cpython/Doc/library/weakref.rst

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:mod:`weakref` --- Weak references
==================================
.. module:: weakref
:synopsis: Support for weak references and weak dictionaries.
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.. 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>
2011-01-27 16:38:46 -04:00
**Source code:** :source:`Lib/weakref.py`
--------------
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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!
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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
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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. However, until the object is actually destroyed the weak
reference may return the object even if there are no strong references to it.
A primary use for weak references is to implement caches or
Merged revisions 59259-59274 via svnmerge from svn+ssh://pythondev@svn.python.org/python/trunk ........ r59260 | lars.gustaebel | 2007-12-01 22:02:12 +0100 (Sat, 01 Dec 2007) | 5 lines Issue #1531: Read fileobj from the current offset, do not seek to the start. (will backport to 2.5) ........ r59262 | georg.brandl | 2007-12-01 23:24:47 +0100 (Sat, 01 Dec 2007) | 4 lines Document PyEval_* functions from ceval.c. Credits to Michael Sloan from GHOP. ........ r59263 | georg.brandl | 2007-12-01 23:27:56 +0100 (Sat, 01 Dec 2007) | 2 lines Add a few refcount data entries. ........ r59264 | georg.brandl | 2007-12-01 23:38:48 +0100 (Sat, 01 Dec 2007) | 4 lines Add test suite for cmd module. Written by Michael Schneider for GHOP. ........ r59265 | georg.brandl | 2007-12-01 23:42:46 +0100 (Sat, 01 Dec 2007) | 3 lines Add examples to the ElementTree documentation. Written by h4wk.cz for GHOP. ........ r59266 | georg.brandl | 2007-12-02 00:12:45 +0100 (Sun, 02 Dec 2007) | 3 lines Add "Using Python on Windows" document, by Robert Lehmann. Written for GHOP. ........ r59271 | georg.brandl | 2007-12-02 15:34:34 +0100 (Sun, 02 Dec 2007) | 3 lines Add example to mmap docs. Written for GHOP by Rafal Rawicki. ........ r59272 | georg.brandl | 2007-12-02 15:37:29 +0100 (Sun, 02 Dec 2007) | 2 lines Convert bdb.rst line endings to Unix style. ........ r59274 | georg.brandl | 2007-12-02 15:58:50 +0100 (Sun, 02 Dec 2007) | 4 lines Add more entries to the glossary. Written by Jeff Wheeler for GHOP. ........
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mappings holding large objects, where it's desired that a large object not be
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See #2414. ........ r61628 | david.wolever | 2008-03-19 17:57:43 +0100 (Mi, 19 M?\195?\164r 2008) | 1 line Added a class for tests which should not run if a particular import is found. ........ r61629 | collin.winter | 2008-03-19 17:58:19 +0100 (Mi, 19 M?\195?\164r 2008) | 1 line Two more relative import fixes in pgen2. ........ r61635 | david.wolever | 2008-03-19 20:16:03 +0100 (Mi, 19 M?\195?\164r 2008) | 1 line Fixed print fixer so it will do the Right Thing when it encounters __future__.print_function. 2to3 gets upset, though, so the tests have been commented out. ........ r61637 | david.wolever | 2008-03-19 21:37:17 +0100 (Mi, 19 M?\195?\164r 2008) | 3 lines Added a fixer for itertools imports (from itertools import imap, ifilterfalse --> from itertools import filterfalse) ........ r61645 | david.wolever | 2008-03-19 23:22:35 +0100 (Mi, 19 M?\195?\164r 2008) | 1 line SVN is happier when you add the files you create... -_-' ........ r61654 | david.wolever | 2008-03-20 01:09:56 +0100 (Do, 20 M?\195?\164r 2008) | 1 line Added an explicit sort order to fixers -- fixes problems like #2427 ........ r61664 | david.wolever | 2008-03-20 04:32:40 +0100 (Do, 20 M?