventually the cyclic gc will clean up the cycle (but leave the objects referenced by the weakref callback alone, since the weakref callback is keeping them alive), then the weakref callback will run since it was a weakref to the original object which has now been collected, the weakref callback will be able to use the specific references it has to do some cleanup, and then most likely both the weakref object, the weakref callback object, and whatever specific objects it held references to will be dropped and eventually collected (though this is not necessarily the case, since the weakref callback could choose to keep the specific objects it referenced (not the object weakly referenced in the first place) alive by putting them into some other living container).

Jean-Paul

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