def check_pid(pid):
    try:
        os.kill(pid, 0)
    except OSError:
        return False
    else:
        return True

pid = None
try:
    pid = int(open("script_name.pid").read())
except:
    pass

if pid:
    if check_pid(pid):
        logging.info("An instance of script_name is already running: PID={}. Bye".format(pid))
        sys.exit(0)

pid = os.getpid()
open("script_name.pid","w").write(str(pid))