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Unfortunately, an str() method would not be that easy. The LaunchConfiguration is a Substitutible object, and doesn't derive from str You're going to need to use the launch.actions.OpaqueFunction action as it will give you a LaunchContext object from which you can perform substitutions:

from pathlib import Path
from launch import LaunchDescription, LaunchContext, LaunchService
from launch.actions import OpaqueFunction, DeclareLaunchArgument, ExecuteProcess
from launch.substitutions import LaunchConfiguration


robot_description_path = Path('destination.urdf')


def render_xacro(context: LaunchContext, support_package):
    support_package_str = context.perform_substitution(support_package)
    # render xacro... just dumping the support_package value in there for example.
    robot_description_config = support_package_str
    robot_description_path.write_text(robot_description_config)
    print(f'wrote robot description to {robot_description_path}')


def generate_launch_description():
    return LaunchDescription([
        DeclareLaunchArgument('support_package', default_value='hello'),

        OpaqueFunction(function=render_xacro, args=[LaunchConfiguration('support_package')]),
        ExecuteProcess(cmd=['cat', str(robot_description_path)],
                       output='screen'),
    ])


if __name__ == '__main__':
    ls = LaunchService()
    ls.include_launch_description(generate_launch_description())
    ls.run()

which outputs:

[INFO] [launch]: Default logging verbosity is set to INFO
wrote robot description to destination.urdf
[INFO] [cat-1]: process started with pid [31440]
[INFO] [cat-1]: process has finished cleanly [pid 31440]
[cat-1] hello