In this lesson, we will create our very own ROS2 Publisher Node using Python. We'll make a simple publisher that publishes a string that says Hello World, followed by a number which increments up every time we publish. Then in a future lesson, we'll create a subscriber to receive the messages we publish.
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For now, let's open our code editor → open folder → open ros2_py_udemy_tutorial workspace folder.
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In the VS Code File Explorer Panel on the left, under the source directory (src) → udemy_ros2_pkg package folder → script folder → Create a new file named publisher.py.
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For more information on ROS2 Python API you can refer to the official rclpy Documentation.
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Add the following code to publisher.py file.
import rclpy # importing rclpy module : Python library for ROS 2 from rclpy.node import Node # importing Node class from rclpy.node module from std_msgs.msg import String # importing message datatype (class) - String - from # std_msgs.msg module class HelloWorldPublisher(Node): # class 'HelloWorldPublisher' is inheriting from 'Node' class # Constructor : Is called when an object of this class is created. def __init__(self): super().__init__("hello_world_pub_node") # Calling the __init__() method # (constructor) of super class 'Node'. # Takes the "name of the node" # as String type argument # Creating a publisher self.pub = self.create_publisher(String, 'hello_world', 10) # self.pub is just an attribute of HelloWorldPublisher class. # We are creating a 'publisher' inside attribute 'self.pub' using the # self.create_publisher() method which is inherited from the parent class 'Node'. # create_publisher() method takes 3 parameters namely - Message Datatype (String), Topic Name ('hello_world'), qos (quality of service) # qos = it is an Integer which tells how many unpublished mesages should the publisher keep in record that are waiting to be sent over the network or delivered to subscribers) self.counter = 0 # self.counter is just another attribute # of HelloWorldPublisher class # Creating a timer self.timer = self.create_timer(0.5, self.publisher_callback_func) # self.create_timer() method is inherited from the parent class 'Node'. # This method will trigger the 'self.publisher_callback_func()' function after every o.5 seconds. # The create_timer method is provided by the Node class and takes two arguments: the timer period (0.5 seconds) and the callback function (self.publisher_callback_func). # This method is the callback function that will be executed by the timer. def publisher_callback_func(self): msg = String() # creating a new 'std_msgs.msg.String' class message object. msg.data = "Hello World" + str(self.counter) # assigning a value to the message object's 'data' field self.pub.publish(msg) # publishing the message using the publish() method of self.pub self.counter += 1 # incrementing the self.counter attribute value by 1. def main(args=None): rclpy.init() # Initializing the ROS 2 communication system my_pub = HelloWorldPublisher() # Creating object of HelloWorldPublisher class print("Publisher Node is running...") try: rclpy.spin(my_pub) # When rclpy.spin() is called with a 'Node' instance ('my_pub' in this case) # - it starts the event loop of the ROS 2 system. # The event loop is a continuous loop that keeps the ROS 2 node active and responsive to various events, such as receiving messages, calling callbacks, and handling other system tasks. # As long as the event loop is active, the publisher node will continue to publish messages (using publisher_callback_func(self) method) at the specified rate (0.5 s) and respond to any incoming events. # As long as the event loop is active, it blocks the further execution of any other code written below. # The event loop runs until it is explicitly terminated. In this code, the termination of the event loop is triggered by a KeyboardInterrupt error below. When a keyboard interrupt is detected, the execution of the event loop stops, and the program flow moves to the exception handling block. except KeyboardInterrupt: # KeyboardInterrupt error is caused on pressing (ctrl + C) to explicitly # terminate a process/operation going on in the console/terminal. print("Terminating publisher...") my_pub.destroy_node() # Destroying the node if __name__=='__main__': # It Allows You to Execute the Code inside it When the File Runs as a Script, but Not When The File is Imported as a Module. main()
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You can run this file from your VS Code in two ways (your VS Code must be opened from your workspace folder)
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Using the Run Python File Button at the top-right corner of your VS Code window.
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Using VS Code Integrated Terminal (or any other terminal)
python3 /home/pritam/Ros2_Workspaces/ros2_py_udemy_tutorial/src/udemy_ros2_pkg/scripts/publisher.py # python3 <complete address of your publisher.py file>python3 /home/pritam/Ros2_Workspaces/ros2_py_udemy_tutorial/src/udemy_ros2_pkg/scripts/publisher.py # python3 <complete address of your publisher.py file>
We cannot see the messages published by the publisher.py file in the current terminal because they are being published/broadcasted over ROS 2 DDS (Data Distribution Service).
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Keep the current terminal open & open a 2nd terminal.
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To get the list of currently active nodes use the following command in the 2nd terminal:
ros2 node list
As we can see the name of our publisher.py file’s node name hello_world_pub_node in the the list of the currently active nodes - this proves that our file publisher.py is running successfully.
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To get the list of currently active topics use the following command in the 2nd terminal:
ros2 topic list
This should list our publisher.py file’s topic name hello_world in the output.
/parameter_events & /rosout are default topics of ROS 2 DDS.
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To see the messages published by our publisher.py file’s hello_world topic, use the following command in the 2nd terminal:
ros2 topic echo /hello_world # ros2 topic echo /topic_name
