Behavior of `output_required` and materialization of downstream assets is unexpected

What's the issue?

From community member Mykola Palamarchuk.

­This part of documentation says that you can return None , so downstream assets will be skipped. But it doesn't work as described (latest Dagster 1.8.11).

from dagster import asset, Definitions, define_asset_job

@asset(output_required=False)
def x():
    return None

@asset
def plus1(x):  # should never materialize
    return x+1 


the_job = define_asset_job(
    name="the_job",
    selection=['*plus1'],
)

defs = Definitions(
    assets=[x, plus1],
    jobs=[the_job]
)

Materializing the_job leads to plus1 asset materialization with error unsupported operand type(s) for +: 'NoneType' and 'int' . There is also a warning that says:

Value "None" returned for non-required output "result" of op "x". This value will be passed to downstream ops. For conditional execution, results must be yielded: ­https://docs.dagster.io/concepts/ops-jobs-graphs/graphs#with-conditional-branching

See original discussion in community Slack:

• https://dagster.slack.com/archives/C01U5LFUZJS/p1728912447272559

What did you expect to happen?

The expected behavior aligns with the docs of ​output­_required​.

Whether the decorated function will always materialize an asset. Defaults to True. If False, the function can return None, which will not be materialized to storage and will halt execution of downstream assets.

How to reproduce?

See example.

Dagster version

1.8.11

Deployment type

Local

Deployment details

No response

Additional information

No response

Message from the maintainers

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