Minimal example of C++ with SWIG for Python in Mac

I am trying to compile a minimal example of a python wrapper for some C++ code with SWIG in OSX.

/* File : example.c */
double My_variable = 3.0;

int fact(int n) {
    if (n <= 1) return 1;
    else return n*fact(n-1);
}

int my_mod(int x, int y) {
    return (x%y);
}

and the interface file:

/* example.i */
%module example
%{
    /* Put header files here or function declarations like below */
    extern int fact(int n);
    extern int my_mod(int x, int y);
%}

extern int fact(int n);
extern int my_mod(int x, int y);

I run the following:

swig -python -o example_wrap.c example.i 
gcc -c -arch x86_64 -fPIC example.cxx -o example.o -I/Library/Frameworks/Python.framework/Versions/2.7/include/python2.7
ld -bundle -macosx_version_min 10.13 -flat_namespace -undefined suppress -o _example.so *.o

All of these correctly creates _example.so, example_wrap.c, example.o and example.py. Then I run python2.7

import example

and I get

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "example.py", line 17, in <module>
    _example = swig_import_helper()
  File "example.py", line 16, in swig_import_helper
    return importlib.import_module('_example')
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/importlib/__init__.py", line 37, in import_module
    __import__(name)
ImportError: dynamic module does not define init function (init_example)

Any idea?