Python 3 extension example

README.md

Python 3 extension example

Build

python3 setup.py build

Output: build/lib.macosx-10.11-x86_64-3.5/hello.cpython-35m-darwin.so

Run

$ cd build/lib.macosx-10.11-x86_64-3.5
$ python3
>>> import hello
>>> hello.hello_world()
Hello, world!
>>> hello.hello('world')
Hello, world!

Tested on OS X 10.11.5, Python 3.5.2.

References

• Python Programming/Extending with C
• Extending Python with C or C++
• Writing extensions for Python 3 in C

hello.c

#include <stdio.h>
#include <Python.h>

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

static PyObject* hello(PyObject *self, PyObject *args) {
    const char* name;
    if (!PyArg_ParseTuple(args, "s", &name)) {
        return NULL;
    }

    printf("Hello, %s!\n", name);
    Py_RETURN_NONE;
}

// Method definition object for this extension, these argumens mean:
// ml_name: The name of the method
// ml_meth: Function pointer to the method implementation
// ml_flags: Flags indicating special features of this method, such as
//          accepting arguments, accepting keyword arguments, being a
//          class method, or being a static method of a class.
// ml_doc:  Contents of this method's docstring
static PyMethodDef hello_methods[] = { 
    {   
        "hello_world", hello_world, METH_NOARGS,
        "Print 'hello world' from a method defined in a C extension."
    },  
    {   
        "hello", hello, METH_VARARGS,
        "Print 'hello xxx' from a method defined in a C extension."
    },  
    {NULL, NULL, 0, NULL}
};

// Module definition
// The arguments of this structure tell Python what to call your extension,
// what it's methods are and where to look for it's method definitions
static struct PyModuleDef hello_definition = { 
    PyModuleDef_HEAD_INIT,
    "hello",
    "A Python module that prints 'hello world' from C code.",
    -1, 
    hello_methods
};

// Module initialization
// Python calls this function when importing your extension. It is important
// that this function is named PyInit_[[your_module_name]] exactly, and matches
// the name keyword argument in setup.py's setup() call.
PyMODINIT_FUNC PyInit_hello(void) {
    Py_Initialize();
    return PyModule_Create(&hello_definition);
}

setup.py

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

from distutils.core import setup, Extension

hello_module = Extension('hello', sources = ['hello.c'])

setup(name='hello',
      version='0.1.0',
      description='Hello world module written in C',
      ext_modules=[hello_module])

It's working good.

Hi ...i am not very familiar with c extension for python.I have some array calculation coding with c extension.but That's not working fine .am got some error when compile that code .anyone can help me...

I am currently trying to implement array multiplication with some formula in c .but don't really know how I would create empty array with input dimension and how to return from c function to extension main function.

My c code return array to main function.
#include "chie.h"

double chie(double m, double b, double *x, double *y, double *yerr, int N,double *result) {
int n;
double diff;
//double result = 0.0;
//static double result[5];
for (n = 0; n < N; n++) {
diff = (y[n] - (m * x[n] + b)) / yerr[n];
result[n]= diff * diff;
}

//return result;

}

then how to i catch the c array returned data to empty array.
this is my c extension main code.

#include <Python.h>
#include <numpy/arrayobject.h>
#include "chie.h"

#define NPY_NO_DEPRECATED_API NPY_1_7_API_VERSION

static char module_docstring[] =
"This module provides an interface for calculating chi-squared using C.";
static char chie_docstring[] =
"Calculate the chi-squared of some data given a model.";

static PyObject *chi2_chi2(PyObject *self, PyObject *args);

static PyObject *chi2_chi2(PyObject *self, PyObject *args)
{
double m, b;
PyObject *x_obj, *y_obj, *yerr_obj;
double *result[5];

/* Parse the input tuple */
if (!PyArg_ParseTuple(args, "ddOOO", &m, &b, &x_obj, &y_obj,
                                    &yerr_obj))
    return NULL;

/* Interpret the input objects as numpy arrays. */
PyObject *x_array = PyArray_FROM_OTF(x_obj, NPY_DOUBLE, NPY_IN_ARRAY);
PyObject *y_array = PyArray_FROM_OTF(y_obj, NPY_DOUBLE, NPY_IN_ARRAY);
PyObject *yerr_array = PyArray_FROM_OTF(yerr_obj, NPY_DOUBLE,
                                        NPY_IN_ARRAY);

/* If that didn't work, throw an exception. */
if (x_array == NULL || y_array == NULL || yerr_array == NULL) {
    Py_XDECREF(x_array);
    Py_XDECREF(y_array);
    Py_XDECREF(yerr_array);
    return NULL;
}

