# 3.3 `weakref` -- Weak references

New in version 2.1.

The `weakref` module allows the Python programmer to create weak references to objects.

XXX -- need to say more here!

Not all objects can be weakly referenced; those objects which do 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.

`ref`(object[, callback])
  Return a weak reference to object. 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. The original object can be retrieved by calling the reference object, if the referent is still alive.

  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 test for equality, but not ordering. If the object is still alive, to references are equal if the objects are equal (regardless of the callback). If the object has been deleted, they are equal iff they are identical.

`proxy`(object[, callback])
  Return a proxy to object 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 either `ProxyType` or `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. callable is the same as the parameter of the same name to the `ref()` function.

`getweakrefcount`(object)
  Return the number of weak references and proxies which refer to object.

`getweakrefs`(object)
  Return a list of all weak reference and proxy objects which refer to object.

class `WeakKeyDictionary`([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.

class `WeakValueDictionary`([dict])
  Mapping class that references values weakly. Entries in the dictionary will be discarded when no strong reference to the value exists anymore.

`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.

exception `ReferenceError`
  Exception raised when a proxy object is used but the underlying object has been collected.

See Also:

PEP 0205, Weak References
  The proposal and rationale for this feature, including links to earlier implementations and information about similar features in other languages.

Subsections

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