[ROS2] Any idea on how to pass parameters (YAML) to a Xacro file via launch python script?

edit

Is your end goal to use xacro for your robot_state_publisher at runtime? Xacro actually has a function (load_yaml()) that allows you to read in a yaml from a file path. In this case you can hand the path of the file through your xacro call using the xacro_arg_name:=param_to_pass notation. If you are running Galactic or later here is how I would do it (with launch configurations for full runtime flexibility):

EDIT:

For foxy you may need to use an opaque functions as described in the second answer of this question

Launch File:

#!/usr/bin/env python3

from launch import LaunchDescription
from launch.actions import DeclareLaunchArgument, LogInfo
from launch.substitutions import LaunchConfiguration, Command, PathJoinSubstitution
from launch_ros.actions import Node
from launch_ros.descriptions import ParameterValue
from launch_ros.substitutions import FindPackageShare

def generate_launch_description():
  this_pkg = FindPackageShare('test_package')

  model_path = LaunchConfiguration('model_path')
  params_path = LaunchConfiguration('params_path')

  return LaunchDescription([
    DeclareLaunchArgument(
      'model_path',
      default_value=PathJoinSubstitution([this_pkg,'xacro','model.xacro']),
      description='Full path to model xacro file including filename and extension'
    ),

    DeclareLaunchArgument(
      'params_path',
      default_value=PathJoinSubstitution([this_pkg,'params','model.yaml']),
      description='Full path to parameter yaml file including filename and extension'
    ),        

    Node(
      package='robot_state_publisher',
      executable='robot_state_publisher',
      name='robot_state_publisher',
      output='screen',
      parameters=[{
        'robot_description': ParameterValue(Command(['xacro ',model_path,' params_path:=',params_path]), value_type=str)
      }],
    )

  ])

Xacro File: model.xacro

<?xml version="1.0"?>
<robot name="model" xmlns:xacro="http://www.ros.org/wiki/xacro">
  <!-- 
    The xacro:arg "params_path" exposes the arg to launch file.
    The xacro:property "params_path" is required to get the evaluated symbol into xacro before "load_yaml"
    The xacro:proterty "mp" (which stands for "model parameters") gets the dictionary from "params_path"
    and is accessed using ${mp['key_name']} which will evaluate to the key value

    The xacro:arg and xacro:property "params_path" shares a name but does not seem to clobber eachother.
    They are not required to have the same name, it was just convenient
  -->
  <xacro:arg name="params_path" default=""/> <!-- Need argument to get from launch file -->
  <xacro:property name="params_path" value="$(arg params_path)"/> <!-- Need seperate property for xacro inorder processing -->
  <xacro:property name="mp" value="${load_yaml(params_path)}"/> <!-- Read in the yaml dict as mp (short for model parameters) -->


  <xacro:property name="PI" value="3.1415926535897931" />

  <xacro:property name="wheel_mass" value="${mp['wheel_mass']}"/>
  <xacro:property name="wheel_width" value="${mp['wheel_width']}"/>
  <xacro:property name="wheel_radius" value="${mp['wheel_radius']}"/>


  <link name="base_link"/>

  <link name="body_link">
    <!-- Visual box geometry -->
    <visual name="visual">
      <geometry>
        <box size="1 1 0.5"/>
      </geometry>
    </visual>

    <!-- Collision box geometry-->
    <collision name="collision">
      <geometry>
        <box size="1 1 0.5"/>
      </geometry>
    </collision>

    <!-- Inertia for a box -->
    <inertial>
      <origin xyz="0 0 0" rpy="0 0 0"/>
      <mass value="1645.0"/>
      <inertia
          ixx="${(1.0/12.0) * 1645.0 * ( 1 * 1 + 0.5 * 0.5 )}"  
          ixy="0" 
          ixz="0" 
          iyy="${(1.0/12.0) * 1645.0 * ( 1 * 1 + 0.5 * 0.5 )}"  
          iyz="0" 
          izz="${(1.0/12.0) * 1645 ...