Set xacro arguments when parsing a file using Python (ROS2)

edit

Is it possible to set arguments when parsing a file using the python xacro library?

In ROS2, a number of packages parse xacro files directly in the launch file using the python xacro library, with something along the lines of:

import xacro, os

xacro_file = "test.urdf.xacro"
doc = xacro.parse(open(xacro_file))
xacro.process_doc(doc)
robot_description_config = doc.toxml()
robot_description = {'robot_description': robot_description_config}

For some context, say I have a file, test.urdf.xacro that takes an argument named input:

<?xml version="1.0"?>
<robot name="test" xmlns:xacro="http://ros.org/wiki/xacro">
    <xacro:arg name="input" defaut="default"/>
    <link name="$(arg input)"/>
</robot>

Using the python code above, the argument would be the default value and the link would be named default. But how can you pass in a custom value?

Using the command line interface, f I wanted to parse the file with some custom value, I could simply type the following in the terminal:

$ xacro test.urdf.xacro input:="new_input"

and get an output where my link is appropriately named new_input:

<?xml version="1.0" ?>
<!-- =================================================================================== -->
<!-- |    This document was autogenerated by xacro from test.urdf.xacro                | -->
<!-- |    EDITING THIS FILE BY HAND IS NOT RECOMMENDED                                 | -->
<!-- =================================================================================== -->
<robot name="test">
  <link name="new_input"/>
</robot>

But I'm at a loss when it comes to achieving the same directly using python. Anyone have any ideas?