I have an asset defined like:

@asset(
    metadata={"data1": ["foo", "bar"], "data2": []},
)
def example_asset(context: OpExecutionContext):

I have an IO manager that gets the metadata from an upstream asset like:

metadata = context.metadata if isinstance(context, OutputContext) else context.upstream_output.metadata

When I get the field

data1 = metadata["data1"]

the value of

data1

is a string

[list] (unserializable)

This works when using the IO manager for output, but doesn’t when getting upstream data for a downstream asset.

after digging into the other bug i was working on, i think it’s unrelated to this one. So i’ll work on debugging this next. just want to confirm the behavior you’re seeing - when you materialize an asset downstream of

example_asset

in the

load_input

function of your IO manager, the value of the metadata

data1

is the literal string

"[list] (unserializable)"

? and in other cases you get the list of strings as expected?

I’m having trouble replicating this. Here’s the test i wrote

def test_metadata_io_manager():
    expected_metadata = {"data1": ["foo", "bar"], "data2": []}
    @asset(
        metadata=expected_metadata,
    )
    def upstream():
        return 1

    @asset
    def downstream(upstream):
        return upstream + 1


    class MyIOManager(IOManager):
        def __init__(self):
            self.values = dict()

        def handle_output(self, context, obj):
            keys = tuple(context.get_identifier())
            self.values[keys] = obj

        def load_input(self, context):
            assert context.upstream_output is not None
            metadata = context.metadata if isinstance(context, OutputContext) else context.upstream_output.metadata
            assert metadata == expected_metadata
            keys = tuple(context.get_identifier())
            return self.values[keys]


    materialize([upstream, downstream], resources={"io_manager":IOManagerDefinition.hardcoded_io_manager(MyIOManager())})

is there something i’m missing in my setup compared to what you’re doing? what version of dagster are you running?

would you be able to copy this code snippet and confirm it’s working for you? just to base-line check that there isn’t something super funky going on

Just reran mine on same env and it failed with the unserializable error.

you can also dm if you don’t want to share the code publicly

following up on this! so it turns out we are overriding list metadata with that string, i talked with the other engineer who worked on the metadata system, and there isn’t a good reason that we do this. we’ll update it in the next release so that list metadata is fully supported. In the meantime, if you need the metadata quickly, you could instead use a dictionary with a single key with the value that’s the list

@asset( 
metadata={"data1": {"val": ["foo", "bar"]}}
)