Create a ROS2 Python Publisher

Lets create a new Python Publisher program

Create a new Python file called pub.py

• Create a new Python file - from the docker terminal type:

touch my_py_pkg/pub.py
chmod -R 777 my_py_pkg

This will create the new empty file, and fix the permissions so we can write to it from VS Code.

Edit pub.py

Cut and paste the following code into the new file:

import rclpy
from rclpy.node import Node
from std_msgs.msg import String

class Talker(Node):

    def __init__(self):
        super().__init__("node_test")
        self.counter_ = 0
        # create_publisher needs 3 parameters: Msg Type, message, and buffer size
        self.publisher = self.create_publisher(String, "Hello", 10)
        self._timer = self.create_timer(0.5, self.publish_hello)
        self.get_logger().info("publishing message")

    def publish_hello(self):
        msg = String()
        msg.data = "Hello World"
        self.publisher.publish(msg)

def main(args=None):
    rclpy.init(args=args)
    node = Talker()
    rclpy.spin(node)
    rclpy.shutdown()

if __name__ == "__main__":
    main()

Add the entry_point to setup.py

We now need to update setup.py to include the entry_point for our code. An entry point is the name of the python scripts that are available to run from with the package, and are shown in the list when you type ros2 run my_py_pkg.

• Update setup.py - From the VS Code open setup.py from with my_py_pkg folder
• Add the entry points - add the following lines to the entry point list:

entry_points={
    'console_scripts' : [
        "pub = my_py_pkg.pub:main",
        "talker = my_py_pkg.talker:main"
    ],
},

Build the package

We can now build out package using colcon build. Do this from the root of the workspace (in this case /ros2/my_py_pkg):

• change to the root of the workspace- From the docker terminal type:

cd /ros2/my_py_pkg

• Build with colcon - From the docker terminal type:

colcon build

Testing the publisher works

Lets test out our code so far…

• Source install/setup.bash - From the docker terminal type:

source install/setup.bash

• Run the package - From the docker terminal type:

ros2 run my_py_pkg pub 

The package should now run and you iwll see the logger message [INFO] [1673801367.468008815] [node_test]: publishing message.

You can also see the messages themselves by querying the ROS2 topic, via the commandline:

• Echo the topic - From the docker terminal type:

ros2 topic echo Hello

This will show the output from the ROS2 topic Hello, which is the topic we are publishing too.

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