\195?\164r 2008) | 3 lines Fixes #2428 -- comments are no longer eatten by __future__ fixer. ........ r61673 | david.wolever | 2008-03-20 17:22:40 +0100 (Do, 20 M?\195?\164r 2008) | 1 line Added 2to3 node pretty-printer ........ r61679 | david.wolever | 2008-03-20 20:50:42 +0100 (Do, 20 M?\195?\164r 2008) | 1 line Made node printing a little bit prettier ........ r61723 | martin.v.loewis | 2008-03-22 00:59:27 +0100 (Sa, 22 M?\195?\164r 2008) | 2 lines Fix whitespace. ........ ................ r61725 | martin.v.loewis | 2008-03-22 01:02:41 +0100 (Sat, 22 Mar 2008) | 2 lines Install lib2to3. ................ r61731 | facundo.batista | 2008-03-22 03:45:37 +0100 (Sat, 22 Mar 2008) | 4 lines Small fix that complicated the test actually when that test failed. ................ r61732 | alexandre.vassalotti | 2008-03-22 05:08:44 +0100 (Sat, 22 Mar 2008) | 2 lines Added warning for the removal of 'hotshot' in Py3k. ................ r61733 | georg.brandl | 2008-03-22 11:07:29 +0100 (Sat, 22 Mar 2008) | 4 lines #1918: document that weak references *to* an object are cleared before the object's __del__ is called, to ensure that the weak reference callback (if any) finds the object healthy. ................ r61734 | georg.brandl | 2008-03-22 11:56:23 +0100 (Sat, 22 Mar 2008) | 2 lines Activate the Sphinx doctest extension and convert howto/functional to use it. ................ r61735 | georg.brandl | 2008-03-22 11:58:38 +0100 (Sat, 22 Mar 2008) | 2 lines Allow giving source names on the cmdline. ................ r61737 | georg.brandl | 2008-03-22 12:00:48 +0100 (Sat, 22 Mar 2008) | 2 lines Fixup this HOWTO's doctest blocks so that they can be run with sphinx' doctest builder. ................ r61739 | georg.brandl | 2008-03-22 12:47:10 +0100 (Sat, 22 Mar 2008) | 2 lines Test decimal.rst doctests as far as possible with sphinx doctest. ................ r61741 | georg.brandl | 2008-03-22 13:04:26 +0100 (Sat, 22 Mar 2008) | 2 lines Make doctests in re docs usable with sphinx' doctest. ................ r61743 | georg.brandl | 2008-03-22 13:59:37 +0100 (Sat, 22 Mar 2008) | 2 lines Make more doctests in pprint docs testable. ................ r61744 | georg.brandl | 2008-03-22 14:07:06 +0100 (Sat, 22 Mar 2008) | 2 lines No need to specify explicit "doctest_block" anymore. ................ r61753 | georg.brandl | 2008-03-22 21:08:43 +0100 (Sat, 22 Mar 2008) | 2 lines Fix-up syntax problems. ................ r61761 | georg.brandl | 2008-03-22 22:06:20 +0100 (Sat, 22 Mar 2008) | 4 lines Make collections' doctests executable. (The <BLANKLINE>s will be stripped from presentation output.) ................ r61765 | georg.brandl | 2008-03-22 22:21:57 +0100 (Sat, 22 Mar 2008) | 2 lines Test doctests in datetime docs. ................ r61766 | georg.brandl | 2008-03-22 22:26:44 +0100 (Sat, 22 Mar 2008) | 2 lines Test doctests in operator docs. ................ r61767 | georg.brandl | 2008-03-22 22:38:33 +0100 (Sat, 22 Mar 2008) | 2 lines Enable doctests in functions.rst. Already found two errors :) ................ r61769 | georg.brandl | 2008-03-22 23:04:10 +0100 (Sat, 22 Mar 2008) | 3 lines Enable doctest running for several other documents. 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From a clean build importing future_builtins would fail since itertools wasn't built yet. ................ r61782 | neal.norwitz | 2008-03-23 07:16:04 +0100 (Sun, 23 Mar 2008) | 1 line Try to prevent the alarm going off early in tearDown ................ r61783 | neal.norwitz | 2008-03-23 07:19:57 +0100 (Sun, 23 Mar 2008) | 4 lines Remove compiler warnings (on Alpha at least) about using chars as array subscripts. Using chars are dangerous b/c they are signed on some platforms and unsigned on others. ................ r61788 | georg.brandl | 2008-03-23 09:05:30 +0100 (Sun, 23 Mar 2008) | 2 lines Make the doctests presentation-friendlier. ................ r61793 | amaury.forgeotdarc | 2008-03-23 10:55:29 +0100 (Sun, 23 Mar 2008) | 4 lines #1477: ur'\U0010FFFF' raised in narrow unicode builds. Corrected the raw-unicode-escape codec to use UTF-16 surrogates in this case, just like the unicode-escape codec. ................ r61796 | raymond.hettinger | 2008-03-23 14:32:32 +0100 (Sun, 23 Mar 2008) | 1 line Issue 1681432: Add triangular distribution the random module. ................ r61807 | raymond.hettinger | 2008-03-23 20:37:53 +0100 (Sun, 23 Mar 2008) | 4 lines Adopt Nick's suggestion for useful default arguments. Clean-up floating point issues by adding true division and float constants. ................ r61813 | gregory.p.smith | 2008-03-23 22:04:43 +0100 (Sun, 23 Mar 2008) | 6 lines Fix gzip to deal with CRC's being signed values in Python 2.x properly and to read 32bit values as unsigned to start with rather than applying signedness fixups allover the place afterwards. This hopefully fixes the test_tarfile failure on the alpha/tru64 buildbot. ................
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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.
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: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. :class:`WeakSet` implements the :class:`set` interface,
but keeps weak references to its elements, just like a
:class:`WeakKeyDictionary` does.
:class:`finalize` provides a straight forward way to register a
cleanup function to be called when an object is garbage collected.
This is simpler to use than setting up a callback function on a raw
weak reference, since the module automatically ensures that the finalizer
remains alive until the object is collected.
Most programs should find that using one of these weak container types
or :class:`finalize` is all they need -- it's not usually necessary to
create your own weak references directly. The low-level machinery is
exposed by the :mod:`weakref` module for the benefit of advanced uses.
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Not all objects can be weakly referenced; those objects which can include class
instances, functions written in Python (but not in C), instance methods, sets,
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frozensets, some :term:`file objects <file object>`, :term:`generators <generator>`,
type objects, sockets, arrays, deques, regular expression pattern objects, and code
objects.
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.. versionchanged:: 3.2
Added support for thread.lock, threading.Lock, and code objects.
Several built-in types such as :class:`list` and :class:`dict` do not directly
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support weak references but can add support through subclassing::
class Dict(dict):
pass
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obj = Dict(red=1, green=2, blue=3) # this object is weak referenceable
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.. impl-detail::
Other built-in types such as :class:`tuple` and :class:`int` do not support weak
references even when subclassed.
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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`.
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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.
This is a subclassable type rather than a factory function.
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.. attribute:: __callback__
This read-only attribute returns the callback currently associated to the
weakref. If there is no callback or if the referent of the weakref is
no longer alive then this attribute will have value ``None``.
.. versionchanged:: 3.4
Added the :attr:`__callback__` attribute.
