python - How to mutate list permanently?

Question

def deep_reverse(L) :
    for i in range (len(L)-1,-1,-1):
        L=L[::-1]
    for i in range (len(L)-1,-1,-1):
        L[i]=L[i][::-1]  

L = [[0, 1, 2], [1, 2, 3], [3, 2, 1], [10, -10, 100]]
deep_reverse(L) 
print(L)

So I have this code here and a function called deep_reverse(L). I am trying to mutate the list L, at the end when i print(L) it prints out L = [[0, 1, 2], [1, 2, 3], [3, 2, 1], [10, -10, 100]]. However, when I print from within the function, I get L= [[2, 1, 0], [3, 2, 1], [1, 2, 3], [100, -10, 10]], which is what I want. How can i permanently mutate L so that it stays that way when I print it outside the function.

question from:https://stackoverflow.com/questions/65928411/how-to-mutate-list-permanently

Answer 1

You're not mutating anything. The function creates a new list and assigns it back to the local variable, which has no effect on the caller's variable.

Also, your first for loop is repeatedly reversing L. If the list has an even number of elements, it will reverse it back to the original order.

Use slice assignment to modify the list in place.

def deep_reverse(L):
    L[:]=L[::-1]
    for i in range (len(L)-1,-1,-1):
        L[i][:]=L[i][::-1] 

Note also that your code only reverses two levels deep. Use recursion to handle any depth.

def deep_reverse(L):
    if isinstance(L, list):
        L[:] = L[::-1]
        for subL in L:
            deep_reverse(subL)