It says I didn't subscribe to /odom - but I have!

Course Support ROS2 Basics in 5 Days Humble (Python)
1

It’s clearly using it when I run it, so maybe it’s a naming error?
Here’s the part in the init section

     self.subscriber2 = self.create_subscription(
            Odometry,
            '/odom',
            self.odom_callback,
            QoSProfile(depth=10, reliability=ReliabilityPolicy.BEST_EFFORT))

Then here’s the callback section:

    # scrubbed

and here’s where I use it:

    # scrubbed
2

This is probably because you did not name the node as specified. Did you check out the things Gradebot asked you to? Here is what it said:

image

To check the name of the node subscribed to /odom,

# launch your quiz in one terminal
ros2 launch topics_quiz topics_quiz.launch.py

And

# Check the subscriptions to `/odom` in another terminal
ros2 topic info /odom -v

If needed, you can share the output here.

You can refer to a similar case here:

3

This was the output - but I’m not sure what it’s supposed to look like? Help?

Subscription count: 1

Node name: topics_quiz_node
Node namespace: /
Topic type: nav_msgs/msg/Odometry
Endpoint type: SUBSCRIPTION
GID: 01.0f.54.de.51.05.e0.05.01.00.00.00.00.00.13.04.00.00.00.00.00.00.00.00
QoS profile:
Reliability: BEST_EFFORT
History (Depth): UNKNOWN
Durability: VOLATILE
Lifespan: Infinite
Deadline: Infinite
Liveliness: AUTOMATIC
Liveliness lease duration: Infinite

4

One thing I noticed is that the reliabilities didn’t match. I’ve fixed this now.

5

Also, I’m assuming that the program is supposed to shut itself down (rather than me typing Ctrl-C). My code matches the examples:

    rclpy.spin(topics_quiz)
    # Explicity destroy the node
    topics_quiz.destroy_node()
    # shutdown the ROS communication
    rclpy.shutdown()

Any idea why my code might not be stopping?

6

This output looks good enough for this test. The reliability bit would be another improvement.

Let me check the Gradebot logs and get back.

7

The error was

Unknown topic '/odom'

That’s strange, because that means it did not find the topic. It could be a temporary error in the simulation, so please just try again. I have restored your “lost” trial due to this.

8

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