Garbage collection

[Nick Craig-Wood]

Hrvoje Niksic <hniksic at xemacs.org> wrote:
>  Simon Pickles <sipickles at googlemail.com> writes:
> 
> > Ken wrote:
> >> What is your __del__ method doing?
> >>   
> > Actually, nothing but printing a message when the object is deleted,
> > just morbid curiosity.
> >
> > I've yet to see one of the destructor messages, tho....
> 
>  Do your objects participate in reference cycles?  In that case they
>  are deallocated by the cycle collector, and the cycle collector
>  doesn't invoke __del__.
> 
> >>> class X(object):
>  ...   def __del__(self): print "gone"
>  ...
> >>> a = X()
> >>> a = 1
>  gone
> >>> b = X()
> >>> b.someslot = b
> >>> b = 1
> >>> import gc
> >>> gc.collect()
>  0
> >>> 

If you want to avoid this particular problem, use a weakref.

  >>> c = X()
  >>> from weakref import proxy
  >>> c.weak_reference = proxy(c)
  >>> c.weak_reference.__del__
  <bound method X.__del__ of <__main__.X object at 0xb7d1e56c>>
  >>> c = 1
  >>> gc.collect()
  gone
  0
  >>>  

Or perhaps slightly more realistically, here is an example of using a
WeakKeyDictionary instead of __del__ methods for keeping an accurate
track of all classes of a given type.

from weakref import WeakKeyDictionary

class Y(object):
    _registry = WeakKeyDictionary()
    def __init__(self):
        self._registry[self] = True
    @classmethod
    def list(cls):
        return cls._registry.keys()

a = Y()
b = Y()
c = Y()
Y.list()
a = 1
c = 1
Y.list()

Which produces the output

[<__main__.Y object at 0xb7d9fc8c>, <__main__.Y object at 0xb7d9fcac>, <__main__.Y object at 0xb7d9fc2c>]
[<__main__.Y object at 0xb7d9fc8c>]

(It behaves slightly differently in the interactive interpreter for
reasons I don't understand - so save it to a file and try it!)

In fact I find most of the times I wanted __del__ can be fixed by
using a weakref.WeakValueDictionary or weakref.WeakKeyDictionary for a
much better result.

-- 
Nick Craig-Wood <nick at craig-wood.com> -- http://www.craig-wood.com/nick