Python: On a function that does not return anything

Question

I was solving the Rotate Array leetcode problem:

Given an array, rotate the array to the right by k steps, where k is non-negative.

I tested the following code on my computer (and it seems to do the required job):

nums = [1,2,3,4,5,6,7]
k = 3
nums = nums[-k:] + nums[:len(nums)-k]
print(nums)

>> [5,6,7,1,2,3,4]

Therefore, I tried the following solution:

class Solution:
    def rotate(self, nums: List[int], k: int) -> None:
        nums = nums[-k:] + nums[:len(nums)-k]

However, getting the following test case wrong:

Actually, this test case was tested successfully when I ran without defining any function (which I provided above). Yet, really, it didn't work when I defined a special function to rotate. Then, I concluded that I might be doing something wrong when defining the function.

@S3DEV The problem states: `Do not return anything, modify nums in-place instead.` — VIVID, Oct 15 '20 at 09:07

score 2 · Accepted Answer · answered Oct 15 '20 at 09:06

2

nums = nums[-k:] + nums[:len(nums)-k]

just rebinds the local variable nums, it does not mutate the original object that nums was referring to before. Do the following instead:

nums[:] = nums[-k:] + nums[:len(nums)-k]

Since slice assignment is a mutation on the object referred to by nums, the rotation will affect the list object that was passed to the function.

answered Oct 15 '20 at 09:06

user2390182

72,016
6
67
89

Thank you, I didn't know about this syntax. Very helpful! – VIVID Oct 15 '20 at 09:12
@user2390182 what is that syntax ( nums[:] ) saying exactly? – 2bon2b Mar 08 '22 at 11:19
@2bon2b It is slice notation and creates a shallow copy. See more: https://stackoverflow.com/questions/509211/understanding-slice-notation – user2390182 Mar 08 '22 at 12:15

Python: On a function that does not return anything

1 Answers1