qsum_single

def qsum_calc(value):
    result = 0
    while value:
        result += value % 10
        value //= 10
    return result


def qsum_single(value):
    while value >= 10:
        value = qsum_calc(value)
    return value


def get_value():
    while True:
        value = input("Please enter a value between 0 and 999: ")
        try:
            value = int(value)
        except ValueError:
            print(f"'{value}' is not a valid integer.")
            continue
        if 0 <= value <= 999:
            return value
        else:
            print(f"'{value}' is not between 0 and 999")


def main():
    while True:
        value = get_value()
        result = qsum_single(value)
        print(f"Result: {result}")


if __name__ == "__main__":
    try:
        main()
    except KeyboardInterrupt:
        print()
        print("Goodbye")