使用python发送需要root权限的linux shell命令

def file_property():
    try:
        filename = "/etc/network/interfaces"
        cmd = "chmod 777 " + filename
        run = pexpect.spawn('su -c "%s" root' %cmd)
        time.sleep(0.5)
        run.sendline('xxx')   
        time.sleep(0.1)     
    except Exception:
        print traceback.format_exc()