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.. 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
Merged revisions 58742-58816 via svnmerge from svn+ssh://pythondev@svn.python.org/python/trunk ........ r58745 | georg.brandl | 2007-11-01 10:19:33 -0700 (Thu, 01 Nov 2007) | 2 lines #1364: os.lstat is available on Windows too, as an alias to os.stat. ........ r58750 | christian.heimes | 2007-11-01 12:48:10 -0700 (Thu, 01 Nov 2007) | 1 line Backport of import tests for bug http://bugs.python.org/issue1293 and bug http://bugs.python.org/issue1342 ........ r58751 | christian.heimes | 2007-11-01 13:11:06 -0700 (Thu, 01 Nov 2007) | 1 line Removed non ASCII text from test as requested by Guido. Sorry :/ ........ r58753 | georg.brandl | 2007-11-01 13:37:02 -0700 (Thu, 01 Nov 2007) | 2 lines Fix markup glitch. ........ r58757 | gregory.p.smith | 2007-11-01 14:08:14 -0700 (Thu, 01 Nov 2007) | 4 lines Fix bug introduced in revision 58385. Database keys could no longer have NULL bytes in them. Replace the errant strdup with a malloc+memcpy. Adds a unit test for the correct behavior. ........ r58758 | gregory.p.smith | 2007-11-01 14:15:36 -0700 (Thu, 01 Nov 2007) | 3 lines Undo revision 58533 58534 fixes. Those were a workaround for a problem introduced by 58385. ........ r58759 | gregory.p.smith | 2007-11-01 14:17:47 -0700 (Thu, 01 Nov 2007) | 2 lines false "fix" undone as correct problem was found and fixed. ........ r58765 | mark.summerfield | 2007-11-02 01:24:59 -0700 (Fri, 02 Nov 2007) | 3 lines Added more file-handling related cross-references. ........ r58766 | nick.coghlan | 2007-11-02 03:09:12 -0700 (Fri, 02 Nov 2007) | 1 line Fix for bug 1705170 - contextmanager swallowing StopIteration (2.5 backport candidate) ........ r58784 | thomas.heller | 2007-11-02 12:10:24 -0700 (Fri, 02 Nov 2007) | 4 lines Issue #1292: On alpha, arm, ppc, and s390 linux systems the --with-system-ffi configure option defaults to "yes" because the bundled libffi sources are too old. ........ r58785 | thomas.heller | 2007-11-02 12:11:23 -0700 (Fri, 02 Nov 2007) | 1 line Enable the full ctypes c_longdouble tests again. ........ r58796 | georg.brandl | 2007-11-02 13:06:17 -0700 (Fri, 02 Nov 2007) | 4 lines Make "hashable" a glossary entry and clarify docs on __cmp__, __eq__ and __hash__. I hope the concept of hashability is better understandable now. Thanks to Tim Hatch for pointing out the flaws here. ........
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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
prevent their use as dictionary keys. *callback* is the same as the parameter
of the same name to the :func:`ref` function.
.. versionchanged:: 3.8
Extended the operator support on proxy objects to include the matrix
multiplication operators ``@`` and ``@=``.
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.. 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.
.. versionchanged:: 3.9
Added support for ``|`` and ``|=`` operators, specified in :pep:`584`.
:class:`WeakKeyDictionary` objects have an additional method that
exposes 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
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.keyrefs()
Return an iterable of the weak references to the keys.
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.. 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.
.. versionchanged:: 3.9
Added support for ``|`` and ``|=`` operators, as specified in :pep:`584`.
:class:`WeakValueDictionary` objects have an additional method that has the
same issues as the :meth:`keyrefs` method of :class:`WeakKeyDictionary`
objects.
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.. method:: WeakValueDictionary.valuerefs()
Return an iterable of the weak references to the values.
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.. class:: WeakSet([elements])
Set class that keeps weak references to its elements. An element will be
discarded when no strong reference to it exists any more.
.. class:: WeakMethod(method)
A custom :class:`ref` subclass which simulates a weak reference to a bound
method (i.e., a method defined on a class and looked up on an instance).
Since a bound method is ephemeral, a standard weak reference cannot keep
hold of it. :class:`WeakMethod` has special code to recreate the bound
method until either the object or the original function dies::
>>> class C:
... def method(self):
... print("method called!")
...
>>> c = C()
>>> r = weakref.ref(c.method)
>>> r()
>>> r = weakref.WeakMethod(c.method)
>>> r()
<bound method C.method of <__main__.C object at 0x7fc859830220>>
>>> r()()
method called!
