[Python-ideas] weakrefs

[Chris Kaynor]

On Thu, May 17, 2012 at 8:44 AM, Antoine Pitrou <solipsis at pitrou.net> wrote:

> On Thu, 17 May 2012 08:10:40 -0700
> Ethan Furman <ethan at stoneleaf.us> wrote:
> >  From the manual [8.11]:
> >
> >  > A weak reference to an object is not enough to keep the object alive:
> >  > when the only remaining references to a referent are weak references,
> >  > garbage collection is free to destroy the referent and reuse its
> >  > memory for something else.
> >
> > This leads to a difference in behaviour between CPython and the other
> > implementations:  CPython will (currently) immediately destroy any
> > objects that only have weak references to them with the result that
> > trying to access said object will require making a new one;
>
> This is only true if the object isn't caught in a reference cycle.


To further this, consider the following example, ran in CPython2.6:

>>> import weakref
>>> import gc
>>>
>>> class O(object):
...     pass
...
>>> a = O()
>>> b = O()
>>> a.x = b
>>> b.x = a
>>>
>>> w = weakref.ref(a)
>>>
>>>
>>> del a, b
>>>
>>> print w()
<__main__.O object at 0x0000000003C78B38>
>>>
>>> gc.collect()
20
>>>
>>> print w()
None


>
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