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Using parameters in a class (Python)

Goal: Create and run a class with ROS parameters using Python (rclpy).

Tutorial level: Beginner

Time: 20 minutes

Contents

Background
Prerequisites
Tasks
Summary
Next steps

Background 

When making your own nodes you will sometimes need to add parameters that can be set from the launch file.

This tutorial will show you how to create those parameters in a Python class, and how to set them in a launch file.

Prerequisites 

In previous tutorials, you learned how to create a workspace and create a package. You have also learned about parameters and their function in a ROS 2 system.

Tasks 

1 Create a package 

Open a new terminal and source your ROS 2 installation so that ros2 commands will work.

Navigate into the dev_ws directory created in a previous tutorial.

Recall that packages should be created in the src directory, not the root of the workspace. Navigate into dev_ws/src and create a new package:

ros2 pkg create --build-type ament_python python_parameters --dependencies rclpy

Your terminal will return a message verifying the creation of your package python_parameters and all its necessary files and folders.

The --dependencies argument will automatically add the necessary dependency lines to package.xml and CMakeLists.txt.

1.1 Update `package.xml`

Because you used the --dependencies option during package creation, you don’t have to manually add dependencies to package.xml or CMakeLists.txt.

As always, though, make sure to add the description, maintainer email and name, and license information to package.xml.

<description>Python parameter tutorial</description>
<maintainer email="you@email.com">Your Name</maintainer>
<license>Apache License 2.0</license>

2 Write the Python node 

Inside the dev_ws/src/python_parameters/python_parameters directory, create a new file called python_parameters_node.py and paste the following code within:

import rclpy
import rclpy.node

class MinimalParam(rclpy.node.Node):
    def __init__(self):
        super().__init__('minimal_param_node')
        timer_period = 2  # seconds
        self.timer = self.create_timer(timer_period, self.timer_callback)

        self.declare_parameter('my_parameter', 'world')

    def timer_callback(self):
        my_param = self.get_parameter('my_parameter').get_parameter_value().string_value

        self.get_logger().info('Hello %s!' % my_param)

        my_new_param = rclpy.parameter.Parameter(
            'my_parameter',
            rclpy.Parameter.Type.STRING,
            'world'
        )
        all_new_parameters = [my_new_param]
        self.set_parameters(all_new_parameters)

def main():
    rclpy.init()
    node = MinimalParam()
    rclpy.spin(node)

if __name__ == '__main__':
    main()

2.1 Examine the code

Note: Declaring a parameter before getting or setting it is compulsory, or a ParameterNotDeclaredException exception will be raised.

The import statements below are used to import the package dependencies.

import rclpy
import rclpy.node

The next piece of code creates the class and the constructor. timer is initialized (with timer_period set as 2 seconds), which causes the timer_callback function to be executed once every two seconds. The line self.declare_parameter('my_parameter', 'world') of the constructor creates a parameter with the name my_parameter and a default value of world. The parameter type is inferred from the default value, so in this case it would be set to a string type.

class MinimalParam(rclpy.node.Node):
    def __init__(self):
        super().__init__('minimal_param_node')
        timer_period = 2  # seconds
        self.timer = self.create_timer(timer_period, self.timer_callback)

        self.declare_parameter('my_parameter', 'world')

The first line of our timer_callback function gets the parameter my_parameter from the node, and stores it in my_param. Next,the get_logger function ensures the message is logged. The set_parameters function then sets the parameter my_parameter back to the default string value world. In the case that the user changed the parameter externally, this ensures it is always reset back to the original.

def timer_callback(self):
    my_param = self.get_parameter('my_parameter').get_parameter_value().string_value

    self.get_logger().info('Hello %s!' % my_param)

    my_new_param = rclpy.parameter.Parameter(
        'my_parameter',
        rclpy.Parameter.Type.STRING,
        'world'
    )
    all_new_parameters = [my_new_param]
    self.set_parameters(all_new_parameters)

Following the timer_callback is the main function where ROS 2 is initialized. Then an instance of the class MinimalParam named node is defined. Finally, rclpy.spin starts processing data from the node.

def main():
    rclpy.init()
    node = MinimalParam()
    rclpy.spin(node)

if __name__ == '__main__':
    main()

2.1.1 (Optional) Add ParameterDescriptor

Optionally, you can set a descriptor for the parameter. Descriptors allow you to specify a text description of the parameter and parameters constraints, like making it read-only, specifying a range, etc. For that to work, the __init__ code has to be changed to:

