A template of writing C or C++ extension for python and numpy

README.md

This gist demonstrates how to setup a python project that process a numpy array from C language.

To compile the project, run

make all

To test it, run

make test

latest version of gist: https://gist.github.com/kanhua/8f1eb7c67f5a031633121b6b187b8dc9

hello.c

#include <stdio.h>
#include <Python.h>
#include "numpy/ndarraytypes.h"
#include "numpy/ufuncobject.h"
#include "numpy/npy_3kcompat.h"

// Module method definitions
static PyObject* hello_world_c(PyObject *self, PyObject *args) {
    printf("Hello, world!\n");
    Py_RETURN_NONE;
}


static PyObject* hello_numpy_c(PyObject *dummy, PyObject *args)
{
    PyObject *arg1=NULL;
    PyObject *arr1=NULL;
    int nd;

    if (!PyArg_ParseTuple(args, "O", &arg1))
        return NULL;

    arr1 = PyArray_FROM_OTF(arg1, NPY_DOUBLE, NPY_IN_ARRAY);
    /*
     * my code starts here
     */
    nd=PyArray_NDIM(arr1);

    npy_intp *sp=PyArray_SHAPE(arr1);

    printf("array dimentsion: %ld\n",*sp);

    printf("Print array elements:\n");
    
    for (int i=0; i<*sp; i++)
    {
        printf("%lf ",*((npy_double*)PyArray_GETPTR1(arr1,i)));
    }

    printf("\n");

    if (arr1 == NULL)
        return NULL;

    nd = PyArray_NDIM(arr1);   //number of dimensions

    Py_DECREF(arr1);

    return PyInt_FromLong(nd);
}


static PyMethodDef hello_methods[] = {
        {
                "hello_python", hello_world_c, METH_VARARGS,
                "Print 'hello xxx'"
        },
        {
                "hello_numpy", hello_numpy_c, METH_VARARGS,
                "numpy function tester",
        },
        {NULL, NULL, 0, NULL}
};


static struct PyModuleDef hello_definition = {
        PyModuleDef_HEAD_INIT,
        "hello",
        "A Python module that prints 'hello world' from C code.",
        -1,
        hello_methods
};


PyMODINIT_FUNC PyInit_hello(void) {
    Py_Initialize();
    import_array();
    return PyModule_Create(&hello_definition);
}

makefile

all: hello.c setup.py
	python3 setup.py build_ext --inplace

test: all
	python3 test_mod.py

setup.py

#!/usr/bin/env python3
# encoding: utf-8

from distutils.core import setup, Extension


def configuration(parent_package='', top_path=None):
      import numpy
      from numpy.distutils.misc_util import Configuration
      from numpy.distutils.misc_util import get_info

      #Necessary for the half-float d-type.
      info = get_info('npymath')

      config = Configuration('',
                             parent_package,
                             top_path)
      config.add_extension('hello',
                           ['hello.c'],
                           extra_info=info)

      return config

if __name__ == "__main__":
      from numpy.distutils.core import setup
      setup(configuration=configuration)

test_mod.py

import hello
import numpy as np

hello.hello_python()
hello.hello_numpy(np.array([1,2,3]))

I am getting this error,

error: cannot convert ‘PyObject*’ {aka ‘_object*’} to ‘PyArrayObject*’ {aka ‘tagPyArrayObject_fields*’}
         npy_intp *sp=PyArray_SHAPE(arr1);

Edit: Ah. You code is in C. I was using C++. In C++, you need to do

PyArrayObject * np_arr1 = (PyArrayObject*)(arr1);
npy_intp *sp=PyArray_SHAPE(np_arr1);