kamai | schwifty programming language | Translation library

serial_number has only one value when you go through your mac_addresses for loop: the last value you set it to in the other for loop. You're going to need to capture all the lines in that loop, with something like serial_numbers = [line.strip() for line in stb_serial_numbers] instead of the first for loop, then make your second loop for serial_number, mac_address in zip(serial_numbers, mac_addresses):.

On another note: you're likely to get messed up output sometimes running this code, because you're writing to the same file from different threads. And similarly, accessing globals from threads is a bad idea. Instead, use a queue to return each thread's results to the main thread and combine them there.