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

Antoine Pitrou solipsis at pitrou.net
Sat May 18 15:47:51 CEST 2013

On Sat, 18 May 2013 06:37:54 -0700 Eli Bendersky <eliben at gmail.com> wrote:

Great PEP, I would really like to see this happen as it defines much saner semantics for finalization than what we currently have. One small question below:

This PEP proposes to turn CI disposal into the following sequence (new > steps are in bold): > > 1. Weakrefs to CI objects are cleared, and their callbacks called. At > this point, the objects are still safe to use. > > 2. The finalizers of all CI objects are called. > > 3. **The CI is traversed again to determine if it is still isolated. > If it is determined that at least one object in CI is now reachable > from outside the CI, this collection is aborted and the whole CI > is resurrected. Otherwise, proceed.** > Not sure if my question is the same as Armin's here, but worth a try: by saying "the CI is traversed again" do you mean the original objects from the CI as discovered earlier, or is a new scan being done? What about a new object entering the CI during step (2)? I.e. the original CI was A->B->A but now one of the finalizers created some C such that B->C and C->A adding it to the connected component?

It is the original CI which is traversed. If a new reference is introduced into the reference chain, the traversal in step 3 will decide to resurrect the CI. This is not necessarily a problem, since the next GC collection will try collecting again.

Reading your description in (3) strictly it says: in this case the collection is aborted. This CI will be disposed next time collection is run. Is this correct?

Yup.

Regards

Antoine.

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