Hi guys, I am brand new to Prefect and trying to get familiar. I want my flow to finish in failed state but with a custom error message in the log and not the full exception detail:

from prefect import flow, task, get_run_logger

from prefect.orion.schemas.states import Failed

def my_function():

raise Exception('This is an exception')

@flow

def run_flow():

logger = get_run_logger()

try:

result = my_function()

<http://logger.info|logger.info>(result)

except Exception as e:

errmsg = f"Error! Type: {e.__class__.__name__} Detail: {e}"

logger.error(errmsg)

return Failed()

if __name__ == "__main__":

run_flow()

However, I get the following error; Exception has occurred: TypeError Unexpected result for failure state: None —— NoneType cannot be resolved into an exception File "main.py", line 21, in <module> run_flow() I'm thinking I might be doing more than one thing wrong here. Can anyone enlighten me, please? Thanks!

Hi Christian, The first thing that stands out to me, is that you are assigning:

result = my_function()

but

my_function()

has no return, which is presumably then

None

I tested your code and this line causes the error

return Failed()

Thanks @Christopher Boyd, but I don't understand why I need the function to return anything. I just need it to throw an exception. The following code throws the same error:

from prefect import flow, task, get_run_logger

from prefect.orion.schemas.states import Failed

@task

def my_function():

return 1

@flow

def run_flow():

logger = get_run_logger()

try:

result = my_function()

<http://logger.info|logger.info>(result)

raise Exception('This is an exception')

except Exception as e:

errmsg = f"Error! Type: {e.__class__.__name__} Detail: {e}"

logger.error(errmsg)

return Failed()

if __name__ == "__main__":

run_flow()

Thanks @Khuyen Tran, I realize that returning the Failed state is causing the error, but I don't know how else I would get my flow to finish in Failed state.

Hi Christian, by default a flow fails with the summary state of all its tasks

I see, but I still don't understand why the exception is thrown. Take this example from https://docs.prefect.io/concepts/flows/#return-a-manual-state If I run the code as is it finishes without errors, but if I change

if y.result() == "success":

to

if y.result() != "success":

it will throw the same exception: Exception has occurred: TypeError Unexpected result for failure state: None —— NoneType cannot be resolved into an exception

You are not returning a return from the exception, so it’s “catching” a none type I think