command-line program that prints out the sum of two non-negative integers

get_sum_of_two_ints.py

# Write a command-line program that prints out the sum of two non-negative integers as input arguments. 
# You must not use any built-in BigInteger library or convert the inputs to integer directly.

import sys

def get_sum(first_num, second_num):
    first_list = [s for s in first_num]
    second_list = [s for s in second_num]

    end = 0
    head = []

    if len(first_list) > len(second_list):
        end = len(second_list)
        head = first_list[:len(first_list) - end]
    elif len(first_list) < len(second_list):
        end = len(first_list)
        head = second_list[:len(second_list) - end]
    else:
        end = len(first_list)

    sum_result = []
    prev = 0
    for i in range(1, end+1):
        current_num = int(first_list[-i]) + int(second_list[-i])
        if prev:
            current_num += 1
        if current_num >= 10:
            prev = 1
            current_num -= 10
        else:
            prev = 0
        sum_result.insert(0, str(current_num))
    
    result = "".join(sum_result)
    
    if prev:
        if not head:
            result = "1" + result
        else:
            for i in range(1, len(head)+1):
                target = int(head[-i]) + 1
                if target >= 10:
                    head[-i] = str(target - 10)
                else:
                    head[-i] = str(target)
                    prev = 0
            if prev:
                head.insert(0, str(1))
    if head:
        result = "".join(head) + result
    print(result)


if __name__ == '__main__':
    get_sum(sys.argv[1], sys.argv[2])