Hi Not sure whether I am doing something wrong or it is a bu

Question

Hi Not sure whether I am doing something wrong or it is a bug but I am trying to run a mapped tasks that returns two values inside my flow an I am getting an error code and trace inside

Tomer Cagan · Answer

Code ``` task nout=2 def load stuff gt Tuple List int List int return 1 2 3 4 5 6 task nout=2 def process stuff low int high int gt Tuple int int return low low high high task def post process lows int highs int print lows print highs with Flow map multi as flow2 v l v h = load stuff p l p h = process stuff map v l v h post process p l p h if name == main flow2 run ``` I am getting an error on `p l p h = process stuff map v l v h ` ```Traceback most recent call last File Users tomercagan dev nextresearch bin run projection py line 98 in lt module gt p l p h = process stuff map v l v h File Users tomercagan dev nsr env lib python3 9 site packages prefect core task py line 998 in iter raise TypeError 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 ``` I have done both type hinting and nout but this fails

Tomer Cagan · Answer

I can change it to this ``` load stuff and process stuff as above and task def post process results List Tuple int int lows highs = list zip results print lows print highs with Flow map multi as flow2 v l v h = load stuff results = process stuff map v l v h post process results ``` And it will work but I am still wondering about the first implementation which I think is more natural

Pim Claessens · Answer

in the task decorater you need to add nout=2 the number of items you return

Pim Claessens · Answer

So in your case it should be ``` task nout=2 def post process results List Tuple int int lows highs = list zip results return lows highs```

Pim Claessens · Answer

Tomer Cagan · Answer

Few things The error message says I can also add a tuple type annotation which I have and it is not working if that s not relevant maybe it s good to remove that from the error Even with `nout=2` it does not work Note that it works for the first tasks load stuff but not for the second task which is mapped over the output of the first Also note that if I change how I capture the output of the mapped task as one return value it properly give me a list of tuples Maybe mapped task cannot properly separate divide the return values of each call

Anna Geller · Answer

The problem with your flow is that the mapped task `process stuff` returns a Tuple Returning tuples doesn t work with mapped tasks you can read more about that in <https github com PrefectHQ prefect issues 4824|this issue> Here is one way how you could handle that ```from prefect import task Flow from typing import List Tuple task nout=2 def load stuff gt Tuple List int List int return 1 2 3 4 5 6 task nout=2 def process stuff low int high int gt List int return low low high high task log stdout=True def post process nr int gt None print nr with Flow map multi as flow v l v h = load stuff results = process stuff map v l v h post process map results if name == main flow run ```

Tomer Cagan · Answer

Hi Thanks the code you shared is what happens anyway though what you did is better as it is explicit I saw that issue you shared but thought to check anyway since I know you guys work fast and resolve things quickly