Very simple CPython code throws segmentation fault in C++ within __Pyx_PyObject_AsStringAndSize function

Question

I want to use a Python function from within C++ code. To this end, I created a short CPython file with the function, compiled this file with Cython to C++ code and header and finally include the header file in a C++ main file.

The CPython file src/mypylib.pyx looks as follows:

# distutils: language=c++

from libcpp.string cimport string

cdef public string lookup(string name):
    return "Hello from Python!"

The build file setup.py looks as follows:

from setuptools import setup, Extension    
from Cython.Build import cythonize         
                                           
setup(                                     
    name='mypylib',                        
    ext_modules=cythonize(                 
        Extension(                         
            "mypylib", ["src/mypylib.pyx"],
        ),                                 
    )                                      
)                                          

The application's main file src/main.cpp looks as follows:

#include <iostream>                           
#include <string>                             
                                              
#include "mypylib.h"                          
                                              
int main() {                                  
  std::string res = lookup("hello");          
  std::cout << "Result: " << res << std::endl;
}                                             

I build and run the code as follows:

$ python setup.py build
$ cd src
$ g++ -fPIC -I/usr/include/python3.6 main.cpp mypylib.cpp -lpython3.6m -lpthread -ldl -lutil -lm -Xlinker -export-dynamic -Wl,-O1 -Wl,-Bsymbolic-function
$ ./a.out
Segmentation fault

Why and where would such a simple application have a segmentation fault? The stack trace is apparently as follows:

__Pyx_PyObject_AsStringAndSize(_object*, long*)
__pyx_convert_string_from_py_std__in_string(_object*)+
lookup
main

My environment:

• Ubuntu 18.04
• Python==3.6.9
• Cython==0.29.21
• g++==8.3.0

I'm honestly unsure where to start to debug this problem. Is this a software problem or did I missed something in my CPython code?

EDIT: Solution

As pointed out in the comments, I forgot to initialize the Python interpreter and I didn't import my CPython module. A working C++ main function would look like this:

#include <iostream>
#include <string>

#include <Python.h>

#include "mypylib.h"

int main() {
  auto err = PyImport_AppendInittab("mypylib", PyInit_mypylib);
  Py_Initialize();
  auto mypylib = PyImport_ImportModule("mypylib");
  std::string res = lookup("hello");
  std::cout << "Result: " << res << std::endl;
  Py_Finalize();
}

PyInit_mypylib is defined in the mypylib.h header file generated by Cython.