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:mod:`weakref` --- Weak references
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==================================
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:synopsis: Support for weak references and weak dictionaries.
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.. moduleauthor:: Fred L. Drake, Jr. <fdrake@acm.org>
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.. moduleauthor:: Neil Schemenauer <nas@arctrix.com>
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.. moduleauthor:: Martin von Löwis <martin@loewis.home.cs.tu-berlin.de>
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.. sectionauthor:: Fred L. Drake, Jr. <fdrake@acm.org>
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The :mod:`weakref` module allows the Python programmer to create :dfn:`weak
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references` to objects.
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.. When making changes to the examples in this file, be sure to update
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Lib/test/test_weakref.py::libreftest too!
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In the following, the term :dfn:`referent` means the object which is referred to
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A weak reference to an object is not enough to keep the object alive: when the
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only remaining references to a referent are weak references,
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:term:`garbage collection` is free to destroy the referent and reuse its memory
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for something else. A primary use for weak references is to implement caches or
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mappings holding large objects, where it's desired that a large object not be
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kept alive solely because it appears in a cache or mapping.
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For example, if you have a number of large binary image objects, you may wish to
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associate a name with each. If you used a Python dictionary to map names to
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images, or images to names, the image objects would remain alive just because
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they appeared as values or keys in the dictionaries. The
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:class:`WeakKeyDictionary` and :class:`WeakValueDictionary` classes supplied by
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the :mod:`weakref` module are an alternative, using weak references to construct
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mappings that don't keep objects alive solely because they appear in the mapping
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objects. If, for example, an image object is a value in a
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:class:`WeakValueDictionary`, then when the last remaining references to that
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image object are the weak references held by weak mappings, garbage collection
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can reclaim the object, and its corresponding entries in weak mappings are
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:class:`WeakKeyDictionary` and :class:`WeakValueDictionary` use weak references
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in their implementation, setting up callback functions on the weak references
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that notify the weak dictionaries when a key or value has been reclaimed by
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garbage collection. :class:`WeakSet` implements the :class:`set` interface,
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but keeps weak references to its elements, just like a
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:class:`WeakKeyDictionary` does.
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Most programs should find that using one of these weak container types is all
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they need -- it's not usually necessary to create your own weak references
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directly. The low-level machinery used by the weak dictionary implementations
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is exposed by the :mod:`weakref` module for the benefit of advanced uses.
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Weak references to an object are cleared before the object's :meth:`__del__`
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is called, to ensure that the weak reference callback (if any) finds the
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Not all objects can be weakly referenced; those objects which can include class
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instances, functions written in Python (but not in C), instance methods, sets,
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frozensets, file objects, :term:`generator`\s, type objects, :class:`DBcursor`
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objects from the :mod:`bsddb` module, sockets, arrays, deques, and regular
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expression pattern objects.
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Several builtin types such as :class:`list` and :class:`dict` do not directly
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support weak references but can add support through subclassing::
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obj = Dict(red=1, green=2, blue=3) # this object is weak referenceable
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Extension types can easily be made to support weak references; see
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:ref:`weakref-support`.
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.. class:: ref(object[, callback])
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Return a weak reference to *object*. The original object can be retrieved by
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calling the reference object if the referent is still alive; if the referent is
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no longer alive, calling the reference object will cause :const:`None` to be
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returned. If *callback* is provided and not :const:`None`, and the returned
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weakref object is still alive, the callback will be called when the object is
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about to be finalized; the weak reference object will be passed as the only
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parameter to the callback; the referent will no longer be available.
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It is allowable for many weak references to be constructed for the same object.
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Callbacks registered for each weak reference will be called from the most
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recently registered callback to the oldest registered callback.
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Exceptions raised by the callback will be noted on the standard error output,
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but cannot be propagated; they are handled in exactly the same way as exceptions
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raised from an object's :meth:`__del__` method.
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Weak references are :term:`hashable` if the *object* is hashable. They will maintain
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their hash value even after the *object* was deleted. If :func:`hash` is called
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the first time only after the *object* was deleted, the call will raise
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Weak references support tests for equality, but not ordering. If the referents
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are still alive, two references have the same equality relationship as their
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referents (regardless of the *callback*). If either referent has been deleted,
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the references are equal only if the reference objects are the same object.
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This is a subclassable type rather than a factory function.
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.. function:: proxy(object[, callback])
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Return a proxy to *object* which uses a weak reference. This supports use of
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the proxy in most contexts instead of requiring the explicit dereferencing used
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with weak reference objects. The returned object will have a type of either
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``ProxyType`` or ``CallableProxyType``, depending on whether *object* is
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callable. Proxy objects are not :term:`hashable` regardless of the referent; this
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avoids a number of problems related to their fundamentally mutable nature, and
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prevent their use as dictionary keys. *callback* is the same as the parameter
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of the same name to the :func:`ref` function.
