python之生成器

廖雪峰教程：http://www.liaoxuefeng.com/wiki/001374738125095c955c1e6d8bb493182103fac9270762a000/00138681965108490cb4c13182e472f8d87830f13be6e88000

http://wklken.me/posts/2013/08/20/python-extra-itertools.html

http://www.cnblogs.com/cython/articles/2169009.html

http://pyzh.readthedocs.org/en/latest/the-python-yield-keyword-explained.html