[Python-3000] weakrefs and cyclic references

[tomer filiba]

dang, you posted before me :)

anyway, please check my implementation as well
http://sebulba.wikispaces.com/recipe+weakattr

i also included some demos.

anyway, i'd like to have this or the other weakattr implementation
included in weakref.py. it's a pretty useful feature to have in the stdlib,
for example:

from weakref import weakattr

class blah(object):
    someattr = weakattr()
    def __init__(self):
        self.someattr = self

just like properties.


-tomer

On 6/2/06, Nick Coghlan <ncoghlan at gmail.com> wrote:
> Greg Ewing wrote:
> > What might be useful is an easier way of *explicitly*
> > creating and using weak references.
> >
> > We already have WeakKeyDictionary and WeakValueDictionary
> > which behave just like ordinary dicts except that they
> > weakly reference things. I'm thinking it would be nice
> > to have a way of declaring any attribute to be a weak
> > reference. Then it could be read and written it in the
> > usual way, without all the code that uses it having
> > to know about its weakness.
> >
> > This could probably be done fairly easily with a suitable
> > property descriptor.
>
> Something like the following? (although you could do a simpler version without
> the callback support) (untested!)
>
> class WeakAttr(object):
>      """Descriptor to define weak instance attributes
>
>      name is the name of the attribute
>      callback is an optional callback function
>
>      If supplied, the callback function is called with the
>      instance and the attribute name as arguments after a currently
>      referenced object is finalized.
>      """
>      def __init__(self, name, callback=None):
>          self._name = name
>          self._callback = callback
>
>      def __get__(self, obj, cls):
>          if obj is None:
>              return self
>          attr_ref = getattr(obj, self._name)
>          if attr_ref is not None:
>              return attr_ref()
>          return None
>
>      def __set__(self, obj, value):
>          name = self._name
>          if value is None:
>              setattr(obj, name, None)
>          else:
>              cb = self._callback
>              if cb is not None:
>                  _cb = cb
>                  def cb(dead_ref):
>                      if dead_ref is getattr(obj, name):
>                          # Object that went away is still
>                          # the one referred to by the
>                          # attribute, so invoke the callback
>                          _cb(obj, name)
>              attr_ref = weakref.ref(value, cb)
>              setattr(obj, self._name, )
>
>      def __delete__(self, obj):
>          delattr(obj, self._name)
>
>
>
>
> --
> Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
> ---------------------------------------------------------------
>              http://www.boredomandlaziness.org
>