>>> del c
>>> gc.collect()
0
>>> r()
>>>
.. versionadded:: 3.4
.. class:: finalize(obj, func, /, *args, **kwargs)
Return a callable finalizer object which will be called when *obj*
is garbage collected. Unlike an ordinary weak reference, a finalizer
will always survive until the reference object is collected, greatly
simplifying lifecycle management.
A finalizer is considered *alive* until it is called (either explicitly
or at garbage collection), and after that it is *dead*. Calling a live
finalizer returns the result of evaluating ``func(*arg, **kwargs)``,
whereas calling a dead finalizer returns :const:`None`.
Exceptions raised by finalizer callbacks during garbage collection
will be shown on the standard error output, but cannot be
propagated. They are handled in the same way as exceptions raised
from an object's :meth:`__del__` method or a weak reference's
callback.
When the program exits, each remaining live finalizer is called
unless its :attr:`atexit` attribute has been set to false. They
are called in reverse order of creation.
A finalizer will never invoke its callback during the later part of
the :term:`interpreter shutdown` when module globals are liable to have
been replaced by :const:`None`.
.. method:: __call__()
If *self* is alive then mark it as dead and return the result of
calling ``func(*args, **kwargs)``. If *self* is dead then return
:const:`None`.
.. method:: detach()
If *self* is alive then mark it as dead and return the tuple
``(obj, func, args, kwargs)``. If *self* is dead then return
:const:`None`.
.. method:: peek()
If *self* is alive then return the tuple ``(obj, func, args,
kwargs)``. If *self* is dead then return :const:`None`.
.. attribute:: alive
Property which is true if the finalizer is alive, false otherwise.
.. attribute:: atexit
A writable boolean property which by default is true. When the
program exits, it calls all remaining live finalizers for which
:attr:`.atexit` is true. They are called in reverse order of
creation.
.. note::
It is important to ensure that *func*, *args* and *kwargs* do
not own any references to *obj*, either directly or indirectly,
since otherwise *obj* will never be garbage collected. In
particular, *func* should not be a bound method of *obj*.
.. versionadded:: 3.4
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.. 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.
.. seealso::
:pep:`205` - Weak References
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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 methods and no attributes besides
:attr:`ref.__callback__`. A weak reference object allows the referent to be
obtained, if it still exists, by calling it:
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>>> 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
Merged revisions 61724-61725,61731-61735,61737,61739,61741,61743-61744,61753,61761,61765-61767,61769,61773,61776-61778,61780-61783,61788,61793,61796,61807,61813 via svnmerge from svn+ssh://pythondev@svn.python.org/python/trunk ................ r61724 | martin.v.loewis | 2008-03-22 01:01:12 +0100 (Sat, 22 Mar 2008) | 49 lines Merged revisions 61602-61723 via svnmerge from svn+ssh://pythondev@svn.python.org/sandbox/trunk/2to3/lib2to3 ........ r61626 | david.wolever | 2008-03-19 17:19:16 +0100 (Mi, 19 M?\195?\164r 2008) | 1 line Added fixer for implicit local imports. See #2414. ........ r61628 | david.wolever | 2008-03-19 17:57:43 +0100 (Mi, 19 M?\195?\164r 2008) | 1 line Added a class for tests which should not run if a particular import is found. ........ r61629 | collin.winter | 2008-03-19 17:58:19 +0100 (Mi, 19 M?\195?\164r 2008) | 1 line Two more relative import fixes in pgen2. ........ r61635 | david.wolever | 2008-03-19 20:16:03 +0100 (Mi, 19 M?\195?