Hi wave what s the recommended approach to map some argument

Question

Hi wave what s the recommended approach to map some arguments and unmapp others for a task that takes kwargs as argument I m getting a `MappingLengthMismatch` error while doing this more details on the thread and I have the impression that kwargs can t be `unmapped` neither explicitly nor implicitly My current work around is to wrap the desired task A with another task that doesn t take kwargs and call task a as a function `task a fn `

Adrian Fernandez · Answer

My environment ```Python 3 8 16 prefect 2 7 8```

Adrian Fernandez · Answer

calling `add xy ` without kwargs ```from prefect import task flow unmapped task def add xy x int y int kwargs print Kwargs kwargs return x + y flow def my flow results = add xy map x= 1 2 y=unmapped 3 print my flow MappingLengthMismatch Received iterable parameters with different lengths Parameters for map must all be the same length Got lengths x 2 kwargs 0 ```

Adrian Fernandez · Answer

Calling `add xy ` with unmapped kwargs ``` flow def my flow 2 results = add xy map x= 1 2 y=unmapped 3 kwargs=unmapped None print my flow 2 MappingLengthMismatch Received iterable parameters with different lengths Parameters for map must all be the same length Got lengths x 2 kwargs 1 ```

Adrian Fernandez · Answer

Work around add a wrapper task ``` task def add wrapper x int y int return add xy fn x=x y=y flow def my flow 3 results = add wrapper map x= 1 2 y=unmapped 3 print results print my flow 3 Completed message=None type=COMPLETED result=4 Completed message=None type=COMPLETED result=5 ```

Adrian Fernandez · Answer

What s the proper way to do this

Zanie · Answer

Hi Thanks for pointing this out This is a bug in our implementation

Zanie · Answer

It s a little tricky to address though hm

Adrian Fernandez · Answer

Ok then in the meantime I ll keep the workaround in place

Adrian Fernandez · Answer

Thanks for the quick response

Ben Ayers-Glassey · Answer

< Zanie> could `inspect signature` be used for this ```import inspect def some func x posonly args kwonly kwargs sig = inspect signature some func assert sig parameters kwargs kind == inspect Parameter VAR KEYWORD for param in sig parameters values print f param name param kind ``` ^ That outputs ```x POSITIONAL ONLY posonly POSITIONAL OR KEYWORD args VAR POSITIONAL kwonly KEYWORD ONLY kwargs VAR KEYWORD```

Ben Ayers-Glassey · Answer

<https docs python org 3 library inspect html introspecting callables with the signature object> or maybe this is tricky for some other reasons But it seems to me like parameters of kind VAR KEYWORD should always be treated as unmapped Edit or in any case the dict itself should be treated as unmapped thinking face

Zanie · Answer

Well you d probably expect us to map over a key in kwargs if it is iterable

Ben Ayers-Glassey · Answer

Yeah But basically it seems like right now the `kwargs` dict itself is being checked for iterability because I believe this error occurs even if you don t pass any kwargs So ideally you d want the values of the dict to be checked for iterability or for whether they were marked with `unmapped` But certainly an empty dict should be allowed Otherwise AFAICT you can t easily write a task which wraps another one unless you explicitly write out all its parameters in the wrapping function s definition and explicitly pass them all along

Zanie · Answer

There s still a bug here that I can t dig into right this moment but this is the approach I m thinking <https github com PrefectHQ prefect pull 8188>

Ben Ayers-Glassey · Answer

Wow I m going to second < Adrian Fernandez> s comment quick response I ll have a look at this and thank you very much slightly smiling face

Zanie · Answer

Ah I found the bug it was dumb it should be working Let me know if you can break it or if you want to add any test cases slightly smiling face