Celery: How to ignore task result in chord or chain?

Question

I'm using celery, I have several tasks which needed to be executed in order.

For example I have this task:

@celery.task def tprint(word):     print word 

And I want to do something like this:

>>> chain(tprint.s('a') | tprint.s('b'))() 

Then I get TypeError: tprint() takes exactly 1 argument (2 given).

The same with chord, in this situation which I need a task to be executed after a group of tasks:

>>> chord([tprint.s('a'), tprint.s('b')])(tprint.s('c')) 

So how to deal with this situation? I don't care the result of each task, but they need to be executed in order.

---

Add a second parameter won't work:

@celery.task def tprint(word, ignore=None):     print word  >>> chain(tprint.s('a', 0) | tprint.s('b'))() 

This will print out 'a' and 'None'.

Answer 1

There is a built-in functionality to ignore result in chaining and others - immutable subtask. You can use .si() shortcut instead of .s() or .subtask(immutable=True)

More details here: http://docs.celeryproject.org/en/master/userguide/canvas.html#immutability

Answer 2

One possible solution has already posted, but I'd like to add further clarification and an alternate solution (and in some cases a superior one).

The error you're seeing, which indicates that your task's signature needs to take into account a second parameter, is due to the fact that when calling tasks in a chain, Celery automatically pushes each tasks result as the first parameter of the following task.

From the docs:

Tasks can be linked together, which in practice means adding a callback task:

>>> res = add.apply_async((2, 2), link=mul.s(16)) >>> res.get() 4 

The linked task will be applied with the result of its parent task as the first argument

Therefore, in your case, you could rewrite your task like this:

@celery.task def tprint(result, word):     print word 

If you're not going to do anything with the result, you may as well ignore it, by changing the decorator thus:

@celery.task(ignore_result=True) 

And then you won't have to change your task signature.

Sorry, that last point needs further research.