When class instances are unpickled, their _init_() method isn't normally called again.
当 unpickle 类的实例时，通常不会再调用它们的 _init_() 方法。
2
First, leave the original class definition intact so that it can be found when existing instances are unpickled.
首先，确保原来的类的定义没有被更改，以便在unpickle现有实例时可以找到它。
3
Once you are sure that all the existing instances have been unpickled, updated, and re-pickled, you can remove the old class definition from the source code module.