Calling C++ extensions in Python

Question

I'm trying to extend Python 3 with C++ in MSVC++ 2010. I'm completely new to this kind of thing, and I'm also not very proficient in C++ yet. Following from the python documentation and help I received here earlier, I've written the following code which compiles and runs successfully:

#include <Python.h>
#include <iostream>
using namespace std;

static PyObject *SpamError;

static PyObject *spam_system(PyObject *self, PyObject *args)
{
    const char *command;
    int sts;

    if (!PyArg_ParseTuple(args, "s", &command))
        return NULL;
    sts = system(command);
    return PyLong_FromLong(sts);
}

static PyMethodDef SpamMethods[] = {
    {"system",  spam_system, METH_VARARGS,
     "Execute a shell command."},
    {NULL, NULL, 0, NULL}        /* Sentinel */
};

static struct PyModuleDef spammodule = {
   PyModuleDef_HEAD_INIT,
   "spam",   /* name of module */
   NULL,     /* module documentation, may be NULL */
   -1,       /* size of per-interpreter state of the module,
                or -1 if the module keeps state in global variables. */
   SpamMethods
};



PyMODINIT_FUNC
PyInit_spam(void)
{
    PyObject *m;

    m = PyModule_Create(&spammodule);
    if (m == NULL)
        return NULL;

    SpamError = PyErr_NewException("spam.error", NULL, NULL);
    Py_INCREF(SpamError);
    PyModule_AddObject(m, "error", SpamError);
    return m;
}

int main(int argc, wchar_t *argv[])
{
    // Add a builtin module, before Py_Initialize
    PyImport_AppendInittab("spam", PyInit_spam);

    // Pass argv[0] to the Python Interpreter
    Py_SetProgramName(argv[0]);

    // Initialise the Python interpreter
    Py_Initialize();

    // Import module
    PyImport_ImportModule("spam");

    cout << "test" << endl;
    system("PAUSE");
}

I'm still unsure about a few things though; do I need to create a header file for this, and how should I go about doing that?

Also, how am I ultimately meant to make it so that I call the extension through the python shell or within a program?

01100110 · Accepted Answer · 2012-02-25 03:39:53Z

2

I've done this with Boost Python. It builds a DLL or shared object (depending on whether the platform is Windows or Linux) as a Python module that you may then import within Python and use like any other module. It's straight-forward and works reliably. You already have your cpp file and header so all you need is to write wrappers exposing the functions/methods/classes that you want to use from Python. I put the wrappers at the bottom on my cpp files. They look like this:

#include <boost/python.hpp>

BOOST_PYTHON_MODULE(my_module)
{
    boost::python::def("function_name", function, boost::python::args("start", "length", "offset", "boundry", "byte", "data", "variable" ), "docstring");
}

That's it.

answered Feb 25, 2012 at 3:39

01100110

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3 Comments

ixe013 Over a year ago

I couldn't agree more. It's quicker to learn boost that the C Python API anyway, and it will open new world of possibilities.

Tagc Over a year ago

Thanks user, but I've tried downloading Boost before and tried again now, and the download always gets interrupted for some reason. It rarely happens with other content I download. I don't suppose anyone knows a reason for this?

Tagc Over a year ago

I think it finally downloaded successfully. I'll give Boost a go, thanks. :)

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