It looks like I can no longer have keyword function params d

Question

It looks like I can no longer have keyword function params defaulted to `None` defined on a mapped task I now get this error `TypeError object of type NoneType has no len ` when trying to map across the subsequent function signature ``` task def get items list client entity page response attr=None path=None params=None ```

Tim Enders · Answer

If I remove the keyword args I don t get the error however I kind of need those there for functionality

Anna Geller · Answer

can you show how you use it with mapping props to you if you could build an example we could use to reproduce I believe the issue may just be that the argument you iterate over must be a list and all others must be wrapped in unmapped

Tim Enders · Answer

Using it with a mapping would be as follows This would use the default `None` values defined in the function signature ```items = get items list map unmapped client unmapped subscriptions pages list ```

Anna Geller · Answer

not sure but you may need to switch the order of arguments ```items = get items list map pages list unmapped client unmapped subscriptions ```

Tim Enders · Answer

No there are 3 positional arguments required and 3 keyword arguments optional However it seems I need to make the keyword arguments default to an empty list ` ` for the mapping to work Which makes the code smelly to me and doesn t seem like very good Python

Alex Wilcoxson · Answer

The issue I saw when debugging is that since the task is mapped Prefect is checking the len on each argument as all the params should have equal len when mapping Since the kwarg is omitted and its default value has no length it throws So to work around it I ve specified all optional kwargs as unmapped It would be nice if Prefect could infer that because the optional is not provided the default should be implicitly unmapped

Tim Enders · Answer

Prefect 1 0 did I think this is a bug

Anna Geller · Answer

thanks to you both for bringing this up and describing the issue I ll investigate more and report back

Anna Geller · Answer

I looked more and I believe this behavior is expected and even really useful there seems to be a tradeoff between allowing default unmapped parameters or allowing to map over multiple values simultaneously and we went for the second option because the first can be easily solved by passing `unmapped None ` in place of those default unmapped arguments as shown here ``` with mapped future = add one map 1 2 3 prefect exceptions MappingLengthMismatch Received parameters with different lengths Parameters for map must all be the same length Got lengths x 3 statis value 6 from prefect import flow task get run logger unmapped task def add one x int statis value str = None gt int logger = get run logger Hello s statis value Current value of x is s x return x + 1 flow def mapping unmapped mapped future = add one map 1 2 3 unmapped None mapped future = add one map 1 2 3 unmapped Anna add one map mapped future unmapped Second mapped loop if name == main mapping unmapped ```

Tim Enders · Answer

Thank you

Anna Geller · Answer

< Tim Enders> + < Alex Wilcoxson> I wasn t entirely wrong but talking more to engineers looks like we will do exactly what you suggested i e we ll treat default values as `unmapped` to keep things consistent for the caller of `map` there is an open issue for that here Again thanks so much for reporting this

Alex Wilcoxson · Answer

cool thanks