boost python - embedding in c++ and passing variables

Introduction

I'm trying to do a simple thing - in theory, but so far not in practise. I have C++ console application, with embedded python script. It has to pass some variables inside python, do calculation, and send it back.

Example code

my cpp file look like this (based on THIS answer):

#include <iostream>
#include <boost/python.hpp>

using namespace boost::python;

int main()
{
    int a = 2;
    int b = 3;

    Py_Initialize();
    try
    {
        object module = import("__main__");
        object name_space = module.attr("__dict__");
        exec_file("Script.py", name_space, name_space);

        object MyFunc = name_space["MyFunc"];
        object result = MyFunc(a,b);

        int sum = extract<int>(result["sum"]);
        std::cout << sum;
    }
    catch (error_already_set)
    {
        PyErr_Print();
    }

    Py_Finalize();
}

My Script.py file is very simple

def MyFunc(a,b):
    result = a+b
    return result

The Issue

I expected, that python will accept two variables, sum it, and let extract it back do my C++. Then C++ print the results. However it ends with following error:

TypeError: 'int' object has no attribute '__getitem__'

I think that MyFunc(a,b) is not correct call, however I can't figure why. Should I pass variables in other way? If yes, pleas explain me how to do this.