Skeleton for SDL2/OpenGL application in Python 3. Game loop has fixed time step, rendering is going at full speed.

sdl2_opengl_skeleton.py

#
# Skeleton for SDL2/OpenGL application in Python
# with fixed logic time step, rendering is going still at full speed.
#
# To turn debug logging messages on, run with -v parameter:
#   python <script_name.py> -v
#

import argparse
import logging
import ctypes
import time

import sdl2
from OpenGL import GL

WIDTH = 450
HEIGHT = 600

LOGIC_FPS = 1.0 / 25.0  # 25 logic frames per second, fixed step


def logic(delta_time):
    """
        Game logic goes here. Delta time is fixed to LOGIC_FPS
    """
    pass


def render(delta_time):
    """
        Render code goes here.
    """
    GL.glClearColor(0.3, 0.3, 0.3, 1)
    GL.glClear(GL.GL_COLOR_BUFFER_BIT | GL.GL_DEPTH_BUFFER_BIT)


def init_args_and_logging():
    parser = argparse.ArgumentParser(
        description='OpenGL skeleton app.')

    parser.add_argument("-v", "--verbose", help="increase output verbosity",
                        action="store_true")

    format_str = '%(asctime)s - %(name)s - %(levelname)s - %(message)s'

    args = parser.parse_args()
    if args.verbose:
        logging.basicConfig(format=format_str, level=logging.DEBUG)
    else:
        logging.basicConfig(format=format_str, level=logging.INFO)


def throw_runtime_sdl_error():
    err = sdl2.SDL_GetError()
    logging.error(err)
    raise RuntimeError(err)


def create_opengl_context(window):
    sdl2.video.SDL_GL_SetAttribute(
        sdl2.video.SDL_GL_CONTEXT_MAJOR_VERSION,
        3)
    sdl2.video.SDL_GL_SetAttribute(
        sdl2.video.SDL_GL_CONTEXT_MINOR_VERSION,
        3)
    sdl2.video.SDL_GL_SetAttribute(
        sdl2.video.SDL_GL_CONTEXT_PROFILE_MASK,
        sdl2.video.SDL_GL_CONTEXT_PROFILE_CORE)
    context = sdl2.SDL_GL_CreateContext(window)

    GL.glEnable(GL.GL_BLEND)
    GL.glBlendFunc(GL.GL_SRC_ALPHA, GL.GL_ONE_MINUS_SRC_ALPHA)

    GL.glEnable(GL.GL_DEPTH_TEST)
    GL.glDepthFunc(GL.GL_LESS)

    return context


def window_title(frames=None):
    if not frames:
        return 'OpenGL Torch demo'.encode('utf-8')
    return 'OpenGL Torch demo FPS:{}'.format(frames).encode('utf-8')


def main():
    init_args_and_logging()

    logging.debug('Application starting...')

    if sdl2.SDL_Init(sdl2.SDL_INIT_VIDEO) != 0:
        throw_runtime_sdl_error()

    logging.debug('SDL Initialized...')

    try:
        window = None
        window = sdl2.SDL_CreateWindow(window_title(),
                                       sdl2.SDL_WINDOWPOS_UNDEFINED,
                                       sdl2.SDL_WINDOWPOS_UNDEFINED,
                                       WIDTH, HEIGHT,
                                       sdl2.SDL_WINDOW_OPENGL)

        if not window:
            throw_runtime_sdl_error()

        logging.debug('SDL Window initialized...')

        context = None
        context = create_opengl_context(window)

        if not context:
            throw_runtime_sdl_error()

        logging.debug('OpenGL context initialized...')

        logging.debug('Entering main loop...')

        accumulator = 0.0

        window_title_timer = 0.0
        frames = 0

        current_time = time.monotonic()

        running = True
        while running:
            event = sdl2.SDL_Event()
            while sdl2.SDL_PollEvent(ctypes.byref(event)) != 0:

                if event.type == sdl2.SDL_QUIT:
                    logging.debug('SDL_QUIT event received, bailing out.')
                    running = False
                elif (event.type == sdl2.SDL_KEYDOWN and
                      event.key.keysym.sym == sdl2.SDLK_ESCAPE):
                    logging.debug('ESC key pressed, bailing out.')
                    running = False

            new_time = time.monotonic()
            delta_time = new_time - current_time
            accumulator += delta_time
            while accumulator >= LOGIC_FPS:
                logic(LOGIC_FPS)
                accumulator -= LOGIC_FPS

            render(delta_time)

            sdl2.SDL_GL_SwapWindow(window)
            sdl2.SDL_Delay(1)

            frames += 1
            window_title_timer += delta_time
            if window_title_timer >= 1.0:
                sdl2.SDL_SetWindowTitle(window, window_title(frames))
                frames = 0
                window_title_timer -= 1.0

            current_time = new_time

    finally:
        logging.debug('Quitting, doing cleanup...')
        if context:
            sdl2.SDL_GL_DeleteContext(context)
        if window:
            sdl2.SDL_DestroyWindow(window)
        sdl2.SDL_Quit()
        logging.debug('Done. Bye.')


main()