Motivation¶

Following are some problems outlined with ROS and rospy that motivates the use of rosonic. However, no rosonic code will be found here.

ROS Nodes¶

When creating a node in ROS, it will typically look something like any of the examples below.

#! /usr/bin/env python

import rospy


## Publisher pattern ##

def main():

    # initialization
    pub = rospy.Publisher(...)
    rate = rospy.Rate(...)

    # main loop
    while not rospy.is_shutdown():
        pub.publish(...)
        rate.sleep()

if __name__ == '__main__':
    rospy.init_node('my_node')

    try:
        main()
    except rospy.ROSInterruptException:
        pass


## Subscriber pattern ##

def callback(msg):
    ...

def main():

    # initialization
    Subscriber(..., callback)

    # main loop
    rospy.spin()

if __name__ == '__main__':
    rospy.init_node('my_node')
    main()


## Pub-Sub pattern ##

def callback(msg, args):
    (pub1, pub2) = args

    pub1.publish(...)

    pub2.publish(...)

def main():

    # initialization
    pub1 = Publisher(...)
    pub2 = Publisher(...)
    Subscriber(..., callback, (pub1, pub2))

    # main loop
    rospy.spin()

if __name__ == '__main__':
    rospy.init_node('my_node')
    main()

Neither of these are very complicated. However, this is mainly since they follow a clear design pattern. When developing, especially in research- oriented projects,following design patterns are often second priority and complexity tend to get out of hand. This is especially true when you start using other ROS concepts more and more.

For new users there is also the overhead of learning what a ROS node is, how it behaves and how they relate to other ROS concepts. It may be easy to explain that ROS starts a separate process that runs your python script as a standalone program, the script declares itself to be ROS node with rospy.init_node, opens communication channels for any of your topics, and then it executes your logic. However, that is probably unclear for any developers/researcher that gets thrown into ROS for the first time.

As a minor example, how should you design a subscribe-publish node? To clarify, you node should subscribe to some topic, augment the incoming data then, on a new topic, publish it. Should you initialize in main? How does callback then have your pub? Maybe you declare pub globally, but that’s a Python anti-pattern! Of course you can solve this neatly and in many ways, but there is nothing in rospy that motivates you to follow best-practice and/or a good design patter. Below is one solution where pub is sent as an argument to callback but you can imagine how that can get out of hand with more publisher and subscribers. The initialization is moved to a dedicated function and, like previously, the node ends on rospy.spin().

#! /usr/bin/env python

import rospy

def callback(msg, pub):
    pub.publish(...)

def init():
    pub = rospy.Publisher(...)
    rospy.Subscriber(..., callback, pub)

if __name__ == '__main__':
    rospy.init_node('my_node')
    init()
    rospy.spin()

ROS Parameters¶

Enough about design patterns! I don’t care about writing nice code, why should I care about rosonic?

Now, let me tell you! Surely the most important problems to solve are those that cause mild inconvenience. rosonic will help you once again before you can say “ROS parameters”! Let’s look at a typical scenario…

#! /usr/bin/env python

import rospy

use_filter = True

def main():
    global use_filter

    # initialize

    has_image2 = rospy.has_param('~image2')
    assert has_image2, 'Parameter "image2" is missing!'

    image1 = rospy.get_param('~image1', 'image1')
    image2 = rospy.get_param('~image2')
    use_filter = rospy.get_param('~use_filter', True)

    pub = rospy.Publisher(image1, ...)
    Subscriber(image2, ...)

    # main loop

    rospy.spin()

def callback(msg):

    if use_filter:
        ...

    ...


if __name__ == '__main__':
    main()

There are multiple problems with passing topic names through parameters, a very typical use case. Firstly, the distinction between '~image1' and 'image1' is not very clear (a problem not solved by rosonic). Many times '~image1' may have the default value 'image1' as well! Secondly, asserting that required parameters, such as '~image2', exists is clunky. Thirdly, parameter values must be passed to different functions/scopes or declared global (same problem as pub before).