Django post_save signal, executing chained tasks

Question

When I save a model I want to create some folders, then download a few things to those folders.

Folders need to be created for the downloads, obviously, downloads can happen in parallel.

HEre's what I have:

@receiver(post_save, sender=Game)
def after_game_model_save(sender, instance, created, **kwargs):
    logger.info("after_game_model_save")
    task = None
    task_id = uuid4()
    tasks_chain = chain(create_game_folder.s(instance.id))

    if created:
        tasks_chain |= download_game.s(instance.id, instance.download_link).set(
            task_id=str(task_id)
        )
    else:
        if instance.tracker.has_changed("screenshots_urls"):
            tasks_group = group(
                [
                    download_game.s(instance.id, instance.download_link).set(
                        task_id=str(task_id)
                    ),
                    download_screenshots.s(instance.id),
                ]
            )
        if instance.tracker.has_changed("download_link"):
            tasks_group = group(
                [
                    download_game_update.s(instance.id, instance.download_link).set(
                        task_id=str(task_id)
                    ),
                    download_screenshots.s(instance.id),
                ]
            )
        tasks_chain |= tasks_group
    tasks_chain.delay()
    try:
        task_obj = Task.objects.get(game=instance)
        task_obj.task_id = str(task_id)
        task_obj.save()
    except Task.DoesNotExist:
        Task.objects.create(game=instance, task_id=str(task_id))

I'm getting the error TypeError: download_game() takes 2 positional arguments but 3 were given

If I interpret the examples correctly, the result of the first chained task get's sent as an argument to the second task? How can I chain tasks so they're executed in order without worrying about this?

The functions return nothing. So I guess right now I end up with something like download_game.s(instance.id, instance.download_link, None)

Update:

@shared_task()
def download_game(game_id, download_link):
    game_folder = Path(settings.GAMES_FOLDER / str(game_id))
    tmp_folder = game_folder / 'tmp'
    logger.info(f"Downloading {download_link} to {tmp_folder}")
    sleep(10)
    with open(tmp_folder/'test.txt', 'w') as fp:
        fp.write("##########################")
        fp.write("##########################")
        fp.write("##########################")
        fp.write("##########################")
        fp.write("##########################")
    
    logger.info(f"Downloaded {download_link} to {tmp_folder}")

Answer 1

You should use .si, if you don't want to use the previous task's output. This internally sets immutable=True in the celery signature, which is the main class that deals with serialisation of messages along with its metadata.

    tasks_chain = chain(create_game_folder.si(instance.id, immutable = True))

    if created:
        tasks_chain |= download_game.si(instance.id, instance.download_link).set(
            task_id=str(task_id)
        )