Project 1: Keyboard Node

In order to get familiar with how python nodes are written in ROS2, go through this tutorial written by the ROS2 developers. Once you have an idea of how to write a python node, you should get started on this project which will have you do exactly that!

Objectives

Keyboard Library

To help keep the focus on learning ROS2 you will use an external python package to handle the low-level details of communicating with the keyboard. To install the package, run python3 -m pip install pynput (or pip install pynput on Windows).

Warning

Install pynput package globally. Do not use python virutal environments since ROS doesn’t work well with them.

All the code you will need to communicate with the keyboard is below but you will still need to write the node and integrate the following code into it:

from pynput import keyboard

listener = keyboard.Listener(on_press=callback_function)
listener.start()

The above code basically makes a call to the user defined callback_function with a KeyCode object passed as a parameter whenever an alphanumeric key is pressed down on your keyboard. It is up to you to write the functionality for def callback_function(self, data) in your node’s class for when a key is pressed and integrate it within a ROS2 node to publish the key being pressed to the /keystrokes topic. Note the data parameter is of type KeyCode which can easily be cast to a string using str(data).

Note

While writing a node is possible with OOP (see any ROS1 node), we will be using OOP as seen in the ROS2 tutorials

You will use a launch file to run your node in order to test it (running a node directly from terminal exists but is bad practice). Follow this tutorial to learn how to write a launch file before making a launch file for your keyboard node.

Test your Node

  • Write a python launch file to run the node

  • Launch your launch file to run the node you wrote

  • Open a seperate terminal and echo the topic you are publishing to

  • Verify the correct keys are being published when you type

Launch Files in Python Packages

The link in the objectives section gives a good overview of using launch files.

Note

Launch files can be written in python, xml, or yaml, but we use python because it is the ROS2 way Launch files can be used to run nodes in either a python package or a CMake (C++) package

Using launch files in python packages has a few quirks that should be noted:

  1. You must define the executable for your node in setup.py

  2. You must also remember to copy over the launch files to the build and install folders by specifying them in setup.py

  3. Launch file names should follow the format <description>.launch.py

Your keyboard package’s setup.py should be modified to address the above quirks:

from setuptools import setup
import os
from glob import glob

...
    # Allows launch files to be found when called
    data_files=[
        ...
        (os.path.join('share', package_name), glob('launch/*.launch.py'))
    ],
...
    # Defines the executables for the node that are used in the launch file
    entry_points={
        'console_scripts': [
                '<executable_name> = <package_name>.<file_name>:main',
        ],
    },