QuickSort实现可以对列表进行排序不会更新列表

Question

I made an implementation of the quicksort algortihm in python3.6 using recursion. It sorts the list in place in increasing order. The problem however is that the list elements order do not change in the code and after the code is done running

If you remove the base case for the recursion and let it fail on maximum recursion depth reached and put print statements in the partition method you can see it changing the elements in the list

def partition(arr, start, end):
    pivot = arr[end]
    ix = start
    for i in range(start, end):
        print("i = ", i)
        if arr[i] <= pivot:
            arr[i], arr[ix] = arr[ix], arr[i]
            ix += 1
    arr[ix], arr[end] = arr[end], arr[ix]
    return ix

def quick_sort(arr, start, end):
    if start < end: return arr
    ix = partition(arr, start, end)
    quick_sort(arr, start, ix-1)
    quick_sort(arr, ix+1, end)

arr = [2,4,7,8,9,1,3,5,6,12,32]

print("before", arr)
print("output", quick_sort(arr, 0, len(ans)-1))
print("after", arr)

OUTPUT 
before [2, 4, 7, 8, 9, 1, 3, 5, 6, 12, 32]
output [2, 4, 7, 8, 9, 1, 3, 5, 6, 12, 32]
after [2, 4, 7, 8, 9, 1, 3, 5, 6, 12, 32]

Answer 1

Here:

if start < end: return arr

The logic is reversed. It should be

if start > end: return arr

However, because your function operates in-place, it shouldn't return anything, so it's best to make it

if start > end: return

You will see that quick_sort(arr, 0, len(ans)-1) will return None , but after the function call, arr will be sorted.