Hi guys, there a seemingly simple thing I can’t manage to do...... I want to cascade mappings: I have two lists: L1=[A,B] L2=[1,2,3] I want to generate 6 tasks, expected execution plan is as follow: Execute (A,1) (A,2) (A,3) (B,1)(B,2)(B,3) Reduce A from first 3 reduce B from last 3 Reduce (A,B) What simple trick am I missing?

Hi @Benjamin Filippi, There's a few ways you could do this. • If the lists

L1

and

L2

are static (fully known at flow build time), you could loop over them and manually construct the tasks without using mapped tasks at all. • If they're not static (result from some task known only at runtime), you might have a task that takes in the lists and generates a list of pairs

(A, 1), (A, 2)...

, then map a task over those pairs. Your reducer function would then need to filter out tasks to apply the reductions only to

A

tasks, only to

B

tasks, etc... If the lists are static and small, I'd probably go with the first option as the code would look more like what you'd write in a simple python script. The latter would work with prefect mapping natively if you need more dynamic tasks.

Yes this is the road I am taking, using list(itertools.product(l1,l2)) And then writing a custom reduce Thanks for your answer