Building Command-line Interfaces (CLIs) for Python Nodes

COM2009 Assignment #1 Checkpoint

It helps if you've already completed Assignment #1 Part 1 before working on this.

Suppose we have a very simple node such as the publisher.py node that you create in Part 1. This node publishes std_msgs/String type messages to a topic called "chatter". Let's have a look at an alternative version of that node that takes in command-line arguments and publishes those to the "chatter" topic instead.

Take a look at the publisher_cli.py node from the tuos_examples package.

Here, we use the Python argparse module to allow us to work with arguments that are passed to the node from the command-line:

import argparse

We instantiate argparse in the __init__() method of the Publisher() class to build a command-line interface (CLI) for the node:

cli = argparse.ArgumentParser(...)
cli.add_argument(...)

(See the code for more details)

Arguments that we define with a - at the front will be optional, i.e. we don't have to provide these every time we run the node. We do, however, need to assign a default value for each optional argument in cases where no value is supplied, e.g.:

cli.add_argument(
    "-colour",
    metavar="COL",
    default="Blue", 
    ...

The final step is to grab any arguments that are passed to this node when it is called. We use the rospy.myargv() method here, so that this works regardless of whether we call the node using rosrun or roslaunch:

self.args = cli.parse_args(rospy.myargv()[1:])

Having defined the CLI above, argparse then automatically generates help text for us! Try running the following command to see this in action:

rosrun tuos_examples publisher_cli.py -h

Warning

You need to have a ROS Master running in order for this to work, so do this in another terminal instance by running the roscore command.

Run the node as it is (using rosrun) and see what happens:

rosrun tuos_examples publisher_cli.py

Stop the node (using Ctrl+C) and then run it again, but this time providing a value for the number variable, via the CLI:

rosrun tuos_examples publisher_cli.py -number 1.5

Info

We can assign values to Python command-line arguments using a space (as above) or the = operator (-number=1.5).

Stop the node again (using Ctrl+C) and also stop the ROS Master that you enabled in another terminal instance.