Sam Redway Sam Redway - 2 years ago 120
Python Question

Compiler can't find Py_InitModule() .. is it deprecated and if so what should I use?

I am attempting to write a C extension for python. With the code (below) I get the compiler warning:

implicit declaration of function ‘Py_InitModule’

And it fails at run time with this error:

undefined symbol: Py_InitModule

I have spent literally hours searching for a solution with no joy. I have tried multiple minor changes to syntax, I even found a post suggesting the method has been deprecated. However I find no replacement.

Here is the code:

#include <Python.h>

//a func to calc fib numbers
int cFib(int n)
if (n<2) return n;
return cFib(n-1) + cFib(n-2);

static PyObject* fib(PyObject* self,PyObject* args)
int n;
if (!PyArg_ParseTuple(args,"i",&n))
return NULL;
return Py_BuildValue("i",cFib(n));

static PyMethodDef module_methods[] = {
{"fib",(PyCFunction) fib, METH_VARARGS,"calculates the fibonachi number"},

PyMODINIT_FUNC initcModPyDem(void)
Py_InitModule("cModPyDem",module_methods,"a module");

If it helps here is my :

from distutils.core import setup, Extension

module = Extension('cModPyDem', sources=['cModPyDem.c'])
setup(name = 'packagename',
description = 'a test package',
ext_modules = [module])

And the test code in :

import cModPyDem

if __name__ == '__main__' :


Any help would be much, much appreciated.

Answer Source

The code you have would work fine in Python 2.x, but Py_InitModule is no longer used in Python 3.x. Nowadays, you create a PyModuleDef structure and then pass a reference to it to PyModule_Create.

The structure would look like:

static struct PyModuleDef cModPyDem =
    "cModPyDem", /* 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. */

And then your PyMODINIT_FUNC function would look like:

PyMODINIT_FUNC PyInit_cModPyDem(void)
    return PyModule_Create(&cModPyDem);

Note that the name of the PyMODINIT_FUNC function must be of the form PyInit_<name> where <name> is the name of your module.

I think it would be worthwhile if you read Extending in the Python 3.x documentation. It has a detailed description of how to build extension modules in modern Python.

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