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

Nick Coghlan ncoghlan at gmail.com
Sat Jun 15 06:57:49 CEST 2013

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

If it isn't easy to fix by changing the way such classes are constructed by typeobject.c (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), I'd say chalk it up as a CPython wart and not worry about it for now.

As far as the test_subtype_resurrection failure goes, I think the associated comment says it all: "(On non-CPython implementations, however, del is typically not called again.)"

The test should just be rewritten to check that del is only called once in the face of resurrection.

Cheers, Nick.

-- Nick Coghlan | ncoghlan at gmail.com | Brisbane, Australia

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