Connect new celery periodic task in django
<p>It's not a question but help to those who will find that the declaration of periodic tasks described in celery 4.0.1 documentation is hard to integrate in django:
<a href="http://docs.celeryproject.org/en/latest/userguide/periodic-tasks.html#entries" rel="noreferrer">http://docs.celeryproject.org/en/latest/userguide/periodic-tasks.html#entries</a></p>

<p>copy paste celery config file <code>main_app/celery.py</code>:</p>

<pre><code>from celery import Celery
from celery.schedules import crontab

app = Celery()

@app.on_after_configure.connect
def setup_periodic_tasks(sender, **kwargs):
    # Calls test('hello') every 10 seconds.
    sender.add_periodic_task(10.0, test.s('hello'), name='add every 10')

    # Calls test('world') every 30 seconds
    sender.add_periodic_task(30.0, test.s('world'), expires=10)

    # Executes every Monday morning at 7:30 a.m.
    sender.add_periodic_task(
        crontab(hour=7, minute=30, day_of_week=1),
        test.s('Happy Mondays!'),
    )

@app.task
def test(arg):
    print(arg)
</code></pre>

<h3>Question</h3>

<p>But what if we use django and our tasks are placed in another app? With celery <code>4.0.1</code> we no longer have <code>@periodic_task</code> decorator. So let's see what we can do. </p>

<h3>First case</h3>

<p>If you prefer to keep tasks and their schedule close to each other:</p>

<p><code>main_app/some_app/tasks.py</code></p>

<pre><code>from main_app.celery import app as celery_app

@celery_app.on_after_configure.connect
    def setup_periodic_tasks(sender, **kwargs):
        # Calls test('hello') every 10 seconds.
        sender.add_periodic_task(10.0, test.s('hello'))

@celery_app.task
def test(arg):
    print(arg)
</code></pre>

<p>We can run <code>beat</code> in debug mode:</p>

<pre><code>celery -A main_app beat -l debug
</code></pre>

<p>and we will see that there's no such periodic task.</p>

<h3>Second case</h3>

<p>We can try to describe all periodic tasks in config file like this:</p>

<p><code>main_app/celery.py</code></p>

<pre><code>...
app = Celery()

@app.on_after_configure.connect
def setup_periodic_tasks(sender, **kwargs):
    # Calls test('hello') every 10 seconds.
    from main_app.some_app.tasks import test
    sender.add_periodic_task(10.0, test.s('hello'))
...
</code></pre>

<p>The result is the same. But it will behave differently that you can see with manual debugging via pdb. In first example <code>setup_periodic_tasks</code> callback will not be fired at all. But in second example we'll get <code>django.core.exceptions.AppRegistryNotReady: Apps aren't loaded yet.</code> (this exception will not be print out)</p>