ros_cython_example

What is this?

This package is an example of ROS package with cython. Cython is great framework to couple python and C very easily.

ROS nodes written in python are sometimes suffered from performance problem, and cython can be a solution.

This package is simplified and self-contained, so I think it is easy to re-use for your own ROS package.

Quick start

On a terminal:

$ roslaunch ros_cython_example server.launch

On another terminal:

$ rosservice list
/add_two_ints
/fibonacci
/mul_two_ints
/sub_two_ints

$ rosservice call /add_two_ints  1 2
result: 3

Now the first terminal screen says the server invoked the python version function of 'add_two_ints'(src/ros_cython_example/add_two_ints.py).

Try another service call as:

$ rosservice call /fibonacci 10
result: 55.0

Now the first terminal screen says the server invoked the C version function of 'fibonacci'(src/fibonacci.c). The C code are called from python through cython framework.

You can easily switch the function version between C and python, by modifing the CMakeLists.txt and rebuild the package.

Contents of the package

The package contains a server written in python (script/server.py). The server has four services:

• add_two_ints
• sub_two_ints
• mul_two_ints
• fibonacci

Each service is corresponding to a function, and the functions are (originally) written in python files in the package directory (src/ros_cython_example).

Assume we want to re-implement 'mul_two_ints' and 'fibonacci' in C because of performance reason. The functions are implemented in C in files:

• src/mul_two_ints.c
• src/fibonacci.c

These C modules are compiled as a shared library (libros_cython_example.so), by CMakeLists.txt on the top level.

You need cython wrapper codes to call the C functions from python. The wrapper codes are src/cython/*.pyx.

For example, mul_two_ints.pyx is:

cdef extern from "ros_cython_example/mul_two_ints.h":
    int c_mul_two_ints(int a, int b)

def mul_two_ints(a, b):
    return c_mul_two_ints(a, b)

The cmake files (src/cython/CMakeLists.txt and top level CMakeLists.txt) specify how to compile cython codes.

The contents of cmake/ directory is from cython_cmake_example project.

Thanks to

• cython_catkin_example by @marcoesposito1988
• cython_cmake_example by @thewtex

Memo

This package owes much to the cython_catkin_example project. I just want to show another simplified and extendable example to fit actual ROS usage.

In this example, I assume fairly standard ROS package contents as follows:

ros_catkin_example/
    cmake/ <-- cmake module files for build cython files
    CMakeLists.txt <-- for top level catkin_make
    setup.py <-- for catkin_make
    include/cython_catkin_example <-- header files for c/c++
    src/ <-- source files 
        ros_catkin_example/ <-- python package source files
	    cython/ <-- cython source files (*.pyx, etc)
		    CMakeLists.txt <-- for building cython, included from top level