New in version 2.1.
The weakref module allows the Python programmer to create weak references to objects.
In the discussion which follows, the term referent means the object which is referred to by a weak reference.
XXX -- need to say more here!
Not all objects can be weakly referenced; those objects which can include class instances, functions written in Python (but not in C), and methods (both bound and unbound). Extension types can easily be made to support weak references; see section 3.3.3, ``Weak References in Extension Types,'' for more information.
Noneto be returned. If callback is provided, it 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 __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.
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.