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#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.
This commit is contained in:
Georg Brandl 2008-03-22 10:07:29 +00:00
parent ab68a3dc6f
commit 86f38c81ae
1 changed files with 24 additions and 16 deletions
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Doc/library/weakref.rst
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@ -26,18 +26,20 @@ 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.
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
@ -48,6 +50,12 @@ 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,
@ -134,11 +142,11 @@ Extension types can easily be made to support weak references; see
.. note::
Caution: Because a :class:`WeakKeyDictionary` is built on top of a Python
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).
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