切片

s = 'abcdefgh'
s[::-1]     #翻转操作
s[::2]      #隔一个取一个

s = 'abcde'
i = -1
for i in range(-1, -len(s), -1):
    print s[:i]

s = 'abcde'
for i in [None] + range(-1, -len(s), -1):
    print s[:i]

摘自《Python核心编程》