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

On Thu, 17 May 2012 08:10:40 -0700
Ethan Furman <ethan@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()

>>>

>>> print w()

None

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