Hi,
Sorry to reply in this old thread. We just noticed this on #pypy:
On 22 October 2016 at 05:32, Nick Coghlan wrote:
> The weakref-before-__del__ ordering change in
> https://www.python.org/dev/peps/pep-0442/#disposal-of-cyclic-isolates
> only applies to cyclic garbage collection,so for normal refcount
> driven object cleanup in CPython, the __del__ still happens first:
>
> >>> class C:
> ... def __del__(self):
> ... print("__del__ called")
> ...
> >>> c = C()
> >>> import weakref
> >>> def cb():
> ... print("weakref callback called")
> ...
> >>> weakref.finalize(c, cb)
>
> >>> del c
> __del__ called
> weakref callback called
Nick, it seems you're suggesting that before PEP 442 (in CPython 3.4)
the __del__ happened before the weakref-clearing operation as well.
That's not the case: before CPython 3.4, weakrefs are always cleared
first. The situation became more muddy in 3.4, where weakrefs are
usually cleared after the __del__ is called---in the absence of
reference cycles (so it's a backward-incompatible change). If there
are reference cycles, then the weakref is cleared before the __del__
is called.
This can be shown in your example by replacing "weakref.finalize(c,
cb)" with an old-style "wr = weakref.ref(c, cb)". Then CPython <= 3.3
and >= 3.4 print the two lines in opposite order.
A bientôt,
Armin.
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