Simple boilerplate code for wrapping C code into either python 2 or 3, taken from the porting guide: https://docs.python.org/3/howto/cporting.html

pythonmodule.c

#include "Python.h"

/*********************************************************************************
 * Taken from the porting guide: https://docs.python.org/3/howto/cporting.html
 * 
 * Change all occurrences of
 * - myextension    => the extension name you want to create,
 * - error_out      => the function(s) you want to wrap
 * - METH_NOARGS    => METH_VARARGS|METH_KWARGS depending on if the function
 *                          takes keyword arguments or not
*********************************************************************************/

struct module_state {
    PyObject *error;
};

#if PY_MAJOR_VERSION >= 3
#define GETSTATE(m) ((struct module_state*)PyModule_GetState(m))
#else
#define GETSTATE(m) (&_state)
static struct module_state _state;
#endif

static PyObject *
error_out(PyObject *m) {
    struct module_state *st = GETSTATE(m);
    PyErr_SetString(st->error, "something bad happened");
    return NULL;
}

static char module_docstring[] = "myextension docstring";
static char error_out_docstring[] = "error_out docstring";

static PyMethodDef myextension_methods[] = {
    {"error_out", (PyCFunction)error_out, METH_NOARGS, error_out_docstring},
    {NULL, NULL}
};

#if PY_MAJOR_VERSION >= 3

static int myextension_traverse(PyObject *m, visitproc visit, void *arg) {
    Py_VISIT(GETSTATE(m)->error);
    return 0;
}

static int myextension_clear(PyObject *m) {
    Py_CLEAR(GETSTATE(m)->error);
    return 0;
}


static struct PyModuleDef moduledef = {
        PyModuleDef_HEAD_INIT,
        "myextension",
        module_docstring,
        sizeof(struct module_state),
        myextension_methods,
        NULL,
        myextension_traverse,
        myextension_clear,
        NULL
};

#define INITERROR return NULL

PyObject *
PyInit_myextension(void)

#else
#define INITERROR return

void
initmyextension(void)
#endif
{
#if PY_MAJOR_VERSION >= 3
    PyObject *module = PyModule_Create(&moduledef);
#else
    PyObject *module = Py_InitModule3("myextension", myextension_methods, module_docstring);
#endif

    if (module == NULL)
        INITERROR;
    struct module_state *st = GETSTATE(module);

    st->error = PyErr_NewException("myextension.Error", NULL, NULL);
    if (st->error == NULL) {
        Py_DECREF(module);
        INITERROR;
    }

#if PY_MAJOR_VERSION >= 3
    return module;
#endif
}