ROS Resources: Documentation | Support | Discussion Forum | Index | Service Status | ros @ Robotics Stack Exchange
Ask Your Question

evaluate launch argument as python variable

asked 2022-07-28 03:54:24 -0500

MaxDio gravatar image

updated 2022-07-28 03:55:09 -0500

Hi I would like to access a launch argument as a python variable:

Say I type

ros2 launch pkg launch_file my_arg:="foo"

In my launch file I have the following:

DeclareLaunchArgument(name="my_arg", default_value="hello")

now I would like to do some python logic with the value I passed. E.g. in an if statement. Jowever I need to define my_arg somehow...

if(my_arg == "hello"):  
     # do something

I have not found anything to get the value of the launch argument. What is the best way to access the value?

edit retag flag offensive close merge delete

1 Answer

Sort by ยป oldest newest most voted

answered 2022-07-28 05:49:39 -0500

William gravatar image

updated 2022-07-28 15:23:08 -0500

There's no convenient way to do this. I recommend you explain what you're trying to do. There's likely a better way than using if(my_arg == "hello"): style logic.

Also, I recommend using XML where you can. The Python API was not designed to be used directly, and as a result it is not very ergonomic.


You can control when things are actually included using "conditionals", for example (based on your comment):

  <arg name="a_not_b" default="true" />
  <!-- argument a_option is required because it does not have a default value -->
  <arg name="a_option" if="$(var a_not_b)" />
  <group unless="$(var a_not_b)">
    <arg name="b_option1" default="true" />
    <arg name="b_option2" default="true" />


In this example, you have a required argument a_option, which if you don't supply it you get:

% ros2 launch /tmp/example.launch.xml
[INFO] [launch]: All log files can be found below /Users/william/.ros/log/2022-07-28-13-17-55-361895-markook.local-67414
[INFO] [launch]: Default logging verbosity is set to INFO
[ERROR] [launch.actions.declare_launch_argument]: Required launch argument "a_option" (description: "no description given") was not provided
[ERROR] [launch]: Caught exception in launch (see debug for traceback): Required launch argument "a_option" was not provided.

But then, due to the condition "if=" attribute on it, you can avoid that with the option a_not_b:

% ros2 launch /tmp/example.launch.xml a_not_b:=false
[INFO] [launch]: All log files can be found below /Users/william/.ros/log/2022-07-28-13-18-03-013500-markook.local-67418
[INFO] [launch]: Default logging verbosity is set to INFO

However, since launch cannot know which arguments will be included until you actually run it, when you ask it to show the arguments, every possible argument will be shown, with only a * for the ones that may be conditionally used:

% ros2 launch /tmp/example.launch.xml --show-arguments
Arguments (pass arguments as '<name>:=<value>'):

        no description given
        (default: 'true')

        no description given

        no description given
        (default: 'true')

        no description given
        (default: 'true')

* argument(s) which are only used if specific conditions occur
edit flag offensive delete link more


Hmm thats a pity. I would like to call a python function based on an argument and define other launch arguments

MaxDio gravatar image MaxDio  ( 2022-07-28 07:10:44 -0500 )edit

You can create a group to contain the "other launch arguments", then condition that group on the value of my_arg. I can add a small example in the answer as an edit.

William gravatar image William  ( 2022-07-28 15:06:33 -0500 )edit

I updated my answer. If you must do this in Python, then that is also possible, though it is much less readable than with XML.

William gravatar image William  ( 2022-07-28 15:24:07 -0500 )edit

Question Tools



Asked: 2022-07-28 03:54:24 -0500

Seen: 184 times

Last updated: Jul 28 '22