(Somewhat related to my question above, but I’ll keep it separate) If a mapped task raises

prefect.engine.signals.SKIP

, does it need to return a

SKIP

as its result? Can I configure the (mapped or mapper) task such that it only returns a result (say, a string) if the task did not skip? Or [does/should] it have to return a

SKIP

?

A bit unclear on the question because

raise SKIP

is a return so if you do:

@task
def abc(x):
    if x == 2:
        raise SKIP
    else:
        return x + 1

this will SKIP if a condition is met. Otherwise it will return a value. Does that answer your question (I think not)?

Hey @Kevin Kho — I think I mean the following outcome:

from prefect import Parameter
from prefect import Flow
from prefect import mapped
from prefect import task
from prefect.engine.signals import SKIP
from prefect.engine.state import State


@task()
def skipper(x: int) -> int:
    if x == 2:
        raise SKIP
    return x + 1


with Flow(name="test") as flow:
    xs = Parameter(name="xs", default=[1, 2, 3])
    ys = skipper(x=mapped(xs))
    zs = skipper(x=mapped(ys))

flow_state: State = flow.run()

print(flow_state.result[ys].result)
# [2, SKIP(None), 4]

print(flow_state.result[zs].result)
# [SKIP(None), None, 5]

Even if you raise

SKIP

you’re still getting a returned object, right? I would expect that raising a

SKIP

would exit without actually returning anything? (Edit: I could see it being helpful if you want to always know that N inputs gives you N outputs. In that case we can just check (in a downstream task) whether or not some input

isinstance(x, SKIP)

and go from there. But just wondering if that’s by design.)

Ah you do because it needs to get submitted to the next task in general. If you return nothing, then the SKIP signal won’t be able to propagate. If you need to though, you can use the

FilterTask

to remove these states. That will collect all the states though and prevent depth first execution.

Gotcha! Thanks again @Kevin Kho! 🙌

I think in general, you need something different from

None

because

None

can be a valid input in some cases