I have defined a

NamedTuple

class called

TableInfo

and created an asset that should return a list of them.

@asset
def tables() -> list[TableInfo]:

I get the following error:

dagster._core.errors.DagsterInvalidDefinitionError: Invalid type: dagster_type must be an instance of DagsterType or a Python type: got list[TableInfo].

I don't see why it shouldn't work. Is this a known limitation or am I doing something wrong?

since the asset function is kind of a meta definition that gets transformed into a proper

AssetsDefinition

instance, I think the type hints are parsed programmatically to link into Dagster's typing and validation functionality (https://docs.dagster.io/concepts/types). So it only accepts Dagster types, which is also what the message is saying. I believe you can make custom ones, but I honestly haven't really made use of it yet.

As far as I understand, any Python type should be accepted, too. But maybe only basic types?

yeah, was just browsing the docs and I agree that appears to be the case, that it should wrap anything else automatically

I tried that already, no luck 😕

hm, maybe it wraps the list type automatically, but then forgets to do it in a nested sense - so if you make a custom type for

TableInfo

it might help? Just guessing at this point 😀

I also tried an

op

with a single return value and that caused the error, too. So I don't think that's it.

and wrapping everything in custom Dagster types doesn't resolve it either?

I haven't tried that yet.

hope you find a solution!

Thanks 🙂

Hi Moritz, what version of dagster are you running? And is it just that code snippet the caused the error? If not, would you mind sharing the full code snippet so I can reproduce?

Just use "list" instead and will do it