342 lines
12 KiB
ReStructuredText
342 lines
12 KiB
ReStructuredText
|
|
:mod:`weakref` --- Weak references
|
|
==================================
|
|
|
|
.. module:: weakref
|
|
:synopsis: Support for weak references and weak dictionaries.
|
|
.. moduleauthor:: Fred L. Drake, Jr. <fdrake@acm.org>
|
|
.. moduleauthor:: Neil Schemenauer <nas@arctrix.com>
|
|
.. moduleauthor:: Martin von Löwis <martin@loewis.home.cs.tu-berlin.de>
|
|
.. sectionauthor:: Fred L. Drake, Jr. <fdrake@acm.org>
|
|
|
|
|
|
.. versionadded:: 2.1
|
|
|
|
The :mod:`weakref` module allows the Python programmer to create :dfn:`weak
|
|
references` to objects.
|
|
|
|
.. When making changes to the examples in this file, be sure to update
|
|
Lib/test/test_weakref.py::libreftest too!
|
|
|
|
In the following, the term :dfn:`referent` means the object which is referred to
|
|
by a weak reference.
|
|
|
|
A weak reference to an object is not enough to keep the object alive: when the
|
|
only remaining references to a referent are weak references,
|
|
:term:`garbage collection` is free to destroy the referent and reuse its memory
|
|
for something else. A primary use for weak references is to implement caches or
|
|
mappings holding large objects, where it's desired that a large object not be
|
|
kept alive solely because it appears in a cache or mapping.
|
|
|
|
For example, if you have a number of large binary image objects, you may wish to
|
|
associate a name with each. If you used a Python dictionary to map names to
|
|
images, or images to names, the image objects would remain alive just because
|
|
they appeared as values or keys in the dictionaries. The
|
|
:class:`WeakKeyDictionary` and :class:`WeakValueDictionary` classes supplied by
|
|
the :mod:`weakref` module are an alternative, using weak references to construct
|
|
mappings that don't keep objects alive solely because they appear in the mapping
|
|
objects. If, for example, an image object is a value in a
|
|
:class:`WeakValueDictionary`, then when the last remaining references to that
|
|
image object are the weak references held by weak mappings, garbage collection
|
|
can reclaim the object, and its corresponding entries in weak mappings are
|
|
simply deleted.
|
|
|
|
:class:`WeakKeyDictionary` and :class:`WeakValueDictionary` use weak references
|
|
in their implementation, setting up callback functions on the weak references
|
|
that notify the weak dictionaries when a key or value has been reclaimed by
|
|
garbage collection. Most programs should find that using one of these weak
|
|
dictionary types is all they need -- it's not usually necessary to create your
|
|
own weak references directly. The low-level machinery used by the weak
|
|
dictionary implementations is exposed by the :mod:`weakref` module for the
|
|
benefit of advanced uses.
|
|
|
|
.. note::
|
|
|
|
Weak references to an object are cleared before the object's :meth:`__del__`
|
|
is called, to ensure that the weak reference callback (if any) finds the
|
|
object still alive.
|
|
|
|
Not all objects can be weakly referenced; those objects which can include class
|
|
instances, functions written in Python (but not in C), methods (both bound and
|
|
unbound), sets, frozensets, file objects, :term:`generator`\s, type objects,
|
|
:class:`DBcursor` objects from the :mod:`bsddb` module, sockets, arrays, deques,
|
|
and regular expression pattern objects.
|
|
|
|
.. versionchanged:: 2.4
|
|
Added support for files, sockets, arrays, and patterns.
|
|
|
|
.. versionchanged:: 2.7
|
|
Added support for thread.lock and threading.Lock.
|
|
|
|
Several built-in types such as :class:`list` and :class:`dict` do not directly
|
|
support weak references but can add support through subclassing::
|
|
|
|
class Dict(dict):
|
|
pass
|
|
|
|
obj = Dict(red=1, green=2, blue=3) # this object is weak referenceable
|
|
|
|
Extension types can easily be made to support weak references; see
|
|
:ref:`weakref-support`.
|
|
|
|
|
|
.. class:: ref(object[, callback])
|
|
|
|
Return a weak reference to *object*. The original object can be retrieved by
|
|
calling the reference object if the referent is still alive; if the referent is
|
|
no longer alive, calling the reference object will cause :const:`None` to be
|
|
returned. If *callback* is provided and not :const:`None`, and the returned
|
|
weakref object is still alive, the callback will be called when the object is
|
|
about to be finalized; the weak reference object will be passed as the only
|
|
parameter to the callback; the referent will no longer be available.
|
|
|
|
It is allowable for many weak references to be constructed for the same object.
|
|
Callbacks registered for each weak reference will be called from the most
|
|
recently registered callback to the oldest registered callback.
|
|
|
|
Exceptions raised by the callback will be noted on the standard error output,
|
|
but cannot be propagated; they are handled in exactly the same way as exceptions
|
|
raised from an object's :meth:`__del__` method.
|
|
|
|
Weak references are :term:`hashable` if the *object* is hashable. They will maintain
|
|
their hash value even after the *object* was deleted. If :func:`hash` is called
|
|
the first time only after the *object* was deleted, the call will raise
|
|
:exc:`TypeError`.
|
|
|
|
Weak references support tests for equality, but not ordering. If the referents
|
|
are still alive, two references have the same equality relationship as their
|
|
referents (regardless of the *callback*). If either referent has been deleted,
|
|
the references are equal only if the reference objects are the same object.
|
|
|
|
.. versionchanged:: 2.4
|
|
This is now a subclassable type rather than a factory function; it derives from
|
|
:class:`object`.
