well I don't know about your `ino` program, but here's how I'd do it:

get_serial.py

    import serial, sys

    with serial.Serial(port=sys.argv[1], baudrate=sys.argv[2]) as ser:
        while ser.isOpen():
            print(ser.readline())

the idea, here, is to print on stdout the data coming on the serial line:

    python get_serial.py /dev/ttyACM0 115200 > output.log

or you can do it this way:


get_serial.py

    import serial, sys

    with open(sys.argv[3]) as f:
        with serial.Serial(port=sys.argv[1], baudrate=sys.argv[2]) as ser:
            while ser.isOpen():
                f.write(ser.readline())

which is ran:

    python get_serial.py /dev/ttyACM0 115200 output.log

HTH