8.11. "weakref" — 弱引用
************************

2.1 新版功能.

**源码：** Lib/weakref.py

======================================================================

"weakref" 模块允许Python程序员创建对象的 *weak references* 。

在下文中，术语 *referent* 表示由弱引用引用的对象。

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.

例如，如果您有许多大型二进制图像对象，则可能希望将名称与每个对象关联起
来。如果您使用Python字典将名称映射到图像，或将图像映射到名称，则图像对
象将保持活动状态，因为它们在字典中显示为值或键。 "weakref" 模块提供的
"WeakKeyDictionary" 和 "WeakValueDictionary" 类可以替代Python字典，使
用弱引用来构造映射，这些映射不会仅仅因为它们出现在映射对象中而使对象保
持存活。例如，如果一个图像对象是 "WeakValueDictionary" 中的值，那么当
对该图像对象的剩余引用是弱映射对象所持有的弱引用时，垃圾回收可以回收该
对象并将其在弱映射对象中相应的条目删除。

"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,
*generator*s, type objects, "DBcursor" objects from the "bsddb"
module, sockets, arrays, deques, regular expression pattern objects,
and code objects.

在 2.4 版更改: Added support for files, sockets, arrays, and patterns.

在 2.7 版更改: 添加了对thread.lock，threading.Lock和代码对象的支持。

几个内建类型如 "list" 和 "dict" 不直接支持弱引用，但可以通过子类化添加
支持:

   class Dict(dict):
       pass

   obj = Dict(red=1, green=2, blue=3)   # this object is weak referenceable

**CPython implementation detail:** Other built-in types such as
"tuple" and "long" do not support weak references even when
subclassed.

Extension types can easily be made to support weak references; see
Weak Reference Support.

class weakref.ref(object[, callback])

   返回对 *对象* 的弱引用。如果原始对象仍然存活，则可以通过调用引用对
   象来检索原始对象；如果引用的原始对象不再存在，则调用引用对象将得到
   "None" 。如果提供了 *回调* 而且值不是 "None" ，并且返回的弱引用对象
   仍然存活，则在对象即将终结时将调用回调;弱引用对象将作为回调的唯一参
   数传递；指示物将不再可用。

   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.

   在 2.4 版更改: This is now a subclassable type rather than a
   factory function; it derives from "object".

weakref.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.

weakref.getweakrefcount(object)

   Return the number of weak references and proxies which refer to
   *object*.

weakref.getweakrefs(object)

   Return a list of all weak reference and proxy objects which refer
   to *object*.

class weakref.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.

   注解: 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.

WeakKeyDictionary.iterkeyrefs()

   Return an iterable of the weak references to the keys.

   2.5 新版功能.

WeakKeyDictionary.keyrefs()

   Return a list of weak references to the keys.

   2.5 新版功能.

class weakref.WeakValueDictionary([dict])

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

   注解: 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 methods have the same issues as the "iterkeyrefs()" and
"keyrefs()" methods of "WeakKeyDictionary" objects.

WeakValueDictionary.itervaluerefs()

   Return an iterable of the weak references to the values.

   2.5 新版功能.

WeakValueDictionary.valuerefs()

   Return a list of weak references to the values.

   2.5 新版功能.

class weakref.WeakSet([elements])

   Set class that keeps weak references to its elements.  An element
   will be discarded when no strong reference to it exists any more.

   2.7 新版功能.

weakref.ReferenceType

   The type object for weak references objects.

weakref.ProxyType

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

weakref.CallableProxyType

   The type object for proxies of callable objects.

weakref.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 weakref.ReferenceError

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

参见:

  **PEP 205** - Weak References
     The proposal and rationale for this feature, including links to
     earlier implementations and information about similar features in
     other languages.


8.11.1. Weak Reference Objects
==============================

Weak reference objects have no attributes or methods, but do allow the
referent to be obtained, if it still exists, by calling it:

>>> import weakref
>>> class Object:
...     pass
...
>>> o = Object()
>>> r = weakref.ref(o)
>>> o2 = r()
>>> o is o2
True

If the referent no longer exists, calling the reference object returns
"None":

>>> del o, o2
>>> print r()
None

Testing that a weak reference object is still live should be done
using the expression "ref() is not None".  Normally, application code
that needs to use a reference object should follow this pattern:

   # r is a weak reference object
   o = r()
   if o is None:
       # referent has been garbage collected
       print "Object has been deallocated; can't frobnicate."
   else:
       print "Object is still live!"
       o.do_something_useful()

Using a separate test for “liveness” creates race conditions in
threaded applications; another thread can cause a weak reference to
become invalidated before the weak reference is called; the idiom
shown above is safe in threaded applications as well as single-
threaded applications.

Specialized versions of "ref" objects can be created through
subclassing. This is used in the implementation of the
"WeakValueDictionary" to reduce the memory overhead for each entry in
the mapping.  This may be most useful to associate additional
information with a reference, but could also be used to insert
additional processing on calls to retrieve the referent.

This example shows how a subclass of "ref" can be used to store
additional information about an object and affect the value that’s
returned when the referent is accessed:

   import weakref

   class ExtendedRef(weakref.ref):
       def __init__(self, ob, callback=None, **annotations):
           super(ExtendedRef, self).__init__(ob, callback)
           self.__counter = 0
           for k, v in annotations.iteritems():
               setattr(self, k, v)

       def __call__(self):
           """Return a pair containing the referent and the number of
           times the reference has been called.
           """
           ob = super(ExtendedRef, self).__call__()
           if ob is not None:
               self.__counter += 1
               ob = (ob, self.__counter)
           return ob


8.11.2. 示例
============

This simple example shows how an application can use object IDs to
retrieve objects that it has seen before.  The IDs of the objects can
then be used in other data structures without forcing the objects to
remain alive, but the objects can still be retrieved by ID if they do.

   import weakref

   _id2obj_dict = weakref.WeakValueDictionary()

   def remember(obj):
       oid = id(obj)
       _id2obj_dict[oid] = obj
       return oid

   def id2obj(oid):
       return _id2obj_dict[oid]
