python协程1：yield的使用
https://mp.weixin.qq.com/s?__biz=MzAwNjI5MjAzNw==&mid=2655751983&idx=1&sn=e4c093c6e5d6e4e8281d76db7c67eb23
python协程2：yield from 从入门到精通
https://segmentfault.com/a/1190000009781688
Python yield 使用浅析 IBM社区分享
https://www.ibm.com/developerworks/cn/opensource/os-cn-python-yield/
Python 3.5 协程究竟是个啥
http://python.jobbole.com/86481/