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In the last lecture, we created a ROS to publisher, so let's create a subscriber node to process the

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data we sent by our publisher.

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In this case, we will just make a simple subscriber which prints the data sent over by the publisher.

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So with Vee's code open, let's create a new file in our scripts folder called Subscriber py.

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Just like our publisher.

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Note we will need to import our ROS client library, our node object and string message type so we can

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just copy those over from the import statements from our publisher script.

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We can also copy the main function as well as the if name equal to main statement.

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From here we're just going to change our my pub variable to my sub.

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I will be creating a class called Hello World Subscriber.

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Rather than saying our publisher node is running, I'll instead say waiting for data to be published

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over the topic.

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And then lastly, in our TRI accept statement, we will change the my pub variables to my sub.

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All right.

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I'll go ahead and close this and let's get to creating our Hello World subscriber class.

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Just like our publisher, we will inherit the node class.

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Then go ahead and reference the node classes in its statement with our node name.

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As you remember, this function takes in our node name, which in this case I will call.

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Hello World Sub node.

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We can now create our subscriber.

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I will just make a variable called sub and use the create subscription function from our node class.

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This function takes in four parameters.

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The first is the message type of the subscriber, which in this case will be our string message we imported

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above.

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The second argument is the topic name to subscribe to.

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If we look back at our publisher, we can see we set it to the string Hello world.

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So you need to make sure that this matches exactly.

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If not, your subscriber node will not receive messages from the right topic.

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The third argument is a callback function.

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The callback function we name here will be run every time we receive a message from our publisher over

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this particular topic.

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In this case, I will create a callback function called subscriber callback.

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Then the last argument is the quality of service profile, which is our subscribers history depth.

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I shouldn't have to worry about this for the simple subscriber, but I'll just set it to ten with the

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subscriber created.

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I'm now going to create the subscriber callback function.

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This function will take in self and the message that comes in over the topic.

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Once we receive the message, we are just going to print it to the terminal screen.

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In this case I will just say received and then append on the content from the messages data attribute.

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All right.

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With that we have finished creating our subscriber script.

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At the top, we imported our Ross Client library, node class and string message type.

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Then we created our Hello World Subscriber class, which is based off of the node class.

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We initialized the node, then created a subscriber which expects a string message type subscribes to

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the hello world topic and triggers our subscriber callback function every time data comes in over the

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topic.

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In this callback function, we took the message received and printed the data attribute of it to the

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terminal.

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Then we have our main function where we initialize our communication pipeline, create an instance of

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our subscriber class print that we are waiting for data to be published and then keep the node running

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until a keyboard interrupt is detected.

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In which case we destroy the node.

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So with that, I've finished my subscriber, although I just notice I'm on spaces for I'll set that

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to tab.

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And I'll go ahead and make sure that this is set correctly.

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All right.

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So now I'm going to go ahead and see what you can do with either the control se keyboard shortcut or

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by hitting file and save.

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And now we can head over to our terminal.

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So let's change it to the directory.

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Our script is located and run the Python three command followed by the name of our subscriber script.

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All right.

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We get the print statement saying that our subscriber node is waiting for data to be published.

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So let's go ahead and open a new terminal and start our publisher script that we made in the last lecture.

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All right.

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So now we see our publisher Node is running and our subscriber node, which is subscribed to that same

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topic, is receiving all of the messages.

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And if I go ahead and stop my publisher node with control C, we stop receiving new messages from our

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publisher node.

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Go ahead and stop our subscriber node.

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And with that you have successfully created your very own ROS two subscriber in Python, which subscribes

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to data sent by our publisher Node.

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Let's quickly review what we learned in this lecture.

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We learned how to create subscribers in Python.

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Within our node class we use the create subscription function to subscribe to a topic with a particular

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message type.

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Every time data is published over that topic, a subscriber callback function runs.

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This callback function takes in the message received over the topic where we can then process it.

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In this lecture we just printed the data attribute of our string message, but you're free to process

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it however you want within your callback function.