New in version 2.1.
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.
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: Added support for files, sockets, arrays, and patterns.
Several builtin types such as list and 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 referencable
Extension types can easily be made to support weak references; see ``Weak Reference Support'' in Extending and Embedding the Python Interpreter.
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 __del__() method.
Weak references are hashable if the object is hashable. They will maintain their hash value even after the object was deleted. If hash() is called the first time only after the object was deleted, the call will raise 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.
Changed in version 2.4: This is now a subclassable type rather than a factory function; it derives from object.
CallableProxyType, depending on whether object is callable. Proxy objects are not 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 ref() function.
Note: Caution: Because a 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 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).
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.
Note: Caution: Because a 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 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).
WeakValueDictionary objects have the following additional methods. These method have the same issues as the iterkeyrefs() and keyrefs() methods of WeakKeyDictionary objects.