python中如何提取列表的前几个元素和后几个元素

>>> a = list(range(1,21))  ##生成列表
>>> a
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
>>> a[:10] ## 提取前10个元素
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
>>> a[-10:]  ## 提取后10个元素
[11, 12, 13, 14, 15, 16, 17, 18, 19, 20]