Exposing a C Interface to Python for Implementation

Objective

Integrate Python implementations of a C interface into an existing C program, allowing Python implementations to be used seamlessly within the larger program.

Interface Definition

Consider the following C interface definition:

class myif {
public:
  virtual float myfunc(float a) = 0;
};

Importing and Extending the Interface in Python

1. Enable Polymorphism with SWIG:

   %module(directors="1") module
   
   %include "myif.h"

2. Create a Python Implementation:

   import module
   
   class MyCl(module.myif):
     def __init__(self):
       module.myif.__init__(self)
     def myfunc(self, a):
       return a * 2.0

Embedding Python in C

1. Initialize Python (main.cc):

   Py_Initialize();

2. Import the Python Module:

   PyObject *module = PyImport_Import(PyString_FromString("mycl"));

3. Create Instance and Execute Function:

   PyObject *instance = PyRun_String("mycl.MyCl()", Py_eval_input, dict, dict);
   double ret = PyFloat_AsDouble(PyObject_CallMethod(instance, "myfunc", (char *)"(O)" ,PyFloat_FromDouble(input)));

4. Finalization:

   Py_Finalize();

Converting Python Object to C Pointer

1. Exposing SWIG Runtime:

   swig -Wall -c   -python -external-runtime runtime.h

2. Recompile SWIG Module:

   g   -DSWIG_TYPE_TABLE=myif -Wall -Wextra -shared -o _module.so myif_wrap.cxx -I/usr/include/python2.7 -lpython2.7

3. Helper Function for Conversion:

   myif *python2interface(PyObject *obj) {
     ...
   }

Final Implementation in main.cc

int main() {
  ...
  myif *inst = python2interface(instance);
  std::cout myfunc(input) 
By following these steps, one can successfully implement Python implementations of a C   interface and seamlessly integrate them into larger C   programs, providing greater flexibility and extensibility.