Running a python node using eval in roslaunch

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In my roslaunch file structure, I have a launch file that stores spawn and goal coordinates in the parameter server. Then I have another launch file that needs those coordinates, but the values need to be stored in an argument tag. I found a Q&A that suggests making a simple python node that retrieves the parameter and writes the value to sysout and running it using eval. However when I tried running the launch file with the syntax that was provided in the Q&A, I get the following error:

while processing /home/csrobot/catkin_ws/src/uml_3d_race/launch/spawn_robot.launch:
Invalid <arg> tag: name 'convert_param_to_arg' is not defined. 

Arg xml is <arg default="$(eval convert_param_to_arg.py /spawn_x)" name="x"/>
The traceback for the exception was written to the log file

Note: I'm trying to run a python script called convert_param_to_arg.py and trying to a pass a string parameter of /spawn_x

My question: how do I make this work? Also, I am using roscpp so are there any cmake changes I need to make?

The launch file in question:

<!-- Assumes Gazebo is already running -->
<launch>
  <!-- Settings (Robot position args are overidden by spawn_world.launch)-->
  <arg name="x" default="$(eval convert_param_to_arg.py /spawn_x)" />
  <arg name="y" default="$(eval convert_param_to_arg.py /spawn_y)" />
  <arg name="z" default="$(eval convert_param_to_arg.py /spawn_z)" />
  <arg name="yaw" default="$(eval convert_param_to_arg.py /spawn_yaw)" /> <!-- Rotation in radians. -->

  <arg name="level" />
  <arg name="navigate" />

  <arg name="model_name" />
  <arg name="model_type" />

  <arg name="invert_goals" />

  <!-- Launch respective robot -->
  <include file="$(find race)/launch/robots/spawn_$(arg model_type).launch">
    <arg name="x" value="$(arg x)" />
    <arg name="y" value="$(arg y)" />
    <arg name="z" value="$(arg z)" />
    <arg name="yaw" value="$(arg yaw)" />
    <!-- Rotation in radians. -->
    <arg name="level" value="$(arg level)" />
    <arg name="navigate" value="$(arg navigate)" />
    <arg name="model_name" value="$(arg model_name)" />
  </include>



<!-- Publish the two different goals needed for the given .world -->
  <node unless="$(arg invert_goals)" name="goal_publisher" pkg="race" type="goal_publisher" output="screen" ns="$(arg model_name)">
    <!-- Goal Positions -->
    <remap from="~goal_x" to="/goal_x" />
    <remap from="~goal_y" to="/goal_y" />
    <remap from="~spawn_x" to="/spawn_x" />
    <remap from="~spawn_y" to="/spawn_y" />
  </node>

</launch>

The python script convert_param_to_arg.py

#!/usr/bin/env python

import rospy
import sys

rospy.init_node('convert_param_to_arg')
param_name = 'param'
if sys.argv.count > 0:
    param_name = sys.argv[0]
value = rospy.get_param(param_name)
sys.stdout.write(value)