Is it wrong to pass list as a task parameter? In the following example the list items order is not preserved.

from prefect import Flow, task


@task
def arrtest(arr):
    return ''.join(arr)


with Flow('ArrayTest') as flow:
    r = arrtest(['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k'])

print(flow.run().result[r].result)

Expected result: abcdefghijk Actual result: ajkbcdefghi

Hi Kamil, thanks for the question. I'm looking into it for you now.

Thank you for quick answer, Jenny!

Hello you guys, I was wondering what are the motivation behind not preserving the order ? In my case I have images and labels, and if I'm modifying my images with a task I'm scared they will not match my labels anymore :s

We love questions here! You are correct in that the list will not reorder if it is passed in as a

Parameter

because Parameters are a special kind of task. What is happening here is a list is being defined directly in the context of the Flow and therefore is being converted into a

Constant

(which is also a special type of task).

@alexandre kempf I think the takeaway here is that not defining parameters as constants means you can't guarantee that tasks will run (or in this case exist) in a particular order. This is because tasks can be run in a distributed environment where certain clusters will pick up certain tasks as they're available

From what I understood, the list in that example is not a

Constant

, and this is why it can be reorder. Can a task (here

r

for example) be reorder if it goes through another task ? (Imagine

r

is a task with a list as a result, and you give it to another task) or task are also a special type ? If this question is not clear I can provide an example, don't hesitate to ask about it if needed 🙂

Nope that list will not be reordered!

That is awesome ! Thank you again 😄

I think it might be instructive to compare the visualization of the constant list and non-constant list version. I think the constant-list version should have one input while the non-constant list version you should see many.