##
3.3.1 Weak Reference Objects

Weak reference objects have no attributes or methods, but do allow the referent to be obtained, if it still exists, by calling it:

    >>> import weakref
    >>> class Object:
    ...     pass
    ...
    >>> o = Object()
    >>> r = weakref.ref(o)
    >>> o2 = r()
    >>> o is o2
    True

If the referent no longer exists, calling the reference object returns `None`:

    >>> del o, o2
    >>> print r()
    None

Testing that a weak reference object is still live should be done using the expression `ref() is not None`. Normally, application code that needs to use a reference object should follow this pattern:

    # r is a weak reference object
    o = r()
    if o is None:
        # referent has been garbage collected
        print "Object has been allocated; can't frobnicate."
    else:
        print "Object is still live!"
        o.do_something_useful()

Using a separate test for ``liveness'' creates race conditions in threaded applications; another thread can cause a weak reference to become invalidated before the weak reference is called; the idiom shown above is safe in threaded applications as well as single-threaded applications.

Specialized versions of `ref` objects can be created through subclassing. This is used in the implementation of the `WeakValueDictionary` to reduce the memory overhead for each entry in the mapping. This may be most useful to associate additional information with a reference, but could also be used to insert additional processing on calls to retrieve the referent.

This example shows how a subclass of `ref` can be used to store additional information about an object and affect the value that's returned when the referent is accessed:

    import weakref

    class ExtendedRef(weakref.ref):
        def __new__(cls, ob, callback=None, **annotations):
            weakref.ref.__new__(cls, ob, callback)
            self.__counter = 0

        def __init__(self, ob, callback=None, **annotations):
            super(ExtendedRef, self).__init__(ob, callback)
            for k, v in annotations:
                setattr(self, k, v)

        def __call__(self):
            """Return a pair containing the referent and the number of
            times the reference has been called.
            """
            ob = super(ExtendedRef, self)()
            if ob is not None:
                self.__counter += 1
                ob = (ob, self.__counter)
            return ob