python - <<Fluent python>> P372 例题8-19 为什么只存在最后一个元素？

Question

import weakref

class Cheese:
  def __init__(self,kind):
    self.kind = kind

  def __repr__(self):
    return "cheese {}".format(self.kind)



stock = weakref.WeakValueDictionary()
catalog = [Cheese("red"),Cheese("block"),Cheese("lock"),Cheese("hehe")]
for cheese in  catalog:
    stock[cheese.kind] = cheese

print(sorted(stock.keys()))
del catalog
print(sorted(stock.keys()))

结果是:

['block', 'hehe', 'lock', 'red']
['hehe']

问题:为什么结果不是:

['block', 'hehe', 'lock', 'red']
['block', 'hehe', 'lock', 'red']

Answer 1

那是因为您正在为 stock 使用 weakref.WeakValueDictionary()。来自docs :

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.

因此销毁 catalog 允许垃圾收集器销毁 stock 内容。

只有剩余元素是最后一个元素，因为它被最后一个循环项引用为 cheese

如果您执行 del cheese，则第二个打印输出完全是空的。