Error in setup file while making C-Extension using SWIG in Python

Question

I'm trying out C to Python Extension using SWIG
For the toy application I've made following files

1. hw.c
2. hw.h
3. hw.i

Contents of hw.h are

double hw1(double r1, double r2);

Contents of hw.c are

#include<hw.h> #include <math.h>
double hw1(double r1, double r2) {
    return sin(r1 + r2);
}

Contents of hw.i are

%module mathModule
%{
 #include <hw.h>   
%}
%include<hw.h>

Can some one please explain me how to write this .i file?
Contents of setup.py

from distutils.core import setup, Extension
name = 'hw'
version = '1.0'
ext_modules_list = [Extension(name = '_mathModule', sources = ["hw.i","hw.c"], include_dirs=['.'])]
setup(name=name, version=version, ext_modules = ext_modules_list)

When I build the module, in iPython it gives me an error when I try following code
from mathModule import hw1
But the same thing works fine when I run in python shell
Also in the last line of setup.py, is setup a method and what is this way of passing parameters?
Also where the name of module is decided
This is the error I get in iPython

no module named _mathModule

Also can anyone say how should I specifyinclude_dirs in Extension class? I tried that it should look in present directory and thus placed . there

Thanks in advance :)

Answer 1

I do not know why you get different behavior in iPython vs python, but I can answer some of your other questions.

Is setup a method and what is this way of passing parameters?

setup is a function provided by distutils.core and it is here called with keyword arguments. I usually write the call like this

setup(
    name = "hw",
    version = "1.0",
    ext_modules = [
        Extension(name = '_mathModule', sources = ["hw.i","hw.c"],
    ]
)

but it is exactly equivalent to your call.

Where the name of module is decided?

The setup file defines two modules, one called "hw" which is a python module, and one called "_mathModule" which is a compiled C extension. Both names are specified using the name keyword.

When you execute python setup.py build, it generates you a directory build with two subdirectories build/temp* and build/lib* where * depends on the systems you compile the code for. If you investigate these directories, you should find build/lib*/hw.py and build/lib*/_mathModule.so. After building, these directories are NOT in your python path, so you need to do an extra step before you can import them in python. This is either

• run python setup.py install to copy the files to a system location where python can find them.
• make symlinks from your main/test directory to the files in your build/lib* directory
• add build/lib* to your PYTHONPATH environment variable
• append the build/lib* folder to sys.path in the python interpreter.

The later three are only for development and testing. When distributing your code, you should always rely on the first option.

How should I specify include_dirs in Extension class?

include_dirs are only used for compiling your extension. Once it is successfully built, the variable does not influence the search path for modules.