Shared library in Python won't open window on macOS

Question

I am currently developing a Reinforcement Learning environment game in C++. The C++ code controls both the graphics and game logic, while I have Python bindings (imported via a shared library) controlling the flow of the game (decision making, stepping, etc). I have no problem running the environment on my Linux machine: it correctly opens a GUI display when I run the bindings in Python. However, on my macOS machine, the environment will not render. Oddly enough, when I compile the C++ code as an executable, rather than a shared library, and run it, the GUI window correctly displays the environment. I suspect there is something different in the way Python handles shared libraries on macOS.

I am using CMake as the build system, SFML as the GUI, and pybind11 for the Python bindings. Does anyone have any ideas on how I could debug this problem?

Here's some snippets of my code to illustrate my point:

C++ Bindings

#include <SFML/Graphics.hpp>                                                       
#include <pybind11/pybind11.h> 
#include <iostream>                                      
                                                                                   
namespace py = pybind11;                                                           
                                                                                   
void openWindow() {                                                                
    sf::RenderWindow window(sf::VideoMode(500, 500), "SFML");                      
    while (window.isOpen())                                                        
    {                                                                              
        // Update the window
        std::cout << 1;                    
        window.clear();                                                            
        window.display();                                                          
    }                                                                              
}                                                                                  
                                                                                   
PYBIND11_MODULE(test, handle) {                                                    
    handle.def("openWindow", &openWindow);                                         
} 

Python Code

from build.test import *                                                           
                                                                                   
def main():                                                                        
    openWindow()                                                                   
                                                                                   
if __name__ == "__main__":                                                         
    main() 

And CMakeLists.txt

make_minimum_required(VERSION 3.10)                                               
                                                                                   
project(main)                                                                      
                                                                                   
find_package(pybind11)                                                             
find_package(SFML 2.5 COMPONENTS REQUIRED graphics system)                         
                                                                                   
pybind11_add_module(test main.cpp)                                                 
target_link_libraries(test PUBLIC sfml-graphics sfml-system)

I know that the loop for rendering the window is working, rather than the program crashing completely, because 1 is outputted throughout the duration of the program's execution. However, I still don't know why the window won't display, or even how to approach debugging this problem.

Answer 1

Here's the solution. Your code needs to poll an SFML event, otherwise the window won't render on macOS.

Insert the following line in your code if you want it render.

sf::Event event;
window.pollEvent(event);

Once again, I'm not sure why my code worked fine without these lines on Linux, but for some reason polling an event is necessary on macOS.