python序列的切片操作

切片操作符：[] [:] [::]

sequence[index]

sequence[starting_index : ending_index]

sequence[starting_index : ending_index : step ]

>>> a='abcdefgh'
>>> a[::0]
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: slice step cannot be zero
>>> a[::-1]
'hgfedcba'
>>> a[::-2]
'hfdb'
>>> a[::-3]
'heb'
>>> a[::-4]
'hd'
>>> a[::2]
'aceg'
>>> a[::3]
'adg'
>>> a='abcde'
>>> print range(-1,-len(a),-1)
[-1, -2, -3, -4]
>>> a[:-1]
'abcd'
>>> a[:-2]
'abc'
>>> a[:-3]
'ab'
>>>

负数索引从最后一个元素开始。

>>> for i in range(-1, -len(a), -1):
...     print a[:i]
...
abcd
abc
ab
a
>>> for i in [None] + range(-1, -len(a), -1):
...     print a[:i]
...
abcde
abcd
abc
ab
a
>>>