I’m practicing merge sort with an exercise from a Youtube tutorial, this is it.
Im just doing the ‘descending = false’ version.

So at first I tried to solve it on my own by editing a block of code of merge sort that works for a list of numbers.

def merge_sort(arr, key, descending=False):
    if len(arr) <= 1:
        return arr

    mid = len(arr)//2

    left_list = merge_sort(arr[0:len(arr) // 2], key)
    right_list = merge_sort(arr[len(arr) // 2:], key)

    sorted_list = merge_two_sorted_lists(left_list, right_list)

    return sorted_list


def merge_two_sorted_lists(a,b):
    merged = []

    len_a = len(a)
    len_b = len(b)

    i = j = k = 0

    while i < len_a and j < len_b:
        if a[i]['time_hours'] <= b[j]['time_hours']:
            merged.append(a[i])
            i+=1
        else:
            merged.append(b[j])
            j+=1
        k+=1

    while i < len_a:
        merged.append(a[i])
        i+=1
        k+=1

    while j < len_b:
        merged.append(b[j])
        j+=1
        k+=1

    return merged

if __name__ == '__main__':
    arr = [
        {'name': 'rajab', 'age': 12, 'time_hours': 3},
        {'name': 'vignesh', 'age': 21, 'time_hours': 2.5},
        {'name': 'chinmay', 'age': 24, 'time_hours': 1.5},
        {'name': 'vedanth', 'age': 17, 'time_hours': 1},
    ]

    new_list = merge_sort(arr, key='time_hours', descending=True)
    print(new_list)

But it didn’t work, here is the output of it

[{'name': 'vedanth', 'age': 17, 'time_hours': 1}, {'name': 'chinmay', 'age': 24, 'time_hours': 1.5}, {'name': 'vignesh', 'age': 21, 'time_hours': 2.5}, {'name': 'rajab', 'age': 12, 'time_hours': 3}]

So I looked at the solution and made some changes on the first ‘merge_sort’ function.

def merge_sort(arr, key, descending=False):
    if len(arr) <= 1:
        return arr

    mid = len(arr)//2

    left_list = merge_sort(arr[0:len(arr) // 2], key)
    right_list = merge_sort(arr[len(arr) // 2:], key)

    sorted_list = merge_two_sorted_lists(left_list, right_list)

    return sorted_list

It worked, but I have no idea how, I tried to debug it several times but still cannot understand why returning some values back made it work. So, I will be delighted if someone explains why I need to return the list when dividing it?