//占位,缓缓写完
http://www.xuebuyuan.com/2117676.html
迭代器与生成器http://www.cnblogs.com/wilber2013/p/4652531.html
https://github.com/lzjun567/note/blob/master/note/python/iterator_generator.md
http://pyzh.readthedocs.org/en/latest/the-python-yield-keyword-explained.html