##
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
      1

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.get() is not None`. Normally, application code that needs to use a reference object should follow this pattern:

      o = ref.get()
      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 `get()` method is called; the idiom shown above is safe in threaded applications as well as single-threaded applications.