2 :mod:`weakref` --- Weak references
3 ==================================
6 :synopsis: Support for weak references and weak dictionaries.
7 .. moduleauthor:: Fred L. Drake, Jr. <fdrake@acm.org>
8 .. moduleauthor:: Neil Schemenauer <nas@arctrix.com>
9 .. moduleauthor:: Martin von Löwis <martin@loewis.home.cs.tu-berlin.de>
10 .. sectionauthor:: Fred L. Drake, Jr. <fdrake@acm.org>
15 The :mod:`weakref` module allows the Python programmer to create :dfn:`weak
16 references` to objects.
18 .. When making changes to the examples in this file, be sure to update
19 Lib/test/test_weakref.py::libreftest too!
21 In the following, the term :dfn:`referent` means the object which is referred to
24 A weak reference to an object is not enough to keep the object alive: when the
25 only remaining references to a referent are weak references,
26 :term:`garbage collection` is free to destroy the referent and reuse its memory
27 for something else. A primary use for weak references is to implement caches or
28 mappings holding large objects, where it's desired that a large object not be
29 kept alive solely because it appears in a cache or mapping.
31 For example, if you have a number of large binary image objects, you may wish to
32 associate a name with each. If you used a Python dictionary to map names to
33 images, or images to names, the image objects would remain alive just because
34 they appeared as values or keys in the dictionaries. The
35 :class:`WeakKeyDictionary` and :class:`WeakValueDictionary` classes supplied by
36 the :mod:`weakref` module are an alternative, using weak references to construct
37 mappings that don't keep objects alive solely because they appear in the mapping
38 objects. If, for example, an image object is a value in a
39 :class:`WeakValueDictionary`, then when the last remaining references to that
40 image object are the weak references held by weak mappings, garbage collection
41 can reclaim the object, and its corresponding entries in weak mappings are
44 :class:`WeakKeyDictionary` and :class:`WeakValueDictionary` use weak references
45 in their implementation, setting up callback functions on the weak references
46 that notify the weak dictionaries when a key or value has been reclaimed by
47 garbage collection. Most programs should find that using one of these weak
48 dictionary types is all they need -- it's not usually necessary to create your
49 own weak references directly. The low-level machinery used by the weak
50 dictionary implementations is exposed by the :mod:`weakref` module for the
51 benefit of advanced uses.
55 Weak references to an object are cleared before the object's :meth:`__del__`
56 is called, to ensure that the weak reference callback (if any) finds the
59 Not all objects can be weakly referenced; those objects which can include class
60 instances, functions written in Python (but not in C), methods (both bound and
61 unbound), sets, frozensets, file objects, :term:`generator`\s, type objects,
62 :class:`DBcursor` objects from the :mod:`bsddb` module, sockets, arrays, deques,
63 and regular expression pattern objects.
65 .. versionchanged:: 2.4
66 Added support for files, sockets, arrays, and patterns.
68 Several builtin types such as :class:`list` and :class:`dict` do not directly
69 support weak references but can add support through subclassing::
74 obj = Dict(red=1, green=2, blue=3) # this object is weak referenceable
76 Extension types can easily be made to support weak references; see
77 :ref:`weakref-support`.
80 .. class:: ref(object[, callback])
82 Return a weak reference to *object*. The original object can be retrieved by
83 calling the reference object if the referent is still alive; if the referent is
84 no longer alive, calling the reference object will cause :const:`None` to be
85 returned. If *callback* is provided and not :const:`None`, and the returned
86 weakref object is still alive, the callback will be called when the object is
87 about to be finalized; the weak reference object will be passed as the only
88 parameter to the callback; the referent will no longer be available.
90 It is allowable for many weak references to be constructed for the same object.
91 Callbacks registered for each weak reference will be called from the most
92 recently registered callback to the oldest registered callback.
94 Exceptions raised by the callback will be noted on the standard error output,
95 but cannot be propagated; they are handled in exactly the same way as exceptions
96 raised from an object's :meth:`__del__` method.
98 Weak references are :term:`hashable` if the *object* is hashable. They will maintain
99 their hash value even after the *object* was deleted. If :func:`hash` is called
100 the first time only after the *object* was deleted, the call will raise
103 Weak references support tests for equality, but not ordering. If the referents
104 are still alive, two references have the same equality relationship as their
105 referents (regardless of the *callback*). If either referent has been deleted,
106 the references are equal only if the reference objects are the same object.
108 .. versionchanged:: 2.4
109 This is now a subclassable type rather than a factory function; it derives from
113 .. function:: proxy(object[, callback])
115 Return a proxy to *object* which uses a weak reference. This supports use of
116 the proxy in most contexts instead of requiring the explicit dereferencing used
117 with weak reference objects. The returned object will have a type of either
118 ``ProxyType`` or ``CallableProxyType``, depending on whether *object* is
119 callable. Proxy objects are not :term:`hashable` regardless of the referent; this
120 avoids a number of problems related to their fundamentally mutable nature, and
121 prevent their use as dictionary keys. *callback* is the same as the parameter
122 of the same name to the :func:`ref` function.
