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Weak references
===============

Support for weak references and weak dictionaries.  This module was
written by Fred L. Drake, Jr. <fdrake@acm.org>.
This module was written by Neil Schemenauer <nas@arctrix.com>.
This module was written by Martin von L"owis
<martin@loewis.home.cs.tu-berlin.de>.
This manual section was written by Fred L. Drake, Jr. <fdrake@acm.org>.
_Added in Python 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 Note: fpectl, "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.  CALLBACK 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.

`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.

`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.

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

See also:
    *PEP0205 Weak References*
          The proposal and rationale for this feature, including links
          to earlier implementations and information about similar
          features in other languages.

Weak Reference Objects

Example

Weak References in Extension Types