test.py

# -*- coding: utf-8 -*-
import serial
from datetime import datetime

ser = serial.Serial(port='/dev/ttyACM0', baudrate=38400, timeout=0.5)
file = 'test.csv'
line = ser.read(1000)

#infinity loop
while True:
    if( ser.inWaiting() > 0 ):
        data = ser.read()
        f = open(file, 'a')
        f.write(data)
        print data
        if( '\n' in data ):
            now = datetime.now().strftime('%Y/%m/%d %H:%M:%S, ')
            f.write(now)
            print now