i’ve got a task that depends on two other prior tasks, how can i get the child task to only start after both parents have finished (i wanted to combine the output of both tasks into a single arg instead of adding a new arg)? i tried using a simple boolean

and

(the output of each task is a bool), but it looks like prefect started the child task when the first parent finished instead of waiting for both:

FIRST_TASK_DONE = first_task()
SECOND_TASK_DONE = second_task()

THIRD_TASK_READY = FIRST_TASK_DONE and SECOND_TASK_DONE
third_task(third_task_ready=THIRD_TASK_READY)

third_task

was supposed to only start after both

first_task

and

second_task

had finished, but instead it only waited for the first one to complete

Hey @Martim Lobao, I think you want to explicitly set both as upstream dependencies.

with Flow(...) as flow:
     a = first_task()
     b = second_task()

     c = third_task(c_inputs, upstream_tasks=[a,b])

alright, so a list will work as intended then?

List for

upstream_tasks

will work yeah. If

a

and

b

are mapped, you may need to wrap them with

unmapped

inside the list

we’re using the functional api btw

I think my example is the functional? You mean imperative (or whatever does not use the Flow block)? Yes it would work. If it doesn’t, you can also do:

with Flow(...) as flow:
     a = first_task()
     b = second_task()

     c = third_task()
     c.set_upstream(b)
     c.set_upstream(a)

ah, got it, i think i was thrown off by the fact that

upstream_tasks

is an argument of

set_dependencies

Oh it isn’t. It’s a reserved keyword added to the

run()

method of tasks

got it, thanks @Kevin Kho 🙂