字符串、列表、元组(tuple)在python中都符合“序列”这一特征，符合“序列”特征的对象都可以使用切片(slice)去访问它们的任意部分，切片操作符在python中的原型是：

[start:stop:step]，    表示为[开始索引:结束索引:步长]

开始索引：从0开始，序列从左向右累增计数，最一个值的索引记为-1；

结束索引：切片操作到此为止，不包含该索引的值；

步长：步长表示切片的间隔，默认步长为1，如果步长为负数，则表示从右向左取，步长不能为0；

常规切片操作>>> L = ["A","B","C","D","E"]

>>> L[::]

["A","B","C","D","E"]

>>> L[0:-1:1]

["A","B","C","D"]

>>> L[0:5]

["A","B","C","D","E"]

>>> L[::2]

["A","C","E"]

>>> L[0:100:2]

["A","C","E"]

>>> L[::-1]