Python 自動進行內存管理（對大多數的對象進行引用計數和垃圾回收——?_garbage collection_?——以循環利用）在最后一個引用消失后，內存會很快釋放。 這個工作方式對大多數應用程序工作良好，但是偶爾會需要跟蹤對象來做一些事。不幸的是，僅僅為跟蹤它們創建引用也會使其長期存在。?weakref?模塊提供了不用創建引用的跟蹤對象工具，一旦對象不再存在，它自動從弱引用表上刪除并觸發回調。典型的應用包括捕獲難以構造的對象: ~~~ >>> 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' ~~~