Boost.Python. Executing C++ inside a python environment

Python is a fantastic scripting language having a simple and easy to learn syntax and style. At LHCb we use Python as a configurable to control the desired functionality that is usually embedded in C++. So why would you want to have two languages? Does that now make things more complicated? Well C++ is inherently much faster to operate so when we have to make decisions which need to be calculated within a few milliseconds we utilise the speed of C++. However, if you have a bunch of classes that are written and just need executing it can be much easier to run a python script to control and setup a job that we want to run as it is a much simpler scripting language. What usually takes many lines of code to do in C++ can be achieved in just a few lines of python. So, I wanted to show an example of a C++ class that can be used as an object in a python instance. The example follows on from the simple vector class (x,y,z) used in the tutorials, With this example we will see how Boost provides tools that converts your class into a fully fledged python object. This means an identical class can be used between platforms, python or C++. To get the source, wget the following:-

tar xzvf boost_python_example.tar.gz
cd boost_python_example
chmod +x

I assume you have cmake, if not a simple “sudo apt-get install cmake” should do the trick. In the build directory created there will be a folder called Vector. The clever part is all done by the Boost.Python template, the one we use in this example is BOOST_PYTHON_MODULE(vectors). Using this wrapper class you can add all the functionality that your class exhibits then behind the scenes the C++ compiler converts this to C-binary understood by python. Running the python script you should see the following:-

matt@matt-W250ENQ-W270ENQ:~/C++/boost_python_example$ ./build/Vectors/ 
vec  = (1, -3.56019, 0.570154)
vec2 = (4.44649, 3, 0.478267)
vec3 = (5.44649, -0.560186, 1.04842)
(1, 1, 1)
(2, 5, 7)
(R, Phi, Theta) = (8.83176086633, 1.19028994968, 0.655744935261)

These are a series of example just to show that the implementation is correct and gives the desired results, we can rotate about the axes, get the length of the vector, change to polar coordinates… Everything we can do in the C++ class. For this vector example we define the following forward declarations of our class:-

// Add the required python headers
#include <boost/python.hpp>
#include <boost/python/operators.hpp>

// The main class definitions
// ...

// Add the python module
using namespace boost::python;
        .def( init<ThreeVector>() )
        .def( init<double, double, double>() )
        .def( self + ThreeVector())
        .def( self - ThreeVector())
        .def( self * ThreeVector())
        .def( self / ThreeVector())
        .def("setXYZ", &ThreeVector::setXYZ)
        .add_property("X", &ThreeVector::getX, &ThreeVector::setX)
        .add_property("Y", &ThreeVector::getY, &ThreeVector::setY)
        .add_property("Z", &ThreeVector::getZ, &ThreeVector::setZ)
        .add_property("R", &ThreeVector::getR, &ThreeVector::setR)
        .add_property("Theta", &ThreeVector::getTheta, &ThreeVector::setTheta)
        .add_property("Phi", &ThreeVector::getPhi, &ThreeVector::setPhi)
        .def("length", &ThreeVector::length)
        .def("rotateX", &ThreeVector::rotateX)
        .def("rotateY", &ThreeVector::rotateY)
        .def("rotateZ", &ThreeVector::rotateZ);

    boost::python::def("arctan", arctan);
    boost::python::def("scalarProduct", scalarProduct);

Firstly we declare the class “ThreeVector”. Then we declare the constructors with the init wrapper, “.def( init() )” followed by the copy assignment operator “.def(self_ns::str(self_ns::self))”. To define operators we use self along with the operator as written above “.def( self + ThreeVector())”. To access getter and setter functions we use the “.add_property(“Name”, &MyClass::getName, &MyClass::setName)”. Any remaining public member functions of the class can be added with the “.def(“func”, &MyClass::func)”. Anyway, that about sums it up for a class. If you have free functions that you want to forward declare for use in python use the def function as shown, “def(“arctan”, arctan);”. It is important to remember the semi-colon at then end of the module declaration and to use full stops after each class attribute that you specify.

Interpolation of a C++ class to a python one is simple with Boost and can be useful for anyone wishing to expand the usability of their code so that python programmers can use it, or simply allow one to write very quick programmes for testing. You can simply open a python terminal import the Vectors object and you’re in business.