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Python Library Reference

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3.3weakref -- Weak references

New in version 2.1.

Theweakref module allows the Python programmer to createweak references to objects.

In the discussion which follows, the termreferent means theobject which is referred to by a weak reference.

XXX -- need to say more here!

Not all objects can be weakly referenced; those objects which doinclude class instances, functions written in Python (but not in C),and methods (both bound and unbound). Extension types can easilybe made to support weak references; see section3.3.3,``Weak References in Extension Types,'' for more information.

ref(object[, callback])

Return a weak reference toobject. 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 causeNone to be returned. Ifcallback 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 theobject is hashable. They will maintain their hash value even after theobject was deleted. Ifhash() is called the first time only after theobject was deleted, the call will raiseTypeError.

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 thecallback). If either referent has been deleted, the references are equal only if the reference objects are the same object.

proxy(object[, callback])

Return a proxy toobject 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 eitherProxyType orCallableProxyType, depending on whetherobject 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 theref() function.

getweakrefcount(object)

Return the number of weak references and proxies which refer toobject.

getweakrefs(object)

Return a list of all weak reference and proxy objects which refer toobject.

classWeakKeyDictionary([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.

classWeakValueDictionary([dict])

Mapping class that references values weakly. Entries in the dictionary will be discarded when no strong reference to the value exists any more.

ReferenceType

The type object for weak references objects.

ProxyType

The type object for proxies of objects which are not callable.

CallableProxyType

The type object for proxies of callable objects.

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.

exceptionReferenceError

Exception raised when a proxy object is used but the underlying object has been collected. This is the same as the standardReferenceError exception.

• 3.3.1 Weak Reference Objects
• 3.3.2 Example
• 3.3.3 Weak References in Extension Types

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Python Library Reference

Previous:3.2 gcUp:3. Python Runtime ServicesNext:3.3.1 Weak Reference Objects

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Release 2.2.3, documentation updated on 30 May 2003.