Building Python C extension

Question

I need to create a new python module as a c extension for both performance and CUDA utilization. I have tried a couple of tutorials for this and was unsuccessful. Here are my files:

hellomodule.c

#include <Python.h>

static PyObject* say_hello(PyObject* self, PyObject* args)
{
    const char* name;

    if (!PyArg_ParseTuple(args, "s", &name))
        return NULL;

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

    Py_RETURN_NONE;
}

static PyMethodDef HelloMethods[] =
{
     {"say_hello", say_hello, METH_VARARGS, "Greet somebody."},
     {NULL, NULL, 0, NULL}
};

PyMODINIT_FUNC inithello(void)
{
     (void) Py_InitModule("hello", HelloMethods);
}

setuphello.py

from distutils.core import setup, Extension

module1 = Extension('hello', sources = ['hellomodule.c'])

setup (name = 'PackageName',
    version = '1.0',
        description = 'This is a demo package',
        ext_modules = [module1])

And here is my result for python setuphello.py build:

running build
running build_ext
building 'hello' extension
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\BIN\cl.exe /c /nologo /Ox /W3 /GL /DNDEBUG /MD -IC:\Anaconda3\include -IC:\Anaconda3\include "-IC:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\INCLUDE" "-IC:\Program Files (x86)\Windows Kits\10\include\10.0.10240.0\ucrt" "-IC:\Program Files (x86)\Windows Kits\NETFXSDK\4.6.1\include\um" "-IC:\Program Files (x86)\Windows Kits\10\include\10.0.10240.0\shared" "-IC:\Program Files (x86)\Windows Kits\10\include\10.0.10240.0\um" "-IC:\Program Files (x86)\Windows Kits\10\include\10.0.10240.0\winrt" "-IC:\Program Files (x86)\IntelSWTools\Trace Analyzer and Collector\9.1.2.024\include" /Tchellomodule.c /Fobuild\temp.win32-3.5\Release\hellomodule.obj
hellomodule.c
hellomodule.c(23): warning C4013: 'Py_InitModule' undefined; assuming extern returning int
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\BIN\link.exe /nologo /INCREMENTAL:NO /LTCG /DLL /MANIFEST:EMBED,ID=2 /MANIFESTUAC:NO /LIBPATH:C:\Anaconda3\libs /LIBPATH:C:\Anaconda3\PCbuild\win32 "/LIBPATH:C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\LIB" "/LIBPATH:C:\Program Files (x86)\Windows Kits\10\lib\10.0.10240.0\ucrt\x86" "/LIBPATH:C:\Program Files (x86)\Windows Kits\NETFXSDK\4.6.1\lib\um\x86" "/LIBPATH:C:\Program Files (x86)\Windows Kits\10\lib\10.0.10240.0\um\x86" /EXPORT:PyInit_hello build\temp.win32-3.5\Release\hellomodule.obj /OUT:build\lib.win32-3.5\hello.cp35-win32.pyd /IMPLIB:build\temp.win32-3.5\Release\hello.cp35-win32.lib
LINK : error LNK2001: unresolved external symbol PyInit_hello
build\temp.win32-3.5\Release\hello.cp35-win32.lib : fatal error LNK1120: 1 unresolved externals
error: command 'C:\\Program Files (x86)\\Microsoft Visual Studio 14.0\\VC\\BIN\\link.exe' failed with exit status 1120

I have looked at the various errors others have had and have tried following their debugging logic but am honestly at a loss as to what is happening behind the scenes that is causing my error. I am using Python 3.5 32-bit (Anaconda) and so have been trying to compile with the Visual C++ build tools and its packaged terminal. This doesn't make any difference however. Can someone please lead me in the right direction?

sokin · Accepted Answer · 2016-07-02 18:38:50Z

2

The problem with your approach is that you relied on tutorials/guides for Python 2.7.x (maybe this?) while you are using a Python 3.5.x interpreter.

The way to construct a C extension has changed in Python 3.x.

So, in order to make your 'hello' module compile you will have to make the appropriate changes in hellomodule.c:

First, add the following struct right above the inithello() function:

static struct PyModuleDef hellomodule = {
    PyModuleDef_HEAD_INIT,
    "hello", /* module name */
    NULL, /* module documentation, may be NULL */
    -1,
    HelloMethods /* the methods array */
};

Then replace the entire inithello() function with this instead:

PyMODINIT_FUNC PyInit_hello(void)
{
    return PyModule_Create(&hellomodule);
}

You do not have to make any changes to the setuphello.py script, which you can run as usual:

python setuphello.py build

You can quickly test your freshly compiled module by going into the build\lib.win32-3.5 directory (or something similar), copy the .pyd file (in my system it is named hello.cp35-win32.pyd) someplace convenient and there use something like this little script (usehello.py):

import hello

def greet(person):
    hello.say_hello(person)

greet("stranger")

Running yields:

c:\>python usehello.py
Hello stranger!

c:\>

For a full description of the extension procedure, you can go to the official documentation for Python 3.5

answered Jul 2, 2016 at 18:38

sokin

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1 Comment

Dan Over a year ago

This worked thank you! All I had to do was look at the 3.5 documentation to figure this out, so that's my bad. Thanks so much for helping me with this!

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