I have a flow that looks a bit like this

Question

Joshua Greenhalgh · Answer

```as flow gcs uri = Parameter name= gcs uri required=True cleaned = clean gcs uri=gcs uri removal = remove existing cleaned insertion = insert cleaned flow set dependencies insertion upstream tasks= removal ``` loads a file from storage into a pandas dataframe does some cleaning of that data frame inserts to a database the `clean` task succeeds and `cleaned` is passed to `remove existing` which looks like ``` task def remove existing cleaned logger = prefect context get logger min = cleaned some col min ``` When this task runs I get occasionally and if I rerun the flow with same input it doesn t happen again ```min = cleaned some col min AttributeError NoneType object has no attribute some col ``` How can that argument cleaned be None when the task proceeding it succeeded and returned a value

Joshua Greenhalgh · Answer

Is there some prefect magic behind the scenes around the argument passing that could go wrong

Kevin Kho · Answer

There have been some people that mentioned this but noone has ever gotten back to me What is the return of `cleaned` You could DM me the code of that task

Joshua Greenhalgh · Answer

Its a pandas dataframe can t share the code really but it reads a file from gcs parses it into a dataframe does some simple cleaning and returns the dataframe I have added logging to the clean function and at the end of that function the return value is not None

Joshua Greenhalgh · Answer

but then it is in the task which the dataframe is passed to

Kevin Kho · Answer

Ah ok no worries is the execution on something like Dask or Local

Joshua Greenhalgh · Answer

its happening on k8s

Kevin Kho · Answer

LocalExecutor on KubernetesRun

Joshua Greenhalgh · Answer

Hey so I am seeing this occur quite frequently now really no idea how to debug it In answer to < Kevin Kho> question sorry didn t reply sooner I am using `KubernetesRun`

Joshua Greenhalgh · Answer

the TLDR of this is that previous task succeeds the value of the parameter to the following task return value of proceeding is None but the previous task cannot return None

Kevin Kho · Answer

If I have to guess this is something like Kubernetes is resubmitting a failed job but the object is no longer in memory Would you have a clue is the original process is running out of memory You can also try configuring `Results` so that it s persisted something I think Kubernetes is just retrying directly WThere really is no magic is you are not using Dask If I have to guess I think the process is actually failing and potentially restarting through Kubernetes restarts

Kevin Kho · Answer

I think the pod logs might be helpful to look at

Joshua Greenhalgh · Answer

< Kevin Kho> thanks very much I shall try to see if there are any correlations with pod failures