# ...

class MinimalParam(rclpy.node.Node):
    def __init__(self):
        super().__init__('minimal_param_node')
        timer_period = 2  # seconds
        self.timer = self.create_timer(timer_period, self.timer_callback)

        from rcl_interfaces.msg import ParameterDescriptor
        my_parameter_descriptor = ParameterDescriptor(description='This parameter is mine!')

        self.declare_parameter('my_parameter',
                               'world',
                               my_parameter_descriptor)

The rest of the code remains the same. Once you run the node, you can then run ros2 param describe /minimal_param_node my_parameter to see the type and description.

2.2 Add an entry point

Open the setup.py file. Again, match the maintainer, maintainer_email, description and license fields to your package.xml:

maintainer='YourName',
maintainer_email='you@email.com',
description='Python parameter tutorial',
license='Apache License 2.0',

Add the following line within the console_scripts brackets of the entry_points field:

entry_points={
    'console_scripts': [
        'param_talker = python_parameters.python_parameters_node:main',
    ],
},

Don’t forget to save.

3 Build and run 

It’s good practice to run rosdep in the root of your workspace (dev_ws) to check for missing dependencies before building:

rosdep install -i --from-path src --rosdistro galactic -y

Navigate back to the root of your workspace, dev_ws, and build your new package:

colcon build --packages-select python_parameters

colcon build --packages-select python_parameters

colcon build --merge-install --packages-select python_parameters

Open a new terminal, navigate to dev_ws, and source the setup files:

. install/setup.bash

. install/setup.bash

call install/setup.bat

Now run the node:

ros2 run python_parameters param_talker

The terminal should return the following message every 2 seconds:

[INFO] [parameter_node]: Hello world!

There are two ways to change the parameter:

3.1 Change via the console

This part will use the knowledge you have gained from the tutoral about parameters and apply it to the node you have just created.

Make sure the node is running:

ros2 run python_parameters param_talker

Open another terminal, source the setup files from inside dev_ws again, and enter the following line:

ros2 param list

There you will see the custom parameter my_parameter. To change it simply run the following line in the console:

ros2 param set /minimal_param_node my_parameter earth

You know it went well if you get the output Set parameter successful. If you look at the other terminal, you should see the output change to [INFO] [minimal_param_node]: Hello earth!

Since the Python talker then set the parameter back to world, further outputs show [INFO] [minimal_param_node]: Hello world!

3.2 Change via a launch file

You can also set parameters in a launch file, but first you will need to add a launch directory. Inside the dev_ws/src/python_parameters/ directory, create a new directory called launch. In there, create a new file called python_parameters_launch.py

from launch import LaunchDescription
from launch_ros.actions import Node

def generate_launch_description():
    return LaunchDescription([
        Node(
            package='python_parameters',
            executable='param_talker',
            name='custom_parameter_node',
            output='screen',
            emulate_tty=True,
            parameters=[
                {'my_parameter': 'earth'}
            ]
        )
    ])

Here you can see that we set my_parameter to earth when we launch our node parameter_node. By adding the two lines below, we ensure our output is printed in our console.

output="screen",
emulate_tty=True,

Now open the setup.py file. Add the import statements to the top of the file, and the other new statement to the data_files parameter to include all launch files:

import os
from glob import glob
# ...

setup(
  # ...
  data_files=[
      # ...
      (os.path.join('share', package_name), glob('launch/*_launch.py')),
    ]
  )

Open a console and navigate to the root of your workspace, dev_ws, and build your new package:

colcon build --packages-select python_parameters

colcon build --packages-select python_parameters

colcon build --merge-install --packages-select python_parameters

Then source the setup files in a new terminal:

. install/setup.bash

. install/setup.bash

call install/setup.bat

Now run the node using the launch file we have just created:

ros2 launch python_parameters python_parameters_launch.py

The terminal should return the following message:

[parameter_node-1] [INFO] [custom_parameter_node]: Hello earth!

Summary 

You created a node with a custom parameter, that can be set either from the launch file or the command line. You wrote the code of a parameter talker: a Python node that declares, and then loops getting and setting a string parameter. You added the entry point so that you could build and run it, and used ros2 param to interact with the parameter talker.

Next steps 

Now that you have some packages and ROS 2 systems of your own, the next tutorial will show you how to examine issues in your environment and systems in case you have problems.