Hello, here's what I've got for this problem - it's a launch file which takes the path to a xacro file as an input argument.

On launch the xacro is converted to urdf (xml) and passed to the robot_state_publisher via the 'robot_description' parameter:

#!/usr/bin/env python3

from launch import LaunchDescription
from launch.actions import DeclareLaunchArgument
from launch.substitutions import LaunchConfiguration, Command
from launch_ros.actions import Node


def generate_launch_description():

    use_sim_time = LaunchConfiguration('use_sim_time', default='false')
    xacro_path = LaunchConfiguration('xacro_path', default=None)

    return LaunchDescription([
        DeclareLaunchArgument(
            'use_sim_time',
            default_value='false',
            description='Use simulation (Gazebo) clock if true'),
        DeclareLaunchArgument(
            'xacro_path',
            default_value=None,
            description='path to urdf.xacro file to publish'),

        Node(
            package='robot_state_publisher',
            executable='robot_state_publisher',
            name='robot_state_publisher',
            output='screen',
            parameters=[{
                'use_sim_time': use_sim_time,
                'robot_description':Command(['xacro',' ', xacro_path])
            }]),
    ])