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<title>3.3.1 Weak Reference Objects
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<H2><A NAME=
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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:
<div class=
"verbatim"><pre>
>>> import weakref
>>> class Object:
>>> o = Object()
>>> r = weakref.ref(o)
If the referent no longer exists, calling the reference object returns
<tt class=
"constant">None
</tt>:
<div class=
"verbatim"><pre>
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
<div class=
"verbatim"><pre>
# r is a weak reference object
# referent has been garbage collected
print
"Object has been deallocated; can't frobnicate."
print
"Object is still live!"
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
<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.
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:
<div class=
"verbatim"><pre>
class ExtendedRef(weakref.ref):
def __init__(self, ob, callback=None, **annotations):
super(ExtendedRef, self).__init__(ob, callback)
for k, v in annotations.iteritems():
"""Return a pair containing the referent and the number of
times the reference has been called.
ob = super(ExtendedRef, self).__call__()
ob = (ob, self.__counter)
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