arduino data stream format
This is the Python function I use for configuring Arduino serial ports. Give it a try:

def configure_port(port_id):
    ser = serial.Serial()
    ser.port = port_id
    ser.baudrate = 9600
    ser.rtscts = True
    ser.dsrdtr = True
    return ser


Usage:

port = configure_port("/dev/ttyACM0")
port.open()


I think setting rtscts and dsrdtr to true is what worked for me. The above code at least works for Pro Micros; you didn't specify which Arduino you are using, so this is my best guess.