Prerequisites: you must have the python package named my_py_pkg in your /ros2_ws/src/

In the newly created file, on the first line itself, mention that you are going to use python3 as ROS2 only works with python3. This needs to be done, as Ubuntu has pre-installed python2 distribution into it.

#!/usr/bin/env python3

This is known as the c-bash script. We are telling our python file that whenever we are executing this script, make sure to run it in python3 compiler (and not in python2). It must be mentioned in the top of every Node file that you create.

Then, define conditional statement as below, (at the bottom of the file)

if __name__== '__main__':
    main()

You use this idiom when you want to create an additional entry point for your script, so that your file is accessible as a stand-alone script as well as an importable module.

We want to use the ROS functionalities, so we import necessary libraries.

import rclpy

rclpy stands for ROS Client Library Python.

Now, define the main() function

def main(args=None):
	rclpy.init(args=args) # first line to write in EVERY ROS2 program
	# whatever we want this function to do, comes here
	rclpy.shutdown() # last line of the program

rclpy.init() basically initializes/starts the ROS2 communication. And, any ROS2 file you write without it will fail.

Now, lets modify the above code a little bit more, adding a NODE to it.

from rclpy.node import Node

def main(args=None):
	rclpy.init(args=args)
	node = Node("py_test") # create a Node, named as 'py_test' node
	node.get_logger().info("Hello, ROS2!") # prints the message
	rclpy.shutdown()

Note that, the python file is NOT the node itself. The node is created inside the python file. And, the name of the node, is NOT the name of the file.

The entire first_py_node.py file would look like this

#!/usr/bin/env python3
import rclpy
from rclpy.node import Node

def main(args=None):
	rclpy.init(args=args)
	node = Node("py_test")
	node.get_logger().info("Hello, ROS2!")
	rclpy.shutdown()

if __name__== '__main__':
  main()