On Sat, 15 Jun 2013 14:57:49 +1000 Nick Coghlan <ncoghlan@gmail.com> wrote:
On 15 June 2013 03:34, Antoine Pitrou <solipsis@pitrou.net> wrote:
On Wed, 5 Jun 2013 09:10:54 -0700 Benjamin Peterson <benjamin@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.