[Python-Dev] PEP 442: Safe object finalization

[Antoine Pitrou]

On Sat, 18 May 2013 14:56:38 +0100
Richard Oudkerk <shibturn at gmail.com> wrote:
> On 18/05/2013 9:59am, Antoine Pitrou wrote:
> > This PEP proposes to turn CI disposal into the following sequence (new
> > steps are in bold):
> >
> > 1. Weakrefs to CI objects are cleared, and their callbacks called. At
> >     this point, the objects are still safe to use.
> >
> > 2. **The finalizers of all CI objects are called.**
> 
> How do you know that one of the finalizers will not do something which 
> causes another to fail?
> 
> Presumably the following would cause an AttributeError to be printed:
> 
>      class Node:
>          def __init__(self):
>              self.next = None
>          def __del__(self):
>              print(self, self.next)
>              del self.next                   # break Node object
> 
>      a = Node()
>      b = Node()
>      a.next = b
>      b.next = a
>      del a, b
>      gc.collect()

It works fine:

$ ./python sbt.py 
<__main__.Node object at 0x7f3acbf8f400> <__main__.Node object at 0x7f3acbf8f878>
<__main__.Node object at 0x7f3acbf8f878> <__main__.Node object at 0x7f3acbf8f400>

The reason is that, when you execute "del self.next", this removes the
last reference to self.next and destroys it immediately.

In essence, you were expecting to see:
- enter a.__del__, destroy b
- leave a.__del__
- enter b.__del__ oops?

But what happens is:
- enter a.__del__, destroy b
  - enter b.__del__
  - leave b.__del__
- leave a.__del__

Regards

Antoine.