How do you unit test a Celery task?

Depends on what exactly you want to be testing.

• Test the task code directly. Don't call "task.delay(...)" just call "task(...)" from your unit tests.
• Use CELERY_ALWAYS_EAGER. This will cause your tasks to be called immediately at the point you say "task.delay(...)", so you can test the whole path (but not any asynchronous behavior).

An update to my seven years old answer:

You can run a worker in a seperate thread via an pytest fixture:

https://docs.celeryq.dev/en/v5.2.6/userguide/testing.html#celery-worker-embed-live-worker

According to the docs you should not use "always_eager" (see top of the page of above link).

---

Old answer:

I use this:

with mock.patch('celeryconfig.CELERY_ALWAYS_EAGER', True, create=True):
    ...

Docs: https://docs.celeryq.dev/en/3.1/configuration.html#celery-always-eager

CELERY_ALWAYS_EAGER lets you run your task synchronous, and you don't need a celery server.

It is possible to test tasks synchronously using any unittest lib out there. I normaly do 2 different test sessions when working with celery tasks. The first one (as I'm suggesting bellow) is completely synchronous and should be the one that makes sure the algorithm does what it should do. The second session uses the whole system (including the broker) and makes sure I'm not having serialization issues or any other distribution, comunication problem.

So:

from celery import Celery

celery = Celery()

@celery.task
def add(x, y):
    return x + y

And your test:

from nose.tools import eq_

def test_add_task():
    rst = add.apply(args=(4, 4)).get()
    eq_(rst, 8)

Hope that helps!