ROS2 Launch - How to Concatenate LaunchConfiguration with String

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It seems the example you linked uses a list of strings and TextSubstitutions in the context of setting the default argument. Using substitutions directly within os.path.join will result in an error, as join() expects a string. In Foxy it results in:

Caught exception when trying to load file of format [py]: expected str, bytes or os.PathLike object, not LaunchConfiguration

In any case, thank you! The method demonstrated in the linked example (creating a new launch argument with a default value based on a previous argument) is a better approach than what I was doing!

As a potential correction to your example, the following works as expected:

joy_config = LaunchConfiguration('joy_config')
joy_config_dec= DeclareLaunchArgument('joy_config', default_value='ps3')
final_string_dec= DeclareLaunchArgument('final_string', default_value=[LaunchConfiguration('joy_config'), '.config.yaml'])

Which would result in LaunchConfiguration('final_string') evaluating to: "ps3.config.yaml" without the explicit use of TextSubstitution. In fact, I am still unclear what TextSubstitution's purpose is - perhaps it is a legacy function from before ROS2 was able to parse a mixed list of strings and LaunchConfigurations?

using a = PythonExpression(["'map", LaunchConfiguration('map_number'), ".world'"]) How can I add the formatted PythonExpression to my DeclareLaunchArgument

return LaunchDescription([
    DeclareLaunchArgument(
        'world',
        default_value=[os.path.join(
            # Change dir below if you want to load different world
            get_package_share_directory('my_bot'), 'worlds', a), ''],
        description='SDF world file'),
    gazebo
])

Or should this be a separate question?

what you currently have will fail because os.path.join has no knowledge of ROS 2 launch substitutions or the launch context, and only supports string inputs.

if memory serves, you can either:

• append the PythonExpression to the default_value list (e.g. default_value=[os.path.join(...), '/', a] and DeclareLaunchArgument will handle the substitution in the launch context at launch time. It will fail when you try to add it. This is a bit sloppier as you need to hard code a path separator
• use PathJoinSubstitution (from launch.substitutions import PathJoinSubstitution) which functions like os.path.join but supports substitutions. e.g.: default_value=PathJoinSubstitution([get_package_share_directory('my_bot'), 'worlds', a]). This approach is probably preferred as it is more portable

Thanks, It worked perfectly. When trying to use the LaunchConfiguration('map_number') as a condition. E.G.: map_number = LaunchConfiguration('map_number') if map_number == '1': # Do something How should we handle that? As it wont compare a LaunchConfiguration to a string. Or is this a seperate question?