How to chain a Celery task that returns a list into a group?

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Question :

How to chain a Celery task that returns a list into a group?

I want to create a group from a list returned by a Celery task, so that for each item in the task result set, one task will be added to the group.

Here’s a simple code example to explain the use case. The ??? should be the result from the previous task.

@celery.task
def get_list(amount):
    # In reality, fetch a list of items from a db
    return [i for i in range(amount)]

@celery.task
def process_item(item):
    #do stuff
    pass

process_list = (get_list.s(10) | group(process_item.s(i) for i in ???))

I’m probably not approaching this correctly, but I’m pretty sure it’s not safe to call tasks from within tasks:

@celery.task
def process_list():
    for i in get_list.delay().get():
        process_item.delay(i)

I don’t need the result from the seconds task.

Answer #1:

You can get this kind of behavior using an intermediate task. Here’s a demonstration of creating a “map” like method that works like you’ve suggested.

from celery import task, subtask, group

@task
def get_list(amount):
    return [i for i in range(amount)]

@task
def process_item(item):
    # do stuff
    pass

@task
def dmap(it, callback):
    # Map a callback over an iterator and return as a group
    callback = subtask(callback)
    return group(callback.clone([arg,]) for arg in it)()

# runs process_item for each item in the return of get_list 
process_list = (get_list.s(10) | dmap.s(process_item.s()))

Credit to Ask Solem for giving me this suggestion when I asked him for help on a similar issue.

Answered By: todd

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