How can I use SWIG to interface a Python class with a C program?

Interfacing a Python Class with C Code

Q: How can I implement a Python class that can be called from within a larger C program?

A: To interface a Python class with C code, there are two key steps:

1. Exposing the Interface in Python:

• Use SWIG to wrap your C interface (e.g., myif.h) and expose it to Python.
• Enable cross-language polymorphism using the director feature in SWIG.

2. Embedding Python in the C Application:

• Initialize Python in the C main function (Py_Initialize()).
• Import the Python module (PyImport_Import()), create an instance of the Python class (PyRun_String()), and call its methods (PyObject_CallMethod()).
• Convert the Python object to a C object using a helper function (myif *python2interface(PyObject *obj)).

Example:

myif.h (C interface):

<code class="cpp">class myif {
   public:
     virtual float myfunc(float a) = 0;
};</code>

mycl.py (Python implementation):

<code class="python">import module

class MyCl(module.myif):
  def myfunc(self,a):
    return a*2.0</code>

main.cc (C embedding Python):

<code class="cpp">#include "runtime.h"

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

int main() {
  Py_Initialize();

  ... // import and call Python class

  myif *inst = python2interface(instance);
  std::cout << inst->myfunc(5.5) << std::endl;

  Py_Finalize();
  return 0;
}</code>

This approach allows you to create Python implementations of your C interface and seamlessly integrate them within the larger C program.

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