[Python-Dev] PEP 442 accepted (original) (raw)

Antoine Pitrou solipsis at pitrou.net
Sat Jun 15 12:55:57 CEST 2013

On Sat, 15 Jun 2013 14:57:49 +1000 Nick Coghlan <ncoghlan at gmail.com> wrote:

On 15 June 2013 03:34, Antoine Pitrou <solipsis at pitrou.net> wrote: > On Wed, 5 Jun 2013 09:10:54 -0700 > Benjamin Peterson <benjamin at python.org> wrote: >> I (and Guido) are accepting PEP 442 (Safe object finalization) on the >> condition that finalizers are only ever called once globally. > > Ok, so there's an issue with that condition: it can't be upholded on > non-GC objects. Creating a non-GC object is quite obscure and rare, > though, since it requires basically a class with no dict and an > empty slots: > > class C: > slots = () > survivors = [] > > def del(self): > self.survivors.append(self) > > > In this case, a C instance's del will be called every time > destruction is attempted, not only once. Is that a realistic problem?

So, to trigger that del() method a second time, such an object would have to be: 1. Defined in the first place (the use cases for stateless objects with destructors seem rare...) 2. Hanging off a reference cycle 3. Which then gets resurrected

They don't need to hang off a reference cycle. You can resurrect a non-cyclic object from del too.

But, yeah, stateless objects must be pretty rare, since by definition they cannot represent anything (except perhaps "nothing").

i.e. force them to be GC-aware when they define a del method, since they may still be hanging off the edge of a reference cycle, even if they can't form one themselves

Objects which hang off the edge of a reference cycle don't need to be GC-aware, since traversing them isn't needed to be break the cycle. I think "chalking it up as a CPython wart" is a reasonable solution, I just wanted to ask.

Regards

Antoine.

More information about the Python-Dev mailing list