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]
['E','D','C','B','A']
>>> L[4:0:-1]
['E','D','C','B']
>>> L[:]
['A','B','C','D','E']
>>> L[-3:-1]
['C','D']
>>> L[-3:]
['C','D','E']使用切片操作对列表插入和修改元素
>>> L = ['A','B','C','D','E']
>>> L[2:0] = ['F']
['A','B','F','C','D','E']
>>> L = ['A','B','C','D','E']
>>> L[2:1]=['F']
['A','B','F','C','D','E']
>>> L = ['A','B','C','D','E']
>>> L[2:2]=['F']
['A','B','F','C','D','E']
>>> L = ['A','B','C','D','E']
>>> L[2:3]=['F']
['A','B','F','D','E']
>>> L = ['A','B','C','D','E']
>>> L[2:4]=['F']
['A','B','F','E']
>>> L = ['A','B','C','D','E']
>>> L[2:5]=['F']
['A','B','F']删除切片
>>> L = ['A','B','C','D','E']
>>> del L[2:2]
['A','B','C','D','E']
>>> L = ['A','B','C','D','E']
>>> del L[2:3]
['A','B','D','E']
>>> L = ['A','B','C','D','E']
>>> del L[2:-1]
['A','B','E']
>>> L = ['A','B','C','D','E']
>>> del L[2:]
['A','B']面对大河我无限惭愧
我年华虚度 空有一身疲倦
和所有以梦为马的诗人一样
岁月易逝 一滴不剩