\164r 2008) | 1 line Fixed print fixer so it will do the Right Thing when it encounters __future__.print_function. 2to3 gets upset, though, so the tests have been commented out. ........ r61637 | david.wolever | 2008-03-19 21:37:17 +0100 (Mi, 19 M?\195?\164r 2008) | 3 lines Added a fixer for itertools imports (from itertools import imap, ifilterfalse --> from itertools import filterfalse) ........ r61645 | david.wolever | 2008-03-19 23:22:35 +0100 (Mi, 19 M?\195?\164r 2008) | 1 line SVN is happier when you add the files you create... -_-' ........ r61654 | david.wolever | 2008-03-20 01:09:56 +0100 (Do, 20 M?\195?\164r 2008) | 1 line Added an explicit sort order to fixers -- fixes problems like #2427 ........ r61664 | david.wolever | 2008-03-20 04:32:40 +0100 (Do, 20 M?\195?\164r 2008) | 3 lines Fixes #2428 -- comments are no longer eatten by __future__ fixer. ........ r61673 | david.wolever | 2008-03-20 17:22:40 +0100 (Do, 20 M?\195?\164r 2008) | 1 line Added 2to3 node pretty-printer ........ r61679 | david.wolever | 2008-03-20 20:50:42 +0100 (Do, 20 M?\195?\164r 2008) | 1 line Made node printing a little bit prettier ........ r61723 | martin.v.loewis | 2008-03-22 00:59:27 +0100 (Sa, 22 M?\195?\164r 2008) | 2 lines Fix whitespace. ........ ................ r61725 | martin.v.loewis | 2008-03-22 01:02:41 +0100 (Sat, 22 Mar 2008) | 2 lines Install lib2to3. ................ r61731 | facundo.batista | 2008-03-22 03:45:37 +0100 (Sat, 22 Mar 2008) | 4 lines Small fix that complicated the test actually when that test failed. ................ r61732 | alexandre.vassalotti | 2008-03-22 05:08:44 +0100 (Sat, 22 Mar 2008) | 2 lines Added warning for the removal of 'hotshot' in Py3k. ................ r61733 | georg.brandl | 2008-03-22 11:07:29 +0100 (Sat, 22 Mar 2008) | 4 lines #1918: document that weak references *to* an object are cleared before the object's __del__ is called, to ensure that the weak reference callback (if any) finds the object healthy. ................ r61734 | georg.brandl | 2008-03-22 11:56:23 +0100 (Sat, 22 Mar 2008) | 2 lines Activate the Sphinx doctest extension and convert howto/functional to use it. ................ r61735 | georg.brandl | 2008-03-22 11:58:38 +0100 (Sat, 22 Mar 2008) | 2 lines Allow giving source names on the cmdline. ................ r61737 | georg.brandl | 2008-03-22 12:00:48 +0100 (Sat, 22 Mar 2008) | 2 lines Fixup this HOWTO's doctest blocks so that they can be run with sphinx' doctest builder. ................ r61739 | georg.brandl | 2008-03-22 12:47:10 +0100 (Sat, 22 Mar 2008) | 2 lines Test decimal.rst doctests as far as possible with sphinx doctest. ................ r61741 | georg.brandl | 2008-03-22 13:04:26 +0100 (Sat, 22 Mar 2008) | 2 lines Make doctests in re docs usable with sphinx' doctest. ................ r61743 | georg.brandl | 2008-03-22 13:59:37 +0100 (Sat, 22 Mar 2008) | 2 lines Make more doctests in pprint docs testable. ................ r61744 | georg.brandl | 2008-03-22 14:07:06 +0100 (Sat, 22 Mar 2008) | 2 lines No need to specify explicit "doctest_block" anymore. ................ r61753 | georg.brandl | 2008-03-22 21:08:43 +0100 (Sat, 22 Mar 2008) | 2 lines Fix-up syntax problems. ................ r61761 | georg.brandl | 2008-03-22 22:06:20 +0100 (Sat, 22 Mar 2008) | 4 lines Make collections' doctests executable. (The <BLANKLINE>s will be stripped from presentation output.) ................ r61765 | georg.brandl | 2008-03-22 22:21:57 +0100 (Sat, 22 Mar 2008) | 2 lines Test doctests in datetime docs. ................ r61766 | georg.brandl | 2008-03-22 22:26:44 +0100 (Sat, 22 Mar 2008) | 2 lines Test doctests in operator docs. ................ r61767 | georg.brandl | 2008-03-22 22:38:33 +0100 (Sat, 22 Mar 2008) | 2 lines Enable doctests in functions.rst. Already found two errors :) ................ r61769 | georg.brandl | 2008-03-22 23:04:10 +0100 (Sat, 22 Mar 2008) | 3 lines Enable doctest running for several other documents. We have now over 640 doctests that are run with "make doctest". ................ r61773 | raymond.hettinger | 2008-03-23 01:55:46 +0100 (Sun, 23 Mar 2008) | 1 line Simplify demo code. ................ r61776 | neal.norwitz | 2008-03-23 04:43:33 +0100 (Sun, 23 Mar 2008) | 7 lines Try to make this test a little more robust and not fail with: timeout (10.0025) is more than 2 seconds more than expected (0.001) I'm assuming this problem is caused by DNS lookup. This change does a DNS lookup of the hostname before trying to connect, so the time is not included. ................ r61777 | neal.norwitz | 2008-03-23 05:08:30 +0100 (Sun, 23 Mar 2008) | 1 line Speed up the test by avoiding socket timeouts. ................ r61778 | neal.norwitz | 2008-03-23 05:43:09 +0100 (Sun, 23 Mar 2008) | 1 line Skip the epoll test if epoll() does not work ................ r61780 | neal.norwitz | 2008-03-23 06:47:20 +0100 (Sun, 23 Mar 2008) | 1 line Suppress failure (to avoid a flaky test) if we cannot connect to svn.python.org ................ r61781 | neal.norwitz | 2008-03-23 07:13:25 +0100 (Sun, 23 Mar 2008) | 4 lines Move itertools before future_builtins since the latter depends on the former. From a clean build importing future_builtins would fail since itertools wasn't built yet. ................ r61782 | neal.norwitz | 2008-03-23 07:16:04 +0100 (Sun, 23 Mar 2008) | 1 line Try to prevent the alarm going off early in tearDown ................ r61783 | neal.norwitz | 2008-03-23 07:19:57 +0100 (Sun, 23 Mar 2008) | 4 lines Remove compiler warnings (on Alpha at least) about using chars as array subscripts. Using chars are dangerous b/c they are signed on some platforms and unsigned on others. ................ r61788 | georg.brandl | 2008-03-23 09:05:30 +0100 (Sun, 23 Mar 2008) | 2 lines Make the doctests presentation-friendlier. ................ r61793 | amaury.forgeotdarc | 2008-03-23 10:55:29 +0100 (Sun, 23 Mar 2008) | 4 lines #1477: ur'\U0010FFFF' raised in narrow unicode builds. Corrected the raw-unicode-escape codec to use UTF-16 surrogates in this case, just like the unicode-escape codec. ................ r61796 | raymond.hettinger | 2008-03-23 14:32:32 +0100 (Sun, 23 Mar 2008) | 1 line Issue 1681432: Add triangular distribution the random module. ................ r61807 | raymond.hettinger | 2008-03-23 20:37:53 +0100 (Sun, 23 Mar 2008) | 4 lines Adopt Nick's suggestion for useful default arguments. Clean-up floating point issues by adding true division and float constants. ................ r61813 | gregory.p.smith | 2008-03-23 22:04:43 +0100 (Sun, 23 Mar 2008) | 6 lines Fix gzip to deal with CRC's being signed values in Python 2.x properly and to read 32bit values as unsigned to start with rather than applying signedness fixups allover the place afterwards. This hopefully fixes the test_tarfile failure on the alpha/tru64 buildbot. ................
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:const:`None`:
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>>> del o, o2
>>> print(r())
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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.")
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else:
print("Object is still live!")
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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):
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super(ExtendedRef, self).__init__(ob, callback)
self.__counter = 0
for k, v in annotations.items():
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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 object IDs to retrieve
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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.
