issue_comments: 472141327

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https://github.com/pydata/xarray/issues/2807#issuecomment-472141327 https://api.github.com/repos/pydata/xarray/issues/2807 472141327 MDEyOklzc3VlQ29tbWVudDQ3MjE0MTMyNw== 306380 2019-03-12T19:09:58Z 2019-03-12T19:09:58Z MEMBER

The challenge is that with dask's lazy evaluation, we don't know the structure of the returned objects until after evaluating the wrapped functions. So we can't rebuild xarray objects unless we require redundantly specify all the coordinates and attributes from the return values.

Typically in Dask we run the user defined function on an empty version of the data and hope that it provides an appropriately shaped output. If it fails during this process, we ask the user to provide sufficient information for us to populate metadata. Maybe something similar would work here? Xarray would construct a dummy Xarray chunk, apply the user defined function onto that chunk, and then extrapolate metadata out from there somehow.

I'm likely glossing over several important details, but hopefully the general gist of what I'm trying to convey above is somewhat sensible, even if not doable.

 
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