Python中的弱引用

weakref模块具有的方法

class weakref.ref(object[, callback])
创建一个弱引用对象，object是被引用的对象，callback是回调函数（当被引用对象被删除时的，会调用改函数）。

weakref.proxy(object[, callback])
创建一个用弱引用实现的代理对象，参数同上。

weakref.getweakrefcount(object)
获取对象object关联的弱引用对象数

weakref.getweakrefs(object)
获取object关联的弱引用对象列表

class weakref.WeakKeyDictionary([dict])
创建key为弱引用对象的字典

class weakref.WeakValueDictionary([dict])
创建value为弱引用对象的字典

class weakref.WeakSet([elements])
创建成员为弱引用对象的集合对象

### 创建弱引用
>>> from socket import *
>>> import weakref
>>> s=socket(AF_INET,SOCK_STREAM)
>>> ref=weakref.ref(s)
>>> s
<socket._socketobject instance at 007B4A94>
>>> ref
<weakref at 0x81195c; to 'instance' at 0x7b4a94> 
>>> ref()    #调用它来访问被引用的对象
<socket.socketobject instance at 007B4A94>
一旦没有了对这个对象的其它的引用，调用弱引用将返回None，因为Python已经销毁了这个对象。 注意：大部分的对象不能通过弱引用来访问。
### 创建代理对象
>>> from socket import*
>>> import weakref
>>> s=socket(AF_INET,SOCK_STREAM)
>>> ref=weakref.proxy(s)
>>> s
<socket._socketobject instance at 007E4874>
>>> ref 
<socket._socketobject instance at 007E4874>
>>> ref.close() #对象的方法同样工作

例子

创建缓存实例
当创建类实例时，我们想反悔一个缓存引用，让其指向上一个用同样参数（如果有的话）创建出的类实例

import sys
class Spam:
    def __init__(self, name):
        self.name = name
import weakref
_spam_cache = weakref.WeakValueDictionary()
def get_spam(name):
    if name not in _spam_cache:
        s = Spam(name)
        _spam_cache[name] = s
    else:
        s = _spam_cache[name]
    return s
if __name__ == '__main__':
    a = get_spam('foo')
    b = get_spam('bar')
    print(sys.getrefcount(a))
    print('a is b:', a is b)
    c = get_spam('foo')
    print(sys.getrefcount(a))
    print('a is c:', a is c)
    # 2
    # a is b: False
    # 3
    # a is c: True
    print(list(_spam_cache))
    del a
    print(list(_spam_cache))
    del c
    print(list(_spam_cache))
    # ['foo', 'bar']
    # ['foo', 'bar']
    # ['bar']

封装

import weakref
class CachedSpamManager:
    def __init__(self):
        self._cache = weakref.WeakValueDictionary()
    def get_spam(self, name):
        if name not in self._cache:
            s = Spam(name)
            self._cache[name] = s
        else:
            s = self._cache[name]
        return s
class Spam:
    def __init__(self, name):
        self.name = name
Spam.manager = CachedSpamManager()
def get_spam(name):
    return Spam.manager.get_spam(name)
if __name__ == '__main__':
    a = get_spam('foo')
    b = get_spam('bar')
    print('a is b:', a is b)
    c = get_spam('foo')
    print('a is c:', a is c)