The weakref module allows the Python programmer to create weak
references to objects.
In the following, the term 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, 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
WeakKeyDictionary and WeakValueDictionary classes supplied by
the 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
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.
WeakKeyDictionary and 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 weakref module for the
benefit of advanced uses.
Note
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 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, generators, type objects,
DBcursor objects from the bsddb module, sockets, arrays, deques,
and regular expression pattern objects.
Changed in version 2.4:
Changed in version 2.4: Added support for files, sockets, arrays, and patterns.