#!/usr/bin/env python
     # encoding: utf-8
     #
     # Copyright (c) 2008 Doug Hellmann All rights reserved.
     #
     """Example using weakref.ref to manage a reference to an object
     with a callback.
     """
     #end_pymotw_header
     
     import weakref
     
     class ExpensiveObject(object):
         def __del__(self):
             print '(Deleting %s)' % self
             
     def callback(reference):
         """Invoked when referenced object is deleted"""
         print 'callback(', reference, ')'
     
     obj = ExpensiveObject()
     r = weakref.ref(obj, callback)
     
     print 'obj:', obj
     print 'ref:', r
     print 'r():', r()
     
     print 'deleting obj'
     del obj
     print 'r():', r()