[Python-Dev] PEP 442: Safe object finalization (original) (raw)

Antoine Pitrou solipsis at pitrou.net
Sat May 18 16:33:15 CEST 2013

On Sat, 18 May 2013 16:22:55 +0200 Armin Rigo <arigo at tunes.org> wrote:

Hi Antoine,

On Sat, May 18, 2013 at 3:45 PM, Antoine Pitrou <solipsis at pitrou.net> wrote: >> How is this done? I don't see a clear way to determine it by looking >> only at the objects in the CI, given that arbitrary modifications of >> the object graph may have occurred. > > The same way a generation is traversed, but restricted to the CI. > > First the gcrefs field of each CI object is initialized to its > obrefcnt (again). > > Then, tptraverse is called on each CI object, and each visited > CI object has its gcrefs decremented. This substracts CI-internal > references from the gcrefs fields. > > At the end of the traversal, if all CI objects have their gcrefs equal > to 0, then the CI has no external reference to it and can be cleared. > If at least one CI object has non-zero gcrefs, the CI cannot be > cleared. Ok, indeed. Then you really should call finalizers only once: in case one of the finalizers in a cycle did a trivial change like I described, the algorithm above will conservatively assume the cycle should be kept alive. At the next GC collection we must not call the finalizer again, because it's likely to just do a similar trivial change.

Well, the finalizer will only be called if the resurrected object is dereferenced again; otherwise the object won't be considered by the GC. So, this will only happen if someone keeps trying to destroy a resurrected object.

Calling finalizers only once is fine with me, but it would be a change in behaviour; I don't know if it may break existing code.

(for example, say someone is using del to manage a freelist)

Regards

Antoine.

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