1.start end step

2.a[：]

>>> #字符串的切片
>>> a='abcdefghijklmn'
>>> a[2]
'c'
>>> a[1:5:1]
'bcde'
>>> #包头不包尾，不唱在最后
>>> a[1:5:2]
'bd'
>>> a[:]
'abcdefghijklmn'
>>> a[2:]
'cdefghijklmn'
>>> a[:2]
'ab'
>>> a[2:4]
'cd'
>>> #当操作数为负数时
>>> a='abcdefghijklmnopqrstuvwxyz'
>>> a[-3:]
'xyz'
>>> a[=8;-3]
SyntaxError: invalid syntax
>>> a[-8:-3]
'stuvw'
>>> a[::-1]
'zyxwvutsrqponmlkjihgfedcba'
>>>