How to put my game loop into a function?

[RogereK]

I think the main problems is due to scope, reinitialization, and event loop structure. So here's to get your "hold key to start game" idea working:

First of all, avoid using too many global variables. Use a Game class to encapsulate the state and logic. Globals will always give you trouble as the program grows.

Now, keep the main loop separate by using this structure:

def main():
    global running

    while running:
        keys = pygame.key.get_pressed()

        for event in pygame.event.get():
            if event.type == pygame.QUIT:
                running = False

        if keys[pygame.K_f]:  # When F is held
            game_loop()

        screen.fill(BLACK)
        font = pygame.font.Font(None, 40)
        message = font.render("Hold 'F' to Start", True, WHITE)
        screen.blit(message, (WIDTH//2 - message.get_width()//2, HEIGHT//2))
        pygame.display.flip()
        clock.tick(60)

Then call main() at the bottom of your script:

if __name__ == "__main__":
    main()
    pygame.quit()