/* How many data points are there? */
int N = (int)PyArray_DIM(x_array, 0);

/* Get pointers to the data as C-types. */
double *x    = (double*)PyArray_DATA(x_array);
double *y    = (double*)PyArray_DATA(y_array);
double *yerr = (double*)PyArray_DATA(yerr_array);

/* Call the external C function to compute the chi-squared. */
chie(m, b, x, y, yerr, N,result);

/* Clean up. */
Py_DECREF(x_array);
Py_DECREF(y_array);
Py_DECREF(yerr_array);

/*if (value < 0.0) {
    PyErr_SetString(PyExc_RuntimeError,
                "Chi-squared returned an impossible value.");
    return NULL;
}*/

/* Build the output tuple */
return PyArray_Return(result);
//PyObject *ret = Py_BuildValue("d", result);
//return ret;

}

static PyMethodDef module_methods[] = {
{"chie", chi2_chi2, METH_VARARGS, chie_docstring},
{NULL, NULL, 0, NULL}
};

static struct PyModuleDef cieModule = {
PyModuleDef_HEAD_INIT,
"cieModule",
"example Module",
-1,
module_methods
};

/*PyMODINIT_FUNC PyInit_myexamp_Module(void)
{
return PyModule_Create(&myexamp_Module);
import_array();
}
*/
PyMODINIT_FUNC PyInit_cieModule(void)
{
PyObject *m;
m = PyModule_Create(&cieModule);
if (!m) {
return NULL;
}
import_array();
return m;
}

Great!

Really nice!

This demo seems to be written for macintosh. Will it works on Windows? Either way, it would be helpful to mention it in the readme and possibly in the title.

Thanks!

Maybe someone here can help me. I do not only want to create a module in C, I also want to use it in C. The following code, however, shows that my embedded method is not read.

#include <stdio.h>
#include <Python.h>

const char* pystart = 
"print('Start')\n\
import eumat\n\
print(dir(eumat))\n\
print(eumat.test())";

static PyObject* ftest (PyObject* self, PyObject* args)
{
	return PyLong_FromLong(1000);
}

static PyMethodDef EmbMethods[] = {
	{
		"test", (PyCFunction)ftest, METH_NOARGS,"Test Function"
	},
	{NULL, NULL, 0, NULL}
};

int main()
{
	printf("Test Programm for Python 3.8\n");

	Py_Initialize();

	static struct PyModuleDef moduledef = {
		PyModuleDef_HEAD_INIT,
		"eumat",
		PyDoc_STR("EMT Module"),
		-1,
		EmbMethods
	};

	PyObject* eumat = PyModule_Create(&moduledef);
	if (!eumat)
	{
		printf("Could not create eumat module in Python.");
		return 0;
	}

	PyObject* pModule = PyImport_AddModule("eumat"); //create main module
	if (!pModule)
	{
		printf("Could not add the module eumat in Python.");
		return 0;
	}

	if (PyRun_SimpleString(pystart) == -1)
	{
		printf("Could not run pystart.");
		return 0;
	}

	return 1;
}

/*
Test Programm for Python 3.8
Start
['__doc__', '__loader__', '__name__', '__package__', '__spec__']
Traceback (most recent call last):
  File "", line 4, in 
AttributeError: module 'eumat' has no attribute 'test'
Could not run pystart.
C:\Users\reneg\source\repos\HalloWorld\x64\Release\HalloWorld.exe (process 30676) exited with code 0.
Press any key to close this window . . .
*/

Just for the records, I solved it:

#include <stdio.h>
#include <Python.h>

const char* pystart = 
"print('Start')\n\
import eumat\n\
print(dir(eumat))\n\
print(eumat.test())";

static PyObject* ftest (PyObject* self, PyObject* args)
{
	return PyLong_FromLong(1000);
}

static PyMethodDef EmbMethods[] = {
	{
		"test", (PyCFunction)ftest, METH_NOARGS,"Test Function"
	},
	{NULL, NULL, 0, NULL}
};

int main()
{
	printf("Test Programm for Python 3.8\n");

	Py_Initialize();

	static struct PyModuleDef moduledef = {
		PyModuleDef_HEAD_INIT,
		"eumat",
		PyDoc_STR("EMT Module"),
		-1,
		EmbMethods
	};


	PyObject* eumat = PyImport_AddModule("eumat"); //create main module
	if (!eumat)
	{
		printf("Could not add the module eumat in Python.");
		return 0;
	}

	PyModule_AddFunctions(eumat, EmbMethods);

	if (PyRun_SimpleString(pystart) == -1)
	{
		printf("Could not run pystart.");
		return 0;
	}

	return 1;
}

Thanks bro！