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| 55 | <H2><A NAME="SECTION005310000000000000000"></A><A NAME="weakref-objects"></A> | |
| 56 | <BR> | |
| 57 | 3.3.1 Weak Reference Objects | |
| 58 | ||
| 59 | </H2> | |
| 60 | ||
| 61 | <P> | |
| 62 | Weak reference objects have no attributes or methods, but do allow the | |
| 63 | referent to be obtained, if it still exists, by calling it: | |
| 64 | ||
| 65 | <P> | |
| 66 | <div class="verbatim"><pre> | |
| 67 | >>> import weakref | |
| 68 | >>> class Object: | |
| 69 | ... pass | |
| 70 | ... | |
| 71 | >>> o = Object() | |
| 72 | >>> r = weakref.ref(o) | |
| 73 | >>> o2 = r() | |
| 74 | >>> o is o2 | |
| 75 | True | |
| 76 | </pre></div> | |
| 77 | ||
| 78 | <P> | |
| 79 | If the referent no longer exists, calling the reference object returns | |
| 80 | <tt class="constant">None</tt>: | |
| 81 | ||
| 82 | <P> | |
| 83 | <div class="verbatim"><pre> | |
| 84 | >>> del o, o2 | |
| 85 | >>> print r() | |
| 86 | None | |
| 87 | </pre></div> | |
| 88 | ||
| 89 | <P> | |
| 90 | Testing that a weak reference object is still live should be done | |
| 91 | using the expression <code><var>ref</var>() is not None</code>. Normally, | |
| 92 | application code that needs to use a reference object should follow | |
| 93 | this pattern: | |
| 94 | ||
| 95 | <P> | |
| 96 | <div class="verbatim"><pre> | |
| 97 | # r is a weak reference object | |
| 98 | o = r() | |
| 99 | if o is None: | |
| 100 | # referent has been garbage collected | |
| 101 | print "Object has been deallocated; can't frobnicate." | |
| 102 | else: | |
| 103 | print "Object is still live!" | |
| 104 | o.do_something_useful() | |
| 105 | </pre></div> | |
| 106 | ||
| 107 | <P> | |
| 108 | Using a separate test for ``liveness'' creates race conditions in | |
| 109 | threaded applications; another thread can cause a weak reference to | |
| 110 | become invalidated before the weak reference is called; the | |
| 111 | idiom shown above is safe in threaded applications as well as | |
| 112 | single-threaded applications. | |
| 113 | ||
| 114 | <P> | |
| 115 | Specialized versions of <tt class="class">ref</tt> objects can be created through | |
| 116 | subclassing. This is used in the implementation of the | |
| 117 | <tt class="class">WeakValueDictionary</tt> to reduce the memory overhead for each | |
| 118 | entry in the mapping. This may be most useful to associate additional | |
| 119 | information with a reference, but could also be used to insert | |
| 120 | additional processing on calls to retrieve the referent. | |
| 121 | ||
| 122 | <P> | |
| 123 | This example shows how a subclass of <tt class="class">ref</tt> can be used to store | |
| 124 | additional information about an object and affect the value that's | |
| 125 | returned when the referent is accessed: | |
| 126 | ||
| 127 | <P> | |
| 128 | <div class="verbatim"><pre> | |
| 129 | import weakref | |
| 130 | ||
| 131 | class ExtendedRef(weakref.ref): | |
| 132 | def __init__(self, ob, callback=None, **annotations): | |
| 133 | super(ExtendedRef, self).__init__(ob, callback) | |
| 134 | self.__counter = 0 | |
| 135 | for k, v in annotations.iteritems(): | |
| 136 | setattr(self, k, v) | |
| 137 | ||
| 138 | def __call__(self): | |
| 139 | """Return a pair containing the referent and the number of | |
| 140 | times the reference has been called. | |
| 141 | """ | |
| 142 | ob = super(ExtendedRef, self).__call__() | |
| 143 | if ob is not None: | |
| 144 | self.__counter += 1 | |
| 145 | ob = (ob, self.__counter) | |
| 146 | return ob | |
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