Arduino Python to Serial command
Majenkos comment (above) solved the problem:

"You need to wait after opening the serial port for the board to finish resetting and the bootloader to run and exit before you can send any commands."

import serial 
import time

ser = serial.Serial(port="/dev/cu.usbmodem144231", baudrate=9600, timeout = 0)
time.sleep(1)
ser.write("200+")
print ser.read()