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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.

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