python - 无法在 __del__() 中引用导入的模块

Question

我正在使用对象的 __del__() 取消订阅事件(使用类似于 this 的事件方案):

import my_enviroment
class MyClass():
    def __del__(self):
        my_environment.events.my_event -= self.event_handler_func

奇怪的是，我在程序运行结束时收到以下错误:

Exception AttributeError: "'NoneType' object has no attribute 'events'" in <bound method MyClass.__del__ of <myclass.MyClass instance at 0x04C54580>> ignored

这怎么可能？! my_environment 是我导入的模块，怎么会是None呢？ (events是一个全局对象，里面有my_event等事件钩子(Hook))

Answer 1

根据python doc about __del__ :

[...] other globals referenced by the __del__() method may already have been deleted or in the process of being torn down (e.g. the import machinery shutting down). For this reason, __del__() methods should do the absolute minimum needed to maintain external invariants.

换句话说，当你的对象调用__del__方法时，my_enviroment可能已经被python“删除”了，所以它可以是None...