Hamilton Open Source

Hello, should I expect something like this to be allowed?

```# subdag.py

def mapping() -> pd.DataFrame:
    ...


@subdag(
    prepare_src1, prepare_src2, prepare_src3,
    inputs={
        "directory": source("directory"),
        "pattern_src1": source("pattern_src1"),
        "pattern_src2": source("pattern_src2"),
        "pattern_src3": source("pattern_src3"),
    },
    external_inputs=source("mapping")
)
def merge_and_stuff(
    prepared_src1: pd.DataFrame,
    prepared_src2: pd.DataFrame,
    prepared_src3: pd.DataFrame,
    mapping: pd.DataFrame
) -> pd.DataFrame:
    ...```
Error:
TypeError: argument of type 'UpstreamDependency' is not iterable

I ended up having to add the mapping to a downstream node. But I had thought this were possible?

<@U0498UD24AC> <https://hamilton.dagworks.io/en/latest/reference/decorators/subdag/#hamilton.function_modifiers.subdag.__init__|external_inputs, is a list>.
So I think:
``` external_inputs=["mapping"]```
Should work. (also looking at the API this should also take `source(...)` as a value too, rather than string).