ENH: warn user if combining dask streamz with not dask streamz

[CJ-Wright]

Problems seem to occur if a dask stream are combined with not a dask stream (causing futures to not be evaluated when running functions). It might be nice to warn users if they have subscribed a dask stream to a non dask stream without a scatter/gather.

[martindurant]

You mean things like union, where multiple upstreams are present?

[CJ-Wright]

Yes (although I ran into this with things like combine_latest combining a dask and not dask stream.

[mrocklin]

If i were to use Stream.combine_latest on a DaskStream i would expect to get a stream with futures in it. Is this what you're getting? If so then is this something that you consider to be an error?

[CJ-Wright]

Yes, that is what I am getting (and hence the confused issue against dask, sorry). I think the use case for this, where users want to operate on futures rather than the actual data is a fairly limited one, and that this issue can be rather difficult to diagnose, especially in larger pipelines. Having the option to warn seemed like a reasonable compromise between outright raising and causing errors inside the operations themselves when they receive futures and expected data.

One could also make an argument for this to go as a gotcha somewhere in the docs.

[mrocklin]

One could also make an argument for this to go as a gotcha somewhere in the docs.

Yeah, if it's reasonable to want to do this then I don't think that we should have it err.

[mrocklin]

I recognize the value of steering users towards common behaviors though. I don't have a good answer on how to balance between these.

[martindurant]

@CJ-Wright , suggest closing, since your "gotcha" was merged.

[CJ-Wright]

Thank you!