weakref demo

weakref.py

import weakref

class ExpensiveObject(object):
    def __init__(self, name):
        self.name = name
    def __del__(self):
        print '(Deleting %s)' % self

obj = ExpensiveObject('Cake')
r = weakref.ref(obj)
p = weakref.proxy(obj)

print 'via obj', obj.name # Cake
print 'via ref', r().name # Cake
print 'via proxy', p.name # Cake
del obj
print 'ref', r() # None
print 'proxy:', p.name # raised