I tend to be very brute force with stuff like this. I do a "ls /dev/tty*", with the device unplugged. Then plug the device in and repeat the ls command. The difference in the two lists should be your device, and usually shows up as ttyUSB0 or ttyACM0 (or in your case, may be a ttyAMA0). BTW, If you already had a serial device connected, it may be named something like ttyAMA0, so your new device would then be called ttyAMA1.
You can also do a "ls /dev/serial/by-*" and look for a reference to the device you are using. If this works, you could add this to your python script to parse the output and auto find the correct serial port.