Python in a Nutshell

Name

WeakValueDictionary

Synopsis

class WeakValueDictionary(adict={  })

A WeakValueDictionary d is a mapping that references its values weakly. When the reference count of a value v in d goes to 0, all items of d such that d [ k ] is v disappear. adict is used to initialize the mapping.

WeakKeyDictionary and WeakValueDictionary are useful when you need to non-invasively associate additional data with objects without changing the objects. Weak mappings are also useful to non-invasively record transient associations between objects and to build caches. In each case, the specific consideration that can make a weak mapping preferable to a normal dictionary is that an object that is otherwise garbage-collectable is not kept alive just by being used in a weak mapping.

A typical use could be a class that keeps track of its instances, but does not keep them alive just in order to keep track of them:

import weakref
class Tracking:
    _instances_dict = weakref.WeakValueDictionary( )
    _num_generated = 0
    def __init__(self):
        Tracking._num_generated += 1
        Tracking._instances_dict[Tracking._num_generated] = self
    def instances( ): return _instances_dict.values( )
    instances = staticmethod(instances)