[ROS2] equivalent of "textfile" argument from ROS1 launch file

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I am developing a ROS2 version of a package, but I am stuck with the launch file. What I had for ROS1 is this:

<arg name="robot_name"/>
<param name="robot_description" textfile="/path/to/$(arg robot_name).urdf"/>

This permitted to read the urdf and to load its content in the robot_description parameter.
In ROS2 xml launches, textfile is not supported anymore.

Options are:

1. Some tricks to still using xml (with the "command" argument?)

2. Using the python launchfile. In this case I have trouble because I can not concatenate LaunchConfiguration and str. From https://docs.ros.org/en/foxy/Tutorial... I tried something like:

   robot_name = LaunchConfiguration('robot_name')
   urdf_path = "/path/to/$(arg robot_name)" urdf = os.path.join("/path/to/", robot_name)
   with open(urdf, 'r') as infp: robot_desc = infp.read()

But I can not join the path with the hand name

Any suggestions? Thanks