Solid Events

A solid can yield a stream of events within its compute_fn to communicate with the Dagster framework. These events must be one of the following types:

• Output
• AssetMaterialization
• ExpectationResult
• TypeCheck
• Failure (meant to be raised)

To return an output from a solid, simply yield an Output event:

@solid
def my_yield_solid(context):
    yield Output(1)

Many solids yield only one output, like the example above. Returning a single value from a solid's compute_fn is equivalent to yielding a single Output event with the default output name "result". For example:

@solid
def return_solid(context):
    return 1


# Is equivalent to
@solid
def yield_solid(context):
    yield Output(1, "result")

Note that you cannot yield a single value without wrapping it in Output. This is because a solid can yield arbitrarily many values, and there's no way for the system to tell which one the author of the solid meant to use as its output. For example:

# This is invalid
@solid
def incorrect_solid(context):
    yield 1

If you want to have multiple outputs for a solid, you cannot return anything from the solid. Instead, you need to yield multiple Output events, each of which is named and defined on output_defs to prevent ambiguity:

@solid(output_defs=[OutputDefinition(name="output_1"), OutputDefinition(name="output_2")])
def multiple_output_solid(context):
    yield Output(1, output_name="output_1")
    yield Output(2, output_name="output_2")