[Python-Dev] PEP 442: Safe object finalization

[Antoine Pitrou]

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.