|
|
|
|
|
|
.. function:: proxy(object[, callback])
|
|
|
|
Return a proxy to *object* which uses a weak reference. This supports use of
|
|
the proxy in most contexts instead of requiring the explicit dereferencing used
|
|
with weak reference objects. The returned object will have a type of either
|
|
``ProxyType`` or ``CallableProxyType``, depending on whether *object* is
|
|
callable. Proxy objects are not :term:`hashable` regardless of the referent; this
|
|
avoids a number of problems related to their fundamentally mutable nature, and
|
|
prevent their use as dictionary keys. *callback* is the same as the parameter
|
|
of the same name to the :func:`ref` function.
|
|
|
|
|
|
.. function:: getweakrefcount(object)
|
|
|
|
Return the number of weak references and proxies which refer to *object*.
|
|
|
|
|
|
.. function:: getweakrefs(object)
|
|
|
|
Return a list of all weak reference and proxy objects which refer to *object*.
|
|
|
|
|
|
.. class:: WeakKeyDictionary([dict])
|
|
|
|
Mapping class that references keys weakly. Entries in the dictionary will be
|
|
discarded when there is no longer a strong reference to the key. This can be
|
|
used to associate additional data with an object owned by other parts of an
|
|
application without adding attributes to those objects. This can be especially
|
|
useful with objects that override attribute accesses.
|
|
|
|
.. note::
|
|
|
|
Caution: Because a :class:`WeakKeyDictionary` is built on top of a Python
|
|
dictionary, it must not change size when iterating over it. This can be
|
|
difficult to ensure for a :class:`WeakKeyDictionary` because actions
|
|
performed by the program during iteration may cause items in the
|
|
dictionary to vanish "by magic" (as a side effect of garbage collection).
|
|
|
|
:class:`WeakKeyDictionary` objects have the following additional methods. These
|
|
expose the internal references directly. The references are not guaranteed to
|
|
be "live" at the time they are used, so the result of calling the references
|
|
needs to be checked before being used. This can be used to avoid creating
|
|
references that will cause the garbage collector to keep the keys around longer
|
|
than needed.
|
|
|
|
|
|
.. method:: WeakKeyDictionary.iterkeyrefs()
|
|
|
|
Return an :term:`iterator` that yields the weak references to the keys.
|
|
|
|
.. versionadded:: 2.5
|
|
|
|
|
|
.. method:: WeakKeyDictionary.keyrefs()
|
|
|
|
Return a list of weak references to the keys.
|
|
|
|
.. versionadded:: 2.5
|
|
|
|
|
|
.. class:: WeakValueDictionary([dict])
|
|
|
|
Mapping class that references values weakly. Entries in the dictionary will be
|
|
discarded when no strong reference to the value exists any more.
|
|
|
|
.. note::
|
|
|
|
Caution: Because a :class:`WeakValueDictionary` is built on top of a Python
|
|
dictionary, it must not change size when iterating over it. This can be
|
|
difficult to ensure for a :class:`WeakValueDictionary` because actions performed
|
|
by the program during iteration may cause items in the dictionary to vanish "by
|
|
magic" (as a side effect of garbage collection).
|
|
|
|
:class:`WeakValueDictionary` objects have the following additional methods.
|
|
These method have the same issues as the :meth:`iterkeyrefs` and :meth:`keyrefs`
|
|
methods of :class:`WeakKeyDictionary` objects.
|
|
|
|
|
|
.. method:: WeakValueDictionary.itervaluerefs()
|
|
|
|
Return an :term:`iterator` that yields the weak references to the values.
|
|
|
|
.. versionadded:: 2.5
|
|
|
|
|
|
.. method:: WeakValueDictionary.valuerefs()
|
|
|
|
Return a list of weak references to the values.
|
|
|
|
.. versionadded:: 2.5
|
|
|
|
|
|
.. data:: ReferenceType
|
|
|
|
The type object for weak references objects.
|
|
|
|
|
|
.. data:: ProxyType
|
|
|
|
The type object for proxies of objects which are not callable.
|
|
|
|
|
|
.. data:: CallableProxyType
|
|
|
|
The type object for proxies of callable objects.
|
|
|
|
|
|
.. data:: ProxyTypes
|
|
|
|
Sequence containing all the type objects for proxies. This can make it simpler
|
|
to test if an object is a proxy without being dependent on naming both proxy
|
|
types.
|
|
|
|
|
|
.. exception:: ReferenceError
|
|
|
|
Exception raised when a proxy object is used but the underlying object has been
|
|
collected. This is the same as the standard :exc:`ReferenceError` exception.
|
|
|
|
|
|
.. seealso::
|
|
|
|
:pep:`0205` - Weak References
|
|
The proposal and rationale for this feature, including links to earlier
|
|
implementations and information about similar features in other languages.
|
|
|
|
|
|
.. _weakref-objects:
|
|
|
|
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
|
|
:const:`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 :class:`ref` objects can be created through subclassing.
|
|
This is used in the implementation of the :class:`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 :class:`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
|
|
|
|
|
|
.. _weakref-example:
|
|
|
|
Example
|
|
-------
|
|
|
|
This simple example shows how an application can use objects IDs to retrieve
|
|
objects that it has seen before. The IDs of the objects can then be used in
|
|
other data structures without forcing the objects to remain alive, but the
|
|
objects can still be retrieved by ID if they do.
|
|
|
|
.. Example contributed by Tim Peters.
|
|
|
|
::
|
|
|
|
import weakref
|
|
|
|
_id2obj_dict = weakref.WeakValueDictionary()
|
|
|
|
def remember(obj):
|
|
oid = id(obj)
|
|
_id2obj_dict[oid] = obj
|
|
return oid
|
|
|
|
def id2obj(oid):
|
|
return _id2obj_dict[oid]
|
|
|