#include <cstdlib>
#include "Python.h"

int main()
{
  // in main thread
  Py_Initialize();
  PyEval_InitThreads();
  PyEval_SaveThread();
  
  // creating a new sub-interpreter, in my real app, this would be created
  //   in a sub-thread.
  PyInterpreterState* const mainInterpreterState = PyInterpreterState_New();
  PyThreadState* const taskState = PyThreadState_New(mainInterpreterState);
  PyEval_AcquireThread(taskState);
  PyThreadState * const sub_interp = Py_NewInterpreter();
  PyThreadState_Swap( taskState );
  PyEval_ReleaseThread( taskState );

  // may execute in another separate sub-thread
  PyThreadState * const tstate = PyThreadState_New(sub_interp->interp);
  PyEval_AcquireThread( tstate );
  PyThreadState_Swap( tstate );
  PyRun_SimpleString( "import sys" ); // these two lines are just for informational only, and can be removed
  PyRun_SimpleString( "print (sys.path)" );  
  PyRun_SimpleString( "import threading" ); // this seems the cause...
  PyThreadState_Clear( tstate );
  PyEval_ReleaseThread( tstate );
  PyThreadState_Delete( tstate );
  
  // may execute in another separate sub-thread
  PyEval_RestoreThread( sub_interp );
  Py_EndInterpreter( sub_interp );// we will get the AssertionError (non-harmful?) error msg: Exception ignored in: <model 'threading'....  
  PyEval_ReleaseLock();

  // in main thread
  PyGILState_Ensure();
  Py_Finalize();
  
  return 0;
}