11.6. 弱引用

Python 自动进行内存管理（对大多数的对象进行引用计数和垃圾回收—— garbage collection ——以循环利用）在最后一个引用消失后，内存会很快释放。

这个工作方式对大多数应用程序工作良好，但是偶尔会需要跟踪对象来做一些事。不幸的是，仅仅为跟踪它们创建引用也会使其长期存在。 weakref 模块提供了不用创建引用的跟踪对象工具，一旦对象不再存在，它自动从弱引用表上删除并触发回调。典型的应用包括捕获难以构造的对象:

>>> import weakref, gc
>>> class A:
...     def __init__(self, value):
...             self.value = value
...     def __repr__(self):
...             return str(self.value)
...
>>> a = A(10)                   # create a reference
>>> d = weakref.WeakValueDictionary()
>>> d['primary'] = a            # does not create a reference
>>> d['primary']                # fetch the object if it is still alive
10
>>> del a                       # remove the one reference
>>> gc.collect()                # run garbage collection right away
0
>>> d['primary']                # entry was automatically removed
Traceback (most recent call last):
  File "", line 1, in 
    d['primary']                # entry was automatically removed
  File "C:/python33/lib/weakref.py", line 46, in __getitem__
    o = self.data[key]()
KeyError: 'primary'