roslaunch use arg to launch serial_node.py

edit

I'd like to roslaunch serial_node.py node with launch file.

I add arg tag in launch file

<arg name="_port" default="/dev/ttyACM0" />
<arg name="_baud" default="9600/>"
<node name="serial_node" pkg="rosserial_python" type="serial_node.py" args="$(arg _port) $(arg _baud)"/>

But after I roslaunch the launch file, it fails. It seems like the two arg do not work at all

[ERROR] [WallTime: 1484219964.751265] Error opening serial: could not open port /dev/ttyUSB0: [Errno 2] No such file or directory: '/dev/ttyUSB0'