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<title>3.3.1 Weak Reference Objects </title>
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<H2><A NAME="SECTION005310000000000000000"></A><A NAME="weakref-objects"></A>
<BR>
3.3.1 Weak Reference Objects
</H2>
<P>
Weak reference objects have no attributes or methods, but do allow the
referent to be obtained, if it still exists, by calling it:
<P>
<div class="verbatim"><pre>
>>> import weakref
>>> class Object:
... pass
...
>>> o = Object()
>>> r = weakref.ref(o)
>>> o2 = r()
>>> o is o2
True
</pre></div>
<P>
If the referent no longer exists, calling the reference object returns
<tt class="constant">None</tt>:
<P>
<div class="verbatim"><pre>
>>> del o, o2
>>> print r()
None
</pre></div>
<P>
Testing that a weak reference object is still live should be done
using the expression <code><var>ref</var>() is not None</code>. Normally,
application code that needs to use a reference object should follow
this pattern:
<P>
<div class="verbatim"><pre>
# r is a weak reference object
o = r()
if o is None:
# referent has been garbage collected
print "Object has been deallocated; can't frobnicate."
else:
print "Object is still live!"
o.do_something_useful()
</pre></div>
<P>
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.
<P>
Specialized versions of <tt class="class">ref</tt> objects can be created through
subclassing. This is used in the implementation of the
<tt class="class">WeakValueDictionary</tt> 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.
<P>
This example shows how a subclass of <tt class="class">ref</tt> can be used to store
additional information about an object and affect the value that's
returned when the referent is accessed:
<P>
<div class="verbatim"><pre>
import weakref
class ExtendedRef(weakref.ref):
def __init__(self, ob, callback=None, **annotations):
super(ExtendedRef, self).__init__(ob, callback)
self.__counter = 0
for k, v in annotations.iteritems():
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).__call__()
if ob is not None:
self.__counter += 1
ob = (ob, self.__counter)
return ob
</pre></div>
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