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It looks like I can no longer have keyword functio...
# ask-community
t
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.
Copy code
@task
def get_items_list(
    client, entity, page,
    response_attr=None, path=None, params=None
):
✅ 1
➕ 1
If I remove the keyword args I don't get the error, however I kind of need those there for functionality
a
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
t
Using it with a mapping would be as follows. This would use the default 
None
values defined in the function signature:
Copy code
items = get_items_list.map(
        unmapped(client), unmapped("subscriptions"), pages_list
    )
a
not sure but you may need to switch the order of arguments:
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items = get_items_list.map(
        pages_list, unmapped(client), unmapped("subscriptions")
    )
t
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.
a
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.
💯 1
t
Prefect 1.0 did. I think this is a bug?
a
thanks to you both for bringing this up and describing the issue - I'll investigate more and report back
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:
Copy code
"""
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) -> int:
    logger = get_run_logger()
    <http://logger.info|logger.info>("Hello, %s!", statis_value)
    <http://logger.info|logger.info>("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()
t
Thank you!
a
@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 https://github.com/PrefectHQ/prefect/issues/6383 Again, thanks so much for reporting this
a
cool, thanks!
🙌 1
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