[ROS2] Substituting a parameter from confg file while keeping others within Python launch file

edit

Hey all,

I am trying to substitute (or prepend) specific parameters from a Python launch file in ROS2. I have filenames specified in launch files, that must be passed as full paths to ROS nodes via launch parameters. Therefore, the filenames from

The problem is, is there a way to prepend some text to specifically one parameters out of the configuration file? All I can think of is to parse the YAML config file in python and then just place it as a dict into the parameters field.

My YAML config file:

tb_lm:
      efficiency_map:
        ros__parameters:
          filename: "PMSynRM.csv"

And this is my launch file:

import os
from ament_index_python.packages import get_package_share_directory
from launch import LaunchDescription
from launch_ros.actions import Node
def generate_launch_description():

    ld = LaunchDescription()
    config = os.path.join(
        get_package_share_directory('loading_motor_dt'),
        'config',
        'params.yaml'
        )

    data_processor=Node(
        package = 'loading_motor_dt',
        namespace = 'tb_lm',
        name = 'efficiency_map',
        executable = 'efficiency_map',
        parameters = [config]
    )
    ld.add_action(data_processor)
    return ld

How can I grab the filename out of config and prepend to it the $HOME path for example (or even a string?) is there a ROS2 way to do this, or better to just transform the YAML into dict, and substitute whatever is required, and pass it to the parameters?

EDIT 1

Here is the update config file for what I am trying to achieve:

import os
from ament_index_python.packages import get_package_share_directory
from launch import LaunchDescription
from launch_ros.actions import Node
import yaml
from pathlib import Path

home = str(Path.home())

"""Used to grab the contents of configuration YAML and
   prepend full paths to all the filenames fields"""
def prepend_to_filenames(config_file, package_name):

    with open(config_file, "r") as f:
        config_yaml = yaml.safe_load(f)

    for ns in config_yaml:
        for node in config_yaml[ns]:
            try:
                filename = config_yaml[ns][node]["ros__parameters"]["filename"]
                config_yaml[ns][node]["ros__parameters"]["filename"] = os.path.join(
                    get_package_share_directory(package_name),'data_files', filename)
            except:
                pass
    # For DEBUG purposes only
    with open(os.path.join(home,"debug.yaml"),'w') as f:
        yaml.dump(config_yaml, f)

    return config_yaml

def generate_launch_description():

    ld = LaunchDescription()
    config = os.path.join(
        get_package_share_directory('loading_motor_dt'),
        'config',
        'params.yaml'
        )

    params_prepended = prepend_to_filenames(config, "loading_motor_dt")

    data_processor=Node(
        package = 'loading_motor_dt',
        namespace = 'tb_lm_left',
        name = 'efficiency_map',
        executable = 'efficiency_map',
        parameters = [params_prepended]
    )
    ld.add_action(data_processor)
    return ld