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Overview
Setting up the Raspberry Pi 4 environment
Setting Docker for ROS
Build the ROS2 Container
Setting up VSCode
First ROS2 Program
ROS2 Python Packages
Docker and X11
RViz2

Create a ROS2 Python Subscriber

Lets create a new Python Subscriber program

By Kevin McAleer,    2 Minutes

Create a new Python file called sub.py

  • Create a new Python file - from the docker terminal type:
touch my_py_pkg/sub.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 sub.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 Listener(Node):

    def __init__(self):
        super().__init__("node_test")
        self.counter_ = 0
        # create_subscriber needs 3 parameters: Msg Type, topic name, the callback, and queue size buffer
        self.subscriber = self.create_subscription(String, "Hello", self.callback_hello, 10)
        self.get_logger().info("Subscribed to Hello topic")

    def callback_hello(self, message):
        self.get_logger().info(message.data)
     
def main(args=None):
    rclpy.init(args=args)
    node = Listener()
    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",
        "listener = my_py_pkg.sub: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 and source the new setup.bash - From the docker terminal type:
colcon build
source install/setup.bash

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