Find inversions in Array

inversion.py

def merge_array(l_array, r_array):
    merged_array = []
    i, j = 0, 0
    local_count = 0
    while(i < len(l_array) and j < len(r_array)):
        if l_array[i] > r_array[j]:
            merged_array.append(r_array[j])
            local_count += len(l_array) - i
            j += 1
        else:
            merged_array.append(l_array[i])
            i += 1
    tail_array = l_array[i:] if i < len(l_array) else r_array[j:]
    return merged_array+tail_array, local_count


def find_inversions_count(input_array, a):
    if len(input_array) > 1:
        l_array, l_count = find_inversions_count(
            input_array[:(len(input_array)+1)/2], a)
        r_array, r_count = find_inversions_count(
            input_array[(len(input_array)+1)/2:], a)
        merged_array, local_count = merge_array(
            l_array, r_array)
        a.append(local_count)
        return merged_array, a
    else:
        return input_array, 0

if __name__ == "__main__":
    input_array = [6, 5, 4, 3, 2, 1]
    assert sum(find_inversions_count(input_array, [])[1]) == 15