boost::python - how to invoke a python function in its own thread from C++?

Question

I have a module written in python. this module is sort of an interface to many different functionalities I implemented in Python:

EmbeddingInterface.py simply imports this module and creates an instance:

import CPPController

cppControllerInstance = CPPController()

I would like to use cppControllerInstance in c++. this is what I have done so far:

#include <Python.h>
#include <boost\python.hpp>

using namespace boost;

python::object createController()
{
    try
    {
        Py_Initialize();

        python::object mainModule = python::import("__main__");
        python::object mainNamespace = mainModule.attr("__dict__");

        python::dict locals;

        python::exec(
            "print \"loading python implementetion:\"\n"
            "import sys\n"
            "sys.path.insert(0, \"C:\\Projects\\Python\\ProjectName\\Panda\")\n"
            "import EmbeddingInterface\n"
            "controller = EmbeddingInterface.cppControllerInstance\n",
            mainNamespace, locals);

            python::object controller = locals["controller"];
            return controller;
    }
    catch(...) {}
}

The Problem:

This 'controller' has some functions which must be called asynchronously. its work is continuous and in addition it can throw exceptions. which is why std::async sounded great.

But it doesn't work:

int main()
{
    python::object controller = createController();
    python::object loadScene = controller.attr("loadScene");
    //loadScene(); // works OK but blocking!
    std::async(loadScene); // non blocking but nothing happens!
    while(true); // do some stuff
}

I tried to invoke the python function 'loadScene' with its own thread but the function seemed to be blocking. It never returns.

What is the proper way of doing that?

Satie · Answer 1 · 2017-08-15T17:44:30.910

Seems you misunderstood the behavior of std::async

a snippet of test code:

#include <iostream>
#include <chrono>
#include <thread>
#include <future>

int doSomething(){
  std::cout << "do something"<<std::endl;
  return 1;
}

int main(){
   auto f = std::async(doSomething);

   std::this_thread::sleep_for(std::chrono::seconds(3));
   std::cout <<"wait a while"<<std::endl;
   f.get();
   return 0;
}

Output:

wait a while
do something

change the line

auto f = std::async(doSomething);

to

auto f = std::async(std::launch::async,doSomething);

Then output:

do something
wait a while

As your example, to run it immediately in another thread, you can try :

std::async(std::launch::async,loadScene);

Indeed, at the time I did not understand the behavior of std::async and the fact the std::future destructor blocks. But regarding calling a python function on a different c++ thread, it is not as simple as you illustrate and requires manipulating with the GIL (python global interpreter lock). — Elad Maimoni, Aug 17 '17 at 11:42

boost::python - how to invoke a python function in its own thread from C++?

1 Answers1