序列类型是其元素被顺序放置的一种数据结构类型，这种方式允许通过下标的方式来获得某一个数据元素，或者通过指定下标范围来获得一组序列的元素。这种访问序列的方式叫做切片。字符串也可以使用切片操作。切片操作符：[] [:] [::]，调用内置函数slice()函数。
以字符串'abcdefg'为例：
|
s |
a |
b |
c |
d |
e |
f |
g |
|
index |
0 |
1 |
2 |
3 |
4 |
5 |
6 |
|
index |
-7 |
-6 |
-5 |
-4 |
-3 |
-2 |
-1 |
一、sequence[index]
类似于其他语言的数组操作。sequence是序列的名字，index是想要访问的元素对应的偏移量。偏移量正负都可以，-len(sequence)<=index<=len(sequence)-1。正索引以序列的开始为起点，负索引以序列的结束为起点。
试图访问一个越界的索引会引发一个IndexError异常。
二、sequence[starting_index:ending_index]
通过这种方式可以得到从起始索引到结束索引（不包括结束索引所对应的元素）之间的元素，起始索引和结束索引都是可选的，如果没有提供或者用None作为索引值，切片操作会从序列的最开始处开始，或者直到序列的最末尾结束。其中，开始和结束的索引值可以超出字符串长度，如：s[-100:100]，返回'abcdefg'.
用一个：时，starting_index应该小于ending_index，否则返回空字符串。
# 返回整个序列，即 'abcdefg'
s
s[:] # 返回 'abc'
s[0:3]
s[:3] # 返回 'cde'
s[2:5]
s[:5] # 返回 'def'
s[-4:-1]
三、sequence[starting_index:ending_index:step]
扩展切片操作，step为步长参数，类似range()里的步长参数。
得到的序列从starting_index（包含starting_index）开始，每次以步长前进，即starting_index + step，直到ending_index结束。
执行：
# 返回 'fedc'
s = 'abcdefg'
s[-2:-6:-1]
(1) starting_index = -2 ending_index = -6 step = -1
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(2) 第一个读sequence[-2]，即 'f'，第二个读starting_index + step，即 -2+(-1)=-3，sequence[-3]，也就是 'e'。
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37kQRjMB0ZIhUmCebkvsDlvv1IgjGYjgyRCpMEc3Jf4HLffiTBGExHhkiFSYI5uS9wuW8/kmAMpiNDpMIkwZzcF7jctx9JMAbTkSFSYZJgTu4LXO7bjyQYg+nIEKkwSTAn9wUu9+1HEozBdGSIVJgkmJP7Apf79iMJxmA6MkQqTBLMyX2By337kQRjMB0ZIhUmCebkvsDlvv1IgjGYjgyRCpMEc3Jf4HLffiSxQAraTyMKwSTBHO3VNQHtCAEAOyzKAACgZHQdAABQMroOAAAoGV0HAACUjK4DAABKRtcBAAAlo+sAAICS0XUAAEDJ/g+2W5sS2osQ5AAAAABJRU5ErkJggg==" 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(3) 按这个顺序继续读，不包括ending_index，即'b'，所以s[-2:-6:-1]返回 'fedc'。
同理，输出s[2:5:2]，返回 'ce'。
有一个经常用到的应用：翻转字符串
# 输出 'gfedcba'
1 s =s'abcdefg'
print s[::-1]