125 .. function:: getweakrefcount(object)
127 Return the number of weak references and proxies which refer to *object*.
130 .. function:: getweakrefs(object)
132 Return a list of all weak reference and proxy objects which refer to *object*.
135 .. class:: WeakKeyDictionary([dict])
137 Mapping class that references keys weakly. Entries in the dictionary will be
138 discarded when there is no longer a strong reference to the key. This can be
139 used to associate additional data with an object owned by other parts of an
140 application without adding attributes to those objects. This can be especially
141 useful with objects that override attribute accesses.
145 Caution: Because a :class:`WeakKeyDictionary` is built on top of a Python
146 dictionary, it must not change size when iterating over it. This can be
147 difficult to ensure for a :class:`WeakKeyDictionary` because actions
148 performed by the program during iteration may cause items in the
149 dictionary to vanish "by magic" (as a side effect of garbage collection).
151 :class:`WeakKeyDictionary` objects have the following additional methods. These
152 expose the internal references directly. The references are not guaranteed to
153 be "live" at the time they are used, so the result of calling the references
154 needs to be checked before being used. This can be used to avoid creating
155 references that will cause the garbage collector to keep the keys around longer
159 .. method:: WeakKeyDictionary.iterkeyrefs()
161 Return an :term:`iterator` that yields the weak references to the keys.
163 .. versionadded:: 2.5
166 .. method:: WeakKeyDictionary.keyrefs()
168 Return a list of weak references to the keys.
170 .. versionadded:: 2.5
173 .. class:: WeakValueDictionary([dict])
175 Mapping class that references values weakly. Entries in the dictionary will be
176 discarded when no strong reference to the value exists any more.
180 Caution: Because a :class:`WeakValueDictionary` is built on top of a Python
181 dictionary, it must not change size when iterating over it. This can be
182 difficult to ensure for a :class:`WeakValueDictionary` because actions performed
183 by the program during iteration may cause items in the dictionary to vanish "by
184 magic" (as a side effect of garbage collection).
186 :class:`WeakValueDictionary` objects have the following additional methods.
187 These method have the same issues as the :meth:`iterkeyrefs` and :meth:`keyrefs`
188 methods of :class:`WeakKeyDictionary` objects.
191 .. method:: WeakValueDictionary.itervaluerefs()
193 Return an :term:`iterator` that yields the weak references to the values.
195 .. versionadded:: 2.5
198 .. method:: WeakValueDictionary.valuerefs()
200 Return a list of weak references to the values.
202 .. versionadded:: 2.5
205 .. data:: ReferenceType
207 The type object for weak references objects.
212 The type object for proxies of objects which are not callable.
215 .. data:: CallableProxyType
217 The type object for proxies of callable objects.
222 Sequence containing all the type objects for proxies. This can make it simpler
223 to test if an object is a proxy without being dependent on naming both proxy
227 .. exception:: ReferenceError
229 Exception raised when a proxy object is used but the underlying object has been
230 collected. This is the same as the standard :exc:`ReferenceError` exception.
235 :pep:`0205` - Weak References
236 The proposal and rationale for this feature, including links to earlier
237 implementations and information about similar features in other languages.
242 Weak Reference Objects
243 ----------------------
245 Weak reference objects have no attributes or methods, but do allow the referent
246 to be obtained, if it still exists, by calling it:
253 >>> r = weakref.ref(o)
258 If the referent no longer exists, calling the reference object returns
265 Testing that a weak reference object is still live should be done using the
266 expression ``ref() is not None``. Normally, application code that needs to use
267 a reference object should follow this pattern::
269 # r is a weak reference object
272 # referent has been garbage collected
273 print "Object has been deallocated; can't frobnicate."
275 print "Object is still live!"
276 o.do_something_useful()
278 Using a separate test for "liveness" creates race conditions in threaded
279 applications; another thread can cause a weak reference to become invalidated
280 before the weak reference is called; the idiom shown above is safe in threaded
281 applications as well as single-threaded applications.
283 Specialized versions of :class:`ref` objects can be created through subclassing.
284 This is used in the implementation of the :class:`WeakValueDictionary` to reduce
285 the memory overhead for each entry in the mapping. This may be most useful to
286 associate additional information with a reference, but could also be used to
287 insert additional processing on calls to retrieve the referent.
289 This example shows how a subclass of :class:`ref` can be used to store
290 additional information about an object and affect the value that's returned when
291 the referent is accessed::
295 class ExtendedRef(weakref.ref):
296 def __init__(self, ob, callback=None, **annotations):
297 super(ExtendedRef, self).__init__(ob, callback)
299 for k, v in annotations.iteritems():
303 """Return a pair containing the referent and the number of
304 times the reference has been called.
306 ob = super(ExtendedRef, self).__call__()
309 ob = (ob, self.__counter)
318 This simple example shows how an application can use objects IDs to retrieve
319 objects that it has seen before. The IDs of the objects can then be used in
320 other data structures without forcing the objects to remain alive, but the
321 objects can still be retrieved by ID if they do.
323 .. Example contributed by Tim Peters.
329 _id2obj_dict = weakref.WeakValueDictionary()
333 _id2obj_dict[oid] = obj
337 return _id2obj_dict[oid]