python中下划线变量的规则和意义

https://michaelyou.github.io/2015/05/05/Python-%E4%B8%AD%E7%9A%84%E4%B8%8B%E5%88%92%E7%BA%BF%E5%91%BD%E5%90%8D%E8%A7%84%E5%88%99/

https://www.jianshu.com/p/a2213c3e1462