Testing SWIG: Using C modules in python

I came across Dave Beazely’s SWIG last week. Opens a whole new world of possiblities in terms of fast code using custom C modules, instead of using python definitions!

Here I define a simple C file containing a function py_by_2() which uses a global variable PI, and divides this value by 2.

The following has worked so far.

Create header and module file

Header file

#define PI 4.0
extern float pi_by_2();

Module file

#include<stdio.h>
#include "pitest.h"

float pi_by_2()
{return PI/2;}

void main(){
  printf("answer is %f\n",pi_by_2());
  return;
}

Now make a .i file that SWIG needs

%module pitest
%{
#include "pitest.h"
%}

#include "pitest.h"
extern float pi_by_2();

Create the wrapper files

swig -python pitest.i # this will generate pitest_wrap.c
gcc -c -fpic pitest.c pitest_wrap.c -I/usr/include/python3.6/
gcc -shared pitest.o pitest_wrap.o -o _pitest.so # not sure what this does

Test

import pitest
print(pitest.pi_by_2())

Finally, run the python script

python3.6 python_call_pitest.py

Result: 2.0

It works!