Hi folks, I have an asset annotated with return value

-> None

because it is not supposed to return anything. I still want to return some metadata of its materialization. This unfortunately doesn't work:

return Output(
        None,
        metadata={
            'my_metadata': my_value,
        }
    )

What would be the correct way to do this?

I believe the decorated return value should be

Output[None]

, not

None

itself.

Thanks for the reply. Will that still ensure that any potential downstream assets understand that it never returns any values? As opposed to receiving an actual value of

None

?

The special return type essentially turns off invoking the I/O Manager. You may use non_argument_dependencies if you want downstream assets to have dependency on this asset but not expecting any input value. https://docs.dagster.io/concepts/assets/software-defined-assets#non-argument-dependencies

Oh yeah, the I/O manager invocation is also what I was worried about. Thanks a lot, this is very helpful!

I'm learning myself too, trying to make sense of the different Dagster concepts and where to use what. It pays to actually read through the Concept section 😅

True that 🙂 Anyway, thanks and good luck!

personally I prefer to use context.add_output_metadata()

I think the issue is less about adding metadata but to avoid asset expectations that may never materialize

It's kind of about both those things, thanks guys, both your advices are helpful 🙂