python报错Nonetype object is not iterable
https://www.cnblogs.com/zhaijiahui/p/8391701.html
参考链接：http://blog.csdn.net/dataspark/article/details/9953225
https://www.cnblogs.com/zhaijiahui/p/8391701.html
参考链接：http://blog.csdn.net/dataspark/article/details/9953225
