A simple example for http://stackoverflow.com/questions/33258718/importing-python-extension-module-loading-already-loaded-shared-library

CMakeLists.txt

cmake_minimum_required(VERSION 3.0.0)

project(so_example)

# needed to link with Python
find_package(PythonLibs REQUIRED)

# a simple shared library
add_library(shared SHARED shared.cpp)

# a simple executable that links with the shared library and embeds Python
add_executable(main main.cpp)
target_link_libraries(main shared)
target_include_directories(main PRIVATE ${PYTHON_INCLUDE_DIRS})
target_link_libraries(main ${PYTHON_LIBRARIES})

# a simple Python extension that links with the shared library
python_add_module(extension SHARED extension.cpp)
target_link_libraries(extension shared)
target_include_directories(extension PUBLIC ${PYTHON_INCLUDE_DIRS})

extension.cpp

// a Python extension that is linked with the shared library

#include <iostream>
#include <Python.h>

#include "shared.h"

static PyObject *extension_error;

static struct PyModuleDef extension_module = {
    PyModuleDef_HEAD_INIT,
    "extension",
    NULL,
    -1,
    NULL
};


PyMODINIT_FUNC
PyInit_extension(void)
{
    std::cout << "PyInit_extension()" << std::endl;
    a_function_from_the_shared_library();

    PyObject *m = PyModule_Create(&extension_module);

    if (!m)
    {
        return NULL;
    }

    return m;
}

main.cpp

#include <iostream>
#include <Python.h>

#include "shared.h"

using namespace std;

int main()
{
    cout << "main()" << endl;
    a_function_from_the_shared_library();

    Py_InitializeEx(1);
    PyEval_InitThreads();

    PyRun_SimpleString("print (\'Python\')");
    PyRun_SimpleString("import extension");

    Py_Finalize();

    return 0;
}

shared.cpp

#include <iostream>

#include "shared.h"

struct global
{
    global()
    {
        std::cout << "global::global() : this = " << this << std::endl;
    }

    ~global()
    {
        std::cout << "global::~global() : this = " << this << std::endl;
    }
};

static global my_global;

void a_function_from_the_shared_library()
{
    std::cout << "a_function_from_the_shared_library()" << std::endl;
}

shared.h

#pragma once

void a_function_from_the_shared_library();