http://python3-cookbook.readthedocs.io/zh_CN/latest/chapters/p08_classes_and_objects.html
描述器
https://www.jianshu.com/p/58f1df955bda
https://segmentfault.com/a/1190000008994270
http://pyzh.readthedocs.io/en/latest/Descriptor-HOW-TO-Guide.html
__dict__
https://www.jianshu.com/p/cf8450b4b537
经典类和新式类