SWIG Python: Derive from C++ base class in Python

CMakeLists.txt

cmake_minimum_required(VERSION 3.0)

project(mytest)

FIND_PACKAGE(SWIG REQUIRED)
INCLUDE(${SWIG_USE_FILE})

if (NOT MSVC)
  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
endif()

FIND_PACKAGE(PythonLibs 2 REQUIRED)
INCLUDE_DIRECTORIES(${PYTHON_INCLUDE_PATH})

INCLUDE_DIRECTORIES(${CMAKE_CURRENT_SOURCE_DIR})

SET(CMAKE_SWIG_FLAGS "-keyword")

SET_SOURCE_FILES_PROPERTIES(mytest.i PROPERTIES CPLUSPLUS ON)
SWIG_ADD_MODULE(mytest python mytest.i)
SWIG_LINK_LIBRARIES(
  mytest
  ${PYTHON_LIBRARIES}
  )

mytest.hpp

#ifndef MYTEST_HPP
#define MYTEST_HPP

#include <iostream>
#include <vector>
#include <memory>

class MyBaseClass {
  public:
    virtual
    double
    eval(const double x) const = 0;
};

class Square: public MyBaseClass {
  public:
    virtual
    double
    eval(const double x) const
    {
      return x*x;
    }
};


void
mytest(const std::shared_ptr<MyBaseClass> & a) {
  std::cout << a->eval(1.0) << std::endl;
  std::cout << a->eval(2.0) << std::endl;
  std::cout << a->eval(3.0) << std::endl;
}

#endif // MYTEST_HPP

mytest.i

%module mytest

%{
#define SWIG_FILE_WITH_INIT
#include "mytest.hpp"
%}

%include <std_shared_ptr.i>
%shared_ptr(MyBaseClass);
%shared_ptr(Square);

%include "mytest.hpp"

test.py

import mytest

a = mytest.Square()
mytest.mytest(a)

class Cube(mytest.MyBaseClass):
    def __init__(self):
        return

    def eval(self, x):
        return x*x*x

c = Cube()
mytest.mytest(c)