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.. function:: getweakrefcount(object)
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Return the number of weak references and proxies which refer to *object*.
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.. function:: getweakrefs(object)
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Return a list of all weak reference and proxy objects which refer to *object*.
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.. class:: WeakKeyDictionary([dict])
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Mapping class that references keys weakly. Entries in the dictionary will be
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discarded when there is no longer a strong reference to the key. This can be
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used to associate additional data with an object owned by other parts of an
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application without adding attributes to those objects. This can be especially
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useful with objects that override attribute accesses.
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Caution: Because a :class:`WeakKeyDictionary` is built on top of a Python
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dictionary, it must not change size when iterating over it. This can be
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difficult to ensure for a :class:`WeakKeyDictionary` because actions
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performed by the program during iteration may cause items in the
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dictionary to vanish "by magic" (as a side effect of garbage collection).
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:class:`WeakKeyDictionary` objects have the following additional methods. These
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expose the internal references directly. The references are not guaranteed to
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be "live" at the time they are used, so the result of calling the references
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needs to be checked before being used. This can be used to avoid creating
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references that will cause the garbage collector to keep the keys around longer
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.. method:: WeakKeyDictionary.keyrefs()
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Return an :term:`iterator` that yields the weak references to the keys.
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.. class:: WeakValueDictionary([dict])
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Mapping class that references values weakly. Entries in the dictionary will be
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discarded when no strong reference to the value exists any more.
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Caution: Because a :class:`WeakValueDictionary` is built on top of a Python
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dictionary, it must not change size when iterating over it. This can be
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difficult to ensure for a :class:`WeakValueDictionary` because actions performed
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by the program during iteration may cause items in the dictionary to vanish "by
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magic" (as a side effect of garbage collection).
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:class:`WeakValueDictionary` objects have the following additional methods.
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These method have the same issues as the and :meth:`keyrefs` method of
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:class:`WeakKeyDictionary` objects.
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.. method:: WeakValueDictionary.valuerefs()
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Return an :term:`iterator` that yields the weak references to the values.
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.. class:: WeakSet([elements])
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Set class that keeps weak references to its elements. An element will be
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discarded when no strong reference to it exists any more.
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.. data:: ReferenceType
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The type object for weak references objects.
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The type object for proxies of objects which are not callable.
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.. data:: CallableProxyType
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The type object for proxies of callable objects.
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Sequence containing all the type objects for proxies. This can make it simpler
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to test if an object is a proxy without being dependent on naming both proxy
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.. exception:: ReferenceError
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Exception raised when a proxy object is used but the underlying object has been
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collected. This is the same as the standard :exc:`ReferenceError` exception.
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:pep:`0205` - Weak References
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The proposal and rationale for this feature, including links to earlier
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implementations and information about similar features in other languages.
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Weak Reference Objects
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----------------------
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Weak reference objects have no attributes or methods, but do allow the referent
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to be obtained, if it still exists, by calling it:
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>>> r = weakref.ref(o)
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If the referent no longer exists, calling the reference object returns
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Testing that a weak reference object is still live should be done using the
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expression ``ref() is not None``. Normally, application code that needs to use
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a reference object should follow this pattern::
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# r is a weak reference object
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# referent has been garbage collected
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print("Object has been deallocated; can't frobnicate.")
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print("Object is still live!")
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o.do_something_useful()
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Using a separate test for "liveness" creates race conditions in threaded
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applications; another thread can cause a weak reference to become invalidated
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before the weak reference is called; the idiom shown above is safe in threaded
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applications as well as single-threaded applications.
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Specialized versions of :class:`ref` objects can be created through subclassing.
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This is used in the implementation of the :class:`WeakValueDictionary` to reduce
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the memory overhead for each entry in the mapping. This may be most useful to
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associate additional information with a reference, but could also be used to
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insert additional processing on calls to retrieve the referent.
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This example shows how a subclass of :class:`ref` can be used to store
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additional information about an object and affect the value that's returned when
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the referent is accessed::
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class ExtendedRef(weakref.ref):
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def __init__(self, ob, callback=None, **annotations):
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super(ExtendedRef, self).__init__(ob, callback)
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for k, v in annotations.items():
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"""Return a pair containing the referent and the number of
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times the reference has been called.
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ob = super(ExtendedRef, self).__call__()
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ob = (ob, self.__counter)
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This simple example shows how an application can use objects IDs to retrieve
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objects that it has seen before. The IDs of the objects can then be used in
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other data structures without forcing the objects to remain alive, but the
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objects can still be retrieved by ID if they do.
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.. Example contributed by Tim Peters.
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_id2obj_dict = weakref.WeakValueDictionary()
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_id2obj_dict[oid] = obj
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return _id2obj_dict[oid]
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Doc/library/weakref.rst
