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() 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.
