communicate between python and C++

Python does not know about the C++ file, it will only be aware of the extension module that is compiled from the C++ file. This extension module is an object file, called a shared library. This file has an interface that looks to Python as if it was a normal Python module.

This object file will only exist after you tell a compiler to compile the C++ file and link it with all the libraries it needs. Of course, the first library needed is Boost.Python itself, which must be available on the system where you are compiling.

You can tell Python to compile the C++ file for you, so that you do not need to mess with the compiler and its library flags. In order to do so, you need a file called setup.py where you use the Setuptools library or the standard Distutils to define how your other Python modules are to be installed on the system. One of the steps for installing is compiling all extension modules, called the build_ext phase.

Let us imagine you have the following directories and files:

hello-world/
├── hello_ext.cpp
└── setup.py

The content of setup.py is:

from distutils.core import setup
from distutils.extension import Extension


hello_ext = Extension(
    'hello_ext',
    sources=['hello_ext.cpp'],
    include_dirs=['/opt/local/include'],
    libraries=['boost_python-mt'],
    library_dirs=['/opt/local/lib'])


setup(
    name='hello-world',
    version='0.1',
    ext_modules=[hello_ext])

As you can see, we are telling Python there is an Extension we want to compile, where the source file is, and where the included libraries are to be found. This is system-dependent. The example shown here is for a Mac OS X system, where Boost libraries were installed via MacPorts.

The content of hello_ext.cpp is as shown in the tutorial, but take care to reorder things so that the BOOST_PYTHON_MODULE macro comes after the definitions of whatever must be exported to Python:

#include <boost/python.hpp>

char const* greet()
{
   return "hello, world";
}

BOOST_PYTHON_MODULE(hello_ext)
{
    using namespace boost::python;
    def("greet", greet);
}

You can then tell Python to compile and link for you by executing the following on the command line:

$ python setup.py build_ext --inplace
running build_ext
building 'hello_ext' extension
/usr/bin/clang -fno-strict-aliasing -fno-common -dynamic -pipe -Os -fwrapv -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes -I/opt/local/include -I/opt/local/Library/Frameworks/Python.framework/Versions/2.7/include/python2.7 -c hello_ext.cpp -o build/temp.macosx-10.9-x86_64-2.7/hello_ext.o
/usr/bin/clang++ -bundle -undefined dynamic_lookup -L/opt/local/lib -Wl,-headerpad_max_install_names -L/opt/local/lib/db46 build/temp.macosx-10.9-x86_64-2.7/hello_ext.o -L/opt/local/lib -lboost_python-mt -o ./hello_ext.so

(The --inplace flag tells Python to leave the products of compilation right next to the source files. The default is to move them to a build directory, to keep the source directory clean.)

After that, you will find a new file called hello_ext.dll (or hello_ext.so on Unix) on the hello-world directory. If you start a Python interpreter in that directory, you will be able to import the module hello_ext and use the function greet, as shown in the Boost tutorial.