Python 的插入返回 None? [英] Python's insert returning None?

问题描述

#!/usr/bin/python

numbers = [1, 2, 3, 5, 6, 7]

clean = numbers.insert(3, 'four')

print clean
# desire results [1, 2, 3, 'four', 5, 6, 7]

我得到无".我做错了什么?

I am getting "None". What am I doing wrong?

推荐答案

列表上的变异方法往往会返回 None，不是 修改后的列表，如您所料 --此类方法通过就地更改列表来实现其效果，而不是通过构建和返回一个新列表.因此，print numbers 而不是 print clean 将显示更改后的列表.

Mutating-methods on lists tend to return None, not the modified list as you expect -- such metods perform their effect by altering the list in-place, not by building and returning a new one. So, print numbers instead of print clean will show you the altered list.

如果您需要保持 numbers 不变，请先制作副本，然后更改副本:

If you need to keep numbers intact, first you make a copy, then you alter the copy:

clean = list(numbers)
clean.insert(3, 'four')

这具有您似乎想要的整体效果:numbers 不变，clean 是更改后的列表.

this has the overall effect you appear to desire: numbers is unchanged, clean is the changed list.

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