Бинарные операции

def choice():
    print('''\n0. Сложение
1. Вычитание''')

    choice = int(input('Выберите операцию (0/1): '))

    if choice:
        return 'substraction'
    else:
        return 'addition'


def numbers_input():
    first = input('Введите первое число: ')
    second = input('Введите второе число: ')

    if len(first) > 8 or len(second) > 8:
        return 'stack overflow'

    first, second = first.zfill(8), second.zfill(8)
    return first, second


def addition(first, second):
    fir = list(first)
    sec = list(second)
    overflow = False                    #переменная, отвечающая за переполнение ячейки

    for i in range(len(fir) - 1, -1, -1):

        if fir[i] == '0':
            if sec[i] == '0':
                if overflow:
                    fir[i] = '1'
                    overflow = False
                else:
                    continue
            elif sec[i] == '1':
                if overflow:
                    fir[i] = '0'
                    overflow = True
                else:
                    fir[i] = '1'
        elif fir[i] == '1':
            if sec[i] == '0':
                if overflow:
                    fir[i] = '0'
                    overflow = True
                else:
                    continue
            elif sec[i] == '1':
                if i == 0:
                    return 'overflow'
                if overflow:
                    fir[i] = '1'
                    overflow = True
                else:
                    fir[i] = '0'
                    overflow = True

    return fir


def substraction(first, second):
    fir = list(first)
    sec = list(second)
    shortage = False                #переменная, отвечающая за заем из старшего разряда

    for i in range(len(fir) - 1, -1, -1):
        if fir[i] == '0':
            if sec[i] == '0':
                continue
            elif sec[i] == '1':
                if shortage:
                    shortage = True
                    fir[i] = 1
                else:
                    shortage = True
                    fir[i] = 1
        elif fir[i] == '1':
            if shortage:
                fir[i] = '0'
                if sec[i] == '0':
                    shortage = False
                    continue
                elif sec[i] == '1':
                    fir[i] = '1'
                    shortage = True
            else:
                if sec[i] == '0':
                    continue
                elif sec[i] == '1':
                    fir[i] = '0'

    return fir


def run():
    numbers = numbers_input()

    if len(numbers) == 1:
        print('\nstack overflow')
    else:
        first, second = numbers
    if choice() == 'addition':
        summ = addition(first, second)
        if summ == 'overflow':
            print('\nstack overflow')
        else:
            print(*summ, sep='')
    else:
        subs = substraction(first, second)
        print(*subs, sep='')


if __name__ == "__main__":
    run()