Hi all I am trying to run a very simple flow It has succeede Prefect Community #ask-community

Hi all, I am trying to run a very simple flow. It ...

Rowen

04/13/2021, 12:54 PM

Hi all, I am trying to run a very simple flow. It has succeeded when I run it locally by adding

flow.run()

at the end of my python file. However, when I trigger the flow in the prefect cloud UI, it fails at the

extract

stage, the error being

At least one upstream state has an unmappable result

. Below is the code. I will elaborate more in the threads

Copy code

@task
def transform(x):
    return x + 30



@task
def extract():
   return [200, 400, 500]



with Flow("flow-name", storage=S3(bucket="bucket_name")) as flow:
    e = extract()
    t = transform.map(e) # fails when i trigger flow in the UI

Rowen

04/13/2021, 12:55 PM

Oddly enough, the code above succeeds in my colleague's machine. Also, after various trials and errors, I realised if I modified the

extract

task to return only two items in a list (e.g.

[200, 400]

), the flow will succeed when triggered in the UI. Modifying my flow to the below will also make the flow succeed:

Copy code

with Flow("flow-name", storage=S3(bucket="bucket_name")) as flow:
    t = transform.map([200, 400, 500])

Is there an explanation for this weird behaviour? (edited)

Rowen

04/13/2021, 12:58 PM

If it helps, I have a prefect agent running on ECS

Zach Angell

04/13/2021, 1:40 PM

Hi @Rowen ! I'm not sure what would be causing this weird behavior. I'm able to run the flow locally and using Prefect Cloud UI I'll set up a test with S3 storage + ECS agent just to be sure

Zach Angell

04/13/2021, 1:51 PM

What versions of Prefect are you using locally and on your ECS agent?

Rowen

04/14/2021, 12:55 AM

My prefect agent uses

0.14.14,

locally it's

0.14.15

Rowen

04/14/2021, 12:55 AM

I am using on macOS Big Sur, if it helps

Rowen

04/14/2021, 1:46 AM

also, i ran the code in isolated python environments using

virtualenv

and

poetry

... not sure if it affects prefect

Zach Angell

04/14/2021, 1:04 PM

Thanks for the additional detail! Do you still see the weird behavior if you upgrade your agent to

0.14.15

Rowen

04/15/2021, 12:06 AM

yeap it still happens 😅

Zach Angell

04/15/2021, 1:32 PM

😅 Hmmm I'm not able to reproduce with ECS Agent and Prefect

0.14.15

I'm registering my flow by running the following script

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from prefect import task, Flow
from prefect.storage import S3

@task
def transform(x):
    return x + 30
@task
def extract():
   return [200, 400, 500]
with Flow("flow-name", storage=S3(bucket="zach-testing")) as flow:
    e = extract()
    t = transform.map(e) # fails when i trigger flow in the UI

flow.register('test')

Zach Angell

04/15/2021, 1:35 PM

I'm using conda to manage python environments, I'll look into whether

virtualenv

poetry

could be affecting this but you're right it shouldn't be an issue with Prefect. Is there anything else about your configuration that I could be missing in my setup?

Zach Angell

04/15/2021, 1:38 PM

From your example, it doesn't look applicable, but in case your prod flow is any different this may be relevant https://github.com/PrefectHQ/prefect/issues/3990

Rowen

04/16/2021, 1:03 AM

@Zach Angell which python version are you using for your flow code? thanks for helping out 😅 ...

Zach Angell

04/16/2021, 1:04 AM

Thanks for your patience! I'm using Python 3.7 in my testing, happy to test other versions

Rowen

04/16/2021, 1:11 AM

I am using

Python 3.9.2

Zach Angell

04/16/2021, 10:25 AM

That could certainly cause problems. I would recommend downgrading Python versions, Prefect support for Python 3.9 is still experimental.

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