python列表切片和嵌套列表取值操作 https://blog.csdn.net/weixin_38082364/article/details/79833348?utm_medium=distribute.pc_relevant.none-task-blog-BlogCommendFromMachineLearnPai2-1.nonecase&depth_1-utm_source=distribute.pc_relevant.none-task-blog-BlogCommendFromMachineLearnPai2-1.nonecase