http://www.cnblogs.com/Xjng/p/3437694.html
http://www.codexiu.cn/python/blog/2729/
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http://www.jianshu.com/p/118e1c41e9f0
http://www.jianshu.com/p/1d09d14976d7
http://www.cnblogs.com/fnng/p/3565912.html
http://www.cnblogs.com/rollenholt/archive/2012/05/29/2524327.html
原文:http://www.cnblogs.com/newpython/p/6371749.html