bar.c

// cc -shared -fPIC -I/usr/include/python3.5m -O -g -o bar.so bar.c
#include "Python.h"

static PyObject* sum_list(PyObject* self, PyObject* args)
{
    Py_ssize_t i, j, n;
    long total = 0, value;
    PyObject *item;
    PyObject *list;
    long *values;

    if (!PyArg_ParseTuple(args, "O!", &PyList_Type, &list)) {
        return NULL;
    }

    n = PyList_Size(list);
    if (n < 0)
        return NULL; /* Not a list */

    values = (long *)malloc(sizeof(long) * n);

    for (i = 0; i < n; i++) {
        item = PyList_GetItem(list, i); /* Can't fail */
        if (!PyLong_Check(item)) continue; /* Skip non-integers */

        // cast to appropriate type
        value = PyLong_AsLong(item);

        if (value == -1 && PyErr_Occurred())
            /* Integer too big to fit in a C long, bail out */
            return NULL;

        values[i] = value;
    }

    for (i = 0; i < n; i++) {
        for (j = 0; j < n; j++) {
            total += values[i] * values[j];
        }
    }

    return Py_BuildValue("i", total);
}

/* Define functions in module */
static PyMethodDef bar_methods[] = {
    {"bar", sum_list, METH_VARARGS, "Sum a list"},
    {NULL, NULL, 0, NULL}  /* Sentinel */
};


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

PyMODINIT_FUNC PyInit_bar(void)
{
    return PyModule_Create(&bar_mod);
}