[Swig-user] Set callback function with userdefined class as return value

[Jörg F. U. <joe...@un...>]

I'd like to write a c++-function that calls a callback function which
isonly defined on the python level (at runtime).
Attached is an example, how I set the callback function and how they are
called. For a simple expression such as a double, I can use
PyFloat_AsDouble to convert the Python float to a c-double or
Py_BuildValue("(d)") to create the python float. However, if a have a
complex class (whose interface has been made available to python via
swig), how to a transfer this to python so that it can directly be used
as input or output of the callback function?

some methods of the CallbackHandlerPython class
void  CallbackHandlerPython::SetCallbackFunction(PyObject *args)
	{
        // check if objective routine is callable
        if (!PyCallable_Check(args))
		{
            exit(0);
        }

        Py_XINCREF(args);
        Py_XDECREF(mCallback);
        mCallback = args;


	}

	double CallbackHandlerPython::evaluate()
	{
        double objective;
        PyObject *arglist = Py_BuildValue("(d)");

        // call external python function
        PyObject *result = PyEval_CallObject(mCallback,arglist);

        if (result==0)
        {
            printf("CallbackHandlerPython::Objective error calling
callback objective.\n");
            exit(0);
        }
        objective = PyFloat_AsDouble(result);

        // decrease reference count for result
        Py_DECREF(result);
        return objective;
    }


Python file
def myCallbackfunction(input):
   print input
   return 2.*input

PythonCallback = CallbackHandlerPython()

PythonCallback.SetCallbackFunctions(myCallbackfunction)

print PythonCallback.evaluate(3.)