def merge(arr, l, m, r):
    merged_arr = []
    i, j = 0, 0
    while (i < len(arr[l:m])) and (j < len(arr[m:r])):
        if arr[l+i] < arr[m+j]:
            merged_arr.append(arr[l+i])
            i += 1
        elif arr[l+i] >= arr[m+j]:
            merged_arr.append(arr[m+j])
            j += 1
    if i == len(arr[l:m]):
        merged_arr.extend(arr[(m+j):r])
    if j == len(arr[m:r]):
        merged_arr.extend(arr[(l+i):m])
    arr[l:r] = merged_arr
    return

def merge_sort(arr, l, r):
    if r > l:
        m = (l + r) // 2
        merge_sort(arr, l, m)
        merge_sort(arr, m+1, r)
        return merge(arr, l, m, r+1)

I am not sure what is wrong with my merge function. The following works well:
print(merge([1,4,7,8,9,20, 0,4,5,15,67,68], 0, 6,12))
However, this does not:
print(merge_sort([1, 5, 4, 2, 33, 4, 2, 44, 22, 0], 0, 10))