参考这篇:
http://python.jobbole.com/86086/?from=timeline&isappinstalled=1&nsukey=MWQG%2B7OI4FvdQWJFFqcknKLNQB6KCvaInsGYZQx2PTPdsy%2BYz%2FZvNGKi%2F1Kov%2FTk0P4N22Yq3C4U34br94xdrLB1faIfeUJS04b%2B55NzJdPjSrixo2l39E4ZT7V2Sai8tHU9rutJH35%2BGA0ZykqIFLnjUTFybzKfbyA5pDGnxJBvIGMi4E2%2FeKmlxu3hO4St