Perform math expressions (or any python expression) on ros topics with data fields (ints & floats)

rosrun topic_op_ros topic_op.py "{'a': '/a', 'b': '/b'}" '(a + b)' /result1 
rostopic pub "a" std_msgs/Float32 "data: 0.5" 
rostopic pub "b" std_msgs/UInt8 "data: 14" 
rostopic echo /result1

Also publish a derivative of a topic with a float or int type:

rosrun topic_op derivative input_topic output_topic

If input_topic varies in time, then the derivative will be published to output_topic. Because the basic types aren't stamped the derivative is based on the timestamp of the reception of the published input topic.

https://github.com/lucasw/topic_op_ros