Using a launch argument as python variable variable

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I would like to be able to use a launch argument as a python variable. What i'm trying to do in this script is load a different model each time by passing it as an argument when i use the launch file. The problem is that i cant get the value of the argument

in the script below the string 'pelican' should be replaced each time i run the launch file with the name of the model i want to view.

def generate_launch_description():

package_dir = get_package_share_directory('rotors_description')
xacro_file = os.path.join(package_dir,'urdf','pelican.xacro')
rviz_file = os.path.join(package_dir,'config.rviz')
robot_description_raw = xacro.process_file(xacro_file,mappings=
                    {'namespace':'pelican'}).toxml()

robot_state_publisher = Node(
    package='robot_state_publisher',
    executable='robot_state_publisher',
    name='robot_state_publisher',
    output='screen',
    parameters=[{'robot_description': robot_description_raw}]
    )

joint_state_publisher = Node(
    package='joint_state_publisher_gui',
    executable='joint_state_publisher_gui',
    name='joint_state_publisher_gui',
    output='screen'
    )

rviz = Node(
    package='rviz2',
    executable='rviz2',
    name='rviz2',
    arguments=['-d',rviz_file],
    output='screen')    

return LaunchDescription([
    robot_state_publisher,
    joint_state_publisher,
    rviz

])