Python中[:]与[::]的用法

巨輪 2020-11-09 22:37:40
Python highlight sequence


Python中[:]与[::]的用法

概述

[:]与[::]语法是通用序列操作(Common Sequence Operations)其中的两个。用[:]或[::]对多数序列类型(可变的或不可变的)(如字符串、列表等)序列中元素进行截取。

[:]的用法

s[i:j]

从下标i到下标j,截取序列s中的元素。

  1. 如果i或j为负数,则下标从序列尾部算起,也就是说,下标i相当于下标len(s)+i,下标j相当于下标len(s)+j。另外,-0依然是0。
  2. 从序列s的下标i的元素(包含)到下标j的元素(不包含)之间所有元素,进行截取。另外,i < j。
    • 若i或j大于len(s),则i或j被len(s)取代。
    • 若i省略或i是None值,则i被0取代。
    • 若j省略或j是None值,则j被len(s)取代。
    • 若i>=j,返回没元素的序列。
>>> a=[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> a[-1:]
[9]
>>> a[-9:-1]
[1, 2, 3, 4, 5, 6, 7, 8]
>>> a[1:20]
[1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> a[:9]
[0, 1, 2, 3, 4, 5, 6, 7, 8]
>>> a[0:]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> a[9:3]
[]
>>>

[::]的用法

s[i:j:k]

从下标i到下标j,步数为k,截取序列s中的元素。

  1. 如果i或j为负数,则下标从序列尾部算起,也就是说,下标i相当于下标len(s)+i,下标j相当于下标len(s)+j。另外,-0依然是0。
  2. 从序列s的下标i的元素(包含)到下标j的元素(不包含)之间,选取下标为i+n*k的元素(0 <= n < (j-i)/k),也就是说,选取下标为i, i+k, i +2*k, i+3*k等等的元素,直到下标大于等于j为止。
    • 当k为正数,下标选取反向从0到len(s)-1。
    • 当k为负数,下标选取反向从len(s)-1到0。
    • 当i或j省略或是None值,则被端下标(0或len(s)-1)所取代(具体看k的正负符号)。
    • k不能为0。若k为None值,则将k看成1。
>>> a = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> a[1:10:2]
[1, 3, 5, 7, 9]
>>> a[1:10:3]
[1, 4, 7]
>>> a[1:10:-2]
[]
>>> a[1:10:-3]
[]
>>> a[10:1:-2]
[9, 7, 5, 3]
>>> a[10:1:-3]
[9, 6, 3]
>>> a[::2]
[0, 2, 4, 6, 8]
>>> a[::-2]
[9, 7, 5, 3, 1]
>>> a[::-1]
[9, 8, 7, 6, 5, 4, 3, 2, 1, 0]
>>> a[-9:-1:2]
[1, 3, 5, 7]
>>> a[-9:-1:-2]
[]
>>> a[-1:-9:-2]
[9, 7, 5, 3]
>>> a[-1::-2]
[9, 7, 5, 3, 1]
>>> a[:-1:2]
[0, 2, 4, 6, 8]
>>>

参考资料

  1. Common Sequence Operations
  2. python 中的[:-1]和[::-1]
  3. What is :: (double colon) in Python when subscripting sequences?
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