Using Boost::Python with catkin

This is a minimal example of how to use Boost::Python in a catkin package. As there is little helpful documentation about this topic on the internet, I try to summarize all necessary steps here.

First you need some C++ code. src/mycpplib.cpp contains a simple hello world function that is made available for Python using Boost::Python. (see documentation of Boost::Python for more details on how to use it).

To compile it and make it available for python, some configuration in the CMakeLists.txt has to be done:

Configure cmake

1. You need Boost::Python and the path to the python include dirs:

   find_package(Boost REQUIRED COMPONENTS python)
   find_package(PythonLibs REQUIRED) # sets ${PYTHON_INCLUDE_DIRS}

   Make sure to add them to the include directories:

   include_directories(
     ${catkin_INCLUDE_DIRS}
     ${Boost_INCLUDE_DIRS}
     ${PYTHON_INCLUDE_DIRS}
   )

2. Uncomment catkin_python_setup(). This will set up the destination path of the python module. You also need a basic setup.py in the packages root directory.

3. Add a library:

   add_library(mycpplib SHARED
     src/mycpplib.cpp
   )
   target_link_libraries(mycpplib
     ${catkin_LIBRARIES}
     ${Boost_LIBRARIES}
   )
   # change output directory, so python can find the module and avoid the 'lib'-prefix
   set_target_properties(mycpplib PROPERTIES
     PREFIX "" 
     LIBRARY_OUTPUT_DIRECTORY ${CATKIN_DEVEL_PREFIX}/${CATKIN_PACKAGE_PYTHON_DESTINATION}
   )

   The last command is important because it will change the destination of the compiled library from /catkin_ws/devel/lib/ to /catkin_ws/devel/lib/python2.7/dist-packages/ so it can be found by python. Also don't forget the SHARED in add_library.

Use the module

After you compiled the package, you should be able to use the wrapped module in Python:

import boostpy_test.libmycpplib as cpp
cpp.hello()

Boost::Python and ROS::NodeHandle

When using ROS, there is one important thing to keep in mind: Assume you have a ROS node written in Python, that uses some C++ code via Boost::Python. If the C++ code needs a ros::NodeHandle, for example to fetch some parameters from the parameter server, it will crash, because the rospy.init_node() does not initialize roscpp!

To get around this, you need the MoveIt ROS planning interface which is available in the ROS repos (sudo apt-get install ros-groovy-moveit-ros-planning-interface). Once installed, add the following code to your Python node:

from moveit_ros_planning_interface._moveit_roscpp_initializer import roscpp_init
roscpp_init('node_name', [])

Now everything should work fine.

References

Some references that were helpful for me in one or the other way:

• This page, that I unfortunatly only found after I figured most of it out by myself, should explain everything... http://wiki.ros.org/ROS/Tutorials/Using%20a%20C%2B%2B%20class%20in%20Python
• Boost::Python documentation: http://www.boost.org/doc/libs/1_57_0/libs/python/doc/index.html
• Using Boost::Python with cmake: https://www.preney.ca/paul/archives/107
• https://github.com/ethz-asl/Schweizer-Messer/wiki/Adding-boost::python-exports-to-your-ROS-package
• http://wiki.ros.org/catkin/CMakeLists.txt, Section 7.2 (the set_target_properties command comes from there)