Can I return a python list from a Task implementation? I'm getting `TypeError: Task is not iterable. If your task returns multiple results, pass

nout

to the task decorator/constructor, or provide a

Tuple

return-type annotation to your task.` but I can't use

nout

because I don't know the result size and it is not a good fit for Tuple (since it's a list of tuple)

I believe this happens when you try read multiple values from a task:

n1, n2 = MyTask()

As you said, this is only useful if you know your output length in advance. Check your code whether if you are doing something similar. Otherwise, there is nothing wrong about returning lists. From there, you can either pass the entire list to downstream tasks, or use mapping to iterate over each list element and create a downstream task per element.

I wasn't... I was returning like this (without

list( ... )

)

I see, imo just put the line that does a set conversion into a custom task.

Wouldn't it make more sense then to annotate tasks'

run

function to return

prefect.Task[TRes]

where TRes is the actually non-wrapped function's return type?

prefect tries its best to infer tasks from builtin python operators. Like

task1 + task2

is resolved as

AddTask(task1, task2)

by prefect. But resolving arbitrary func calls like

set(task1)

to a prefect task is not possible at this point

On this separate thread of discussion of what to implicitly cast: wouldn't it be more "normal" to provide wrappers for all built-in collections?

There are a bunch of wrappers, check out: https://github.com/PrefectHQ/prefect/tree/master/src/prefect/tasks/core

Ok, I must be misunderstanding then. The docs for these say basically "these are implicit and you don't have to use thejm"?

I believe

{task1}

would get resolved into a

Set(task1)

task definition, in this sense they are implicit. calling pythons default

set

function cannot be detected by prefect however

So does that mean, had I created a

def handle(xs: Set[Thing]) -> None:

I would have gotten the set of things automatically cast?

The users will not use these tasks by hand bit works like below:

@task
def ret1():
    return 1
@task
def ret2():
    return 2
@task
def print_content(x):
    print(x)
with Flow("ListTest") as f:
    print_content([ret1, ret2])

Ok, I see

a

list

call however would try to iterate over tasks, because the

list

function assumes its inputs are iterable. You will simply get the above error, even before building a python list of tasks. Because task objects are not iterable.