这是python的难点和特点, 有很多的和c/cpp不一样的地方. 它的灵活易用性造成不习惯之处.
refer to:
http://blog.csdn.net/cc7756789w/article/details/46635383
https://docs.lvrui.io/2016/07/12/Python%E7%9A%84%E5%8F%98%E9%87%8F%E4%BD%9C%E7%94%A8%E5%9F%9F/
http://www.cnblogs.com/frydsh/archive/2012/08/12/2602100.html