Using xacro with ROS 2 python launch

xacro-launch.py

The goal here is to be able to use the ROS 2 python launch system to generate URDF files from xacro files, utilizing substitution.  All of the comments below are an attempt to solve this problem.

# Robot state publisher
In order to make this work, it is easiest to use the refactored robot_state_publisher branch https://github.com/ros/robot_state_publisher/tree/ros2-refactor .  The reason for this is that the refactor takes the `robot_description` as a parameter rather than as an argument on the command-line.  This branch isn't currently reviewed or released into any ROS 2 distribution, so it needs to be built from source.  (I'll note that it is probably *possible* to make all of this work with the version of robot_state_publisher released into Eloquent, but the rest of this comment will assume you are using the ros2-refactor branch).

```
source /opt/ros/eloquent/setup.bash
mkdir -p rsp_ws/src
cd rsp_ws/src
git clone https://github.com/ros/robot_state_publisher -b ros2-refactor
cd ..
colcon build
```

# Xacro file
The rest of the comments will assume the simple xacro file as follows.  More complex ones will probably work but have not been tested:

```
<?xml version="1.0"?>
<robot xmlns:xacro="http://www.ros.org/wiki/xacro" name="test">
  <xacro:arg name="length" default="0.6"/>
  <xacro:property name="length" value="$(arg length)"/>
  <xacro:arg name="radius" default="0.2"/>
  <xacro:property name="radius" value="$(arg radius)"/>
  <link name="base_link">
    <visual>
      <geometry>
        <cylinder length="${length}" radius="${radius}"/>
      </geometry>
      <material name="blue"/>
    </visual>
  </link>
</robot>
```

# Launch file
Now we can put the above together and create a launch file that will take the xacro file, create a URDF file from it (incorporating any mappings from the user):

```
import launch
import launch_ros.actions
import xacro

def generate_launch_description():
    doc = xacro.process_file('/home/ubuntu/rsp_ws/test-desc.urdf.xacro', mappings={'radius': '0.9'})
    robot_desc = doc.toprettyxml(indent='  ')
    params = {'robot_description': robot_desc}
    rsp = launch_ros.actions.Node(package='robot_state_publisher',
                                  node_executable='robot_state_publisher_node',
                                  output='both',
                                  parameters=[params])

    return launch.LaunchDescription([rsp])
```

Note that the mappings parameter is a dictionary of keys to values, so as many mapping as necessary can be substituted in.

# Output
The output will be as follows:

```
<?xml version=\"1.0\" ?>
<!-- =================================================================================== -->
<!-- |    This document was autogenerated by xacro from /home/ubuntu/rsp_ws/test-desc.urdf.xacro | -->
<!-- |    EDITING THIS FILE BY HAND IS NOT RECOMMENDED                                 | -->
<!-- =================================================================================== -->
<robot name="test">
  <link name="base_link">
    <visual>
      <geometry>
        <cylinder length="0.6" radius="0.9"/>
      </geometry>
      <material name="blue"/>
    </visual>
  </link>
</robot>
```

Thanks for the detailed explanation. One question, how can I pass LaunchArguments to the xacro files?

@superseppl you could do something like this:

https://github.com/Greenroom-Robotics/launch_ext/blob/main/launch_ext/substitutions/xacro.py#L17

There is a property in my xacro file that I would like to retrieve the value of inside the python launch file. Any idea how I could achieve that? (Yes, I know the other option is to pass that propery value as arguments into the xacro file.)