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::
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]
.. _finalize-examples:
Finalizer Objects
-----------------
The main benefit of using :class:`finalize` is that it makes it simple
to register a callback without needing to preserve the returned finalizer
object. For instance
>>> import weakref
>>> class Object:
... pass
...
>>> kenny = Object()
>>> weakref.finalize(kenny, print, "You killed Kenny!") #doctest:+ELLIPSIS
<finalize object at ...; for 'Object' at ...>
>>> del kenny
You killed Kenny!
The finalizer can be called directly as well. However the finalizer
will invoke the callback at most once.
>>> def callback(x, y, z):
... print("CALLBACK")
... return x + y + z
...
>>> obj = Object()
>>> f = weakref.finalize(obj, callback, 1, 2, z=3)
>>> assert f.alive
>>> assert f() == 6
CALLBACK
>>> assert not f.alive
>>> f() # callback not called because finalizer dead
>>> del obj # callback not called because finalizer dead
You can unregister a finalizer using its :meth:`~finalize.detach`
method. This kills the finalizer and returns the arguments passed to
the constructor when it was created.
>>> obj = Object()
>>> f = weakref.finalize(obj, callback, 1, 2, z=3)
>>> f.detach() #doctest:+ELLIPSIS
(<...Object object ...>, <function callback ...>, (1, 2), {'z': 3})
>>> newobj, func, args, kwargs = _
>>> assert not f.alive
>>> assert newobj is obj
>>> assert func(*args, **kwargs) == 6
CALLBACK
Unless you set the :attr:`~finalize.atexit` attribute to
:const:`False`, a finalizer will be called when the program exits if it
is still alive. For instance
.. doctest::
:options: +SKIP
>>> obj = Object()
>>> weakref.finalize(obj, print, "obj dead or exiting")
<finalize object at ...; for 'Object' at ...>
>>> exit()
obj dead or exiting
Comparing finalizers with :meth:`__del__` methods
-------------------------------------------------
Suppose we want to create a class whose instances represent temporary
directories. The directories should be deleted with their contents
when the first of the following events occurs:
* the object is garbage collected,
* the object's :meth:`remove` method is called, or
* the program exits.
We might try to implement the class using a :meth:`__del__` method as
follows::
class TempDir:
def __init__(self):
self.name = tempfile.mkdtemp()
def remove(self):
if self.name is not None:
shutil.rmtree(self.name)
self.name = None
@property
def removed(self):
return self.name is None
def __del__(self):
self.remove()
Starting with Python 3.4, :meth:`__del__` methods no longer prevent
reference cycles from being garbage collected, and module globals are
no longer forced to :const:`None` during :term:`interpreter shutdown`.
So this code should work without any issues on CPython.
However, handling of :meth:`__del__` methods is notoriously implementation
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specific, since it depends on internal details of the interpreter's garbage
collector implementation.
A more robust alternative can be to define a finalizer which only references
the specific functions and objects that it needs, rather than having access
to the full state of the object::
class TempDir:
def __init__(self):
self.name = tempfile.mkdtemp()
self._finalizer = weakref.finalize(self, shutil.rmtree, self.name)
def remove(self):
self._finalizer()
@property
def removed(self):
return not self._finalizer.alive
Defined like this, our finalizer only receives a reference to the details
it needs to clean up the directory appropriately. If the object never gets
garbage collected the finalizer will still be called at exit.
The other advantage of weakref based finalizers is that they can be used to
register finalizers for classes where the definition is controlled by a
third party, such as running code when a module is unloaded::
import weakref, sys
def unloading_module():
# implicit reference to the module globals from the function body
weakref.finalize(sys.modules[__name__], unloading_module)
.. note::
If you create a finalizer object in a daemonic thread just as the program
exits then there is the possibility that the finalizer
does not get called at exit. However, in a daemonic thread
:func:`atexit.register`, ``try: ... finally: ...`` and ``with: ...``
do not guarantee that cleanup occurs either.