5.11.1 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 deallocated; 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.
Specialized versions of ref objects can be created through
subclassing. This is used in the implementation of the
WeakValueDictionary 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 ref can be used to store
additional information about an object and affect the value that's
returned when the referent is accessed:
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
Release 2.5.2, documentation updated on 21st February, 2008.
See About this document... for information on suggesting changes.
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