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705078481 https://github.com/pydata/xarray/pull/4484#issuecomment-705078481 https://api.github.com/repos/pydata/xarray/issues/4484 MDEyOklzc3VlQ29tbWVudDcwNTA3ODQ4MQ== shoyer 1217238 2020-10-07T17:17:48Z 2020-10-07T17:17:48Z MEMBER

I think some version of xarray.map could indeed be pretty useful more generally. In particular, it could implement each of the ways to "align" variables between different datasets, e.g., should we use the intersection of the variables, or the union, inserting dummy variables with fill values? The dataset_join argument to xarray.apply_ufunc controls this behavior, but it would be nice to have a self-contained version of this.

One challenge for using this internally in xarray (vs. implementing things only on Dataset objects) is that the coordinates on the DataArray objects inside a Dataset can be redundant, so mapping over DataArrays may be less efficient, due to duplicate work on coordinates. In contrast, once everything is in Dataset objects, all the variables/coordinates are pre-aligned and de-deduplicated.

 
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   xarray.map 714228717
704647248 https://github.com/pydata/xarray/pull/4484#issuecomment-704647248 https://api.github.com/repos/pydata/xarray/issues/4484 MDEyOklzc3VlQ29tbWVudDcwNDY0NzI0OA== dcherian 2448579 2020-10-07T01:59:59Z 2020-10-07T01:59:59Z MEMBER

Are there many other cases outside of xr.dot which only operate on DataArrays? If not, we could update that function to take a Dataset

I think we should do this. 

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   xarray.map 714228717
704632636 https://github.com/pydata/xarray/pull/4484#issuecomment-704632636 https://api.github.com/repos/pydata/xarray/issues/4484 MDEyOklzc3VlQ29tbWVudDcwNDYzMjYzNg== max-sixty 5635139 2020-10-07T01:05:37Z 2020-10-07T01:05:37Z MEMBER

The motivating use case was that I wanted to compute the dot-product of two DataSets (=all of their matching variables).

Thanks, that's a good case.

A couple of thoughts: - Are there many other cases outside of xr.dot which only operate on DataArrays? If not, we could update that function to take a Dataset - Along those lines, I wonder whether this becomes an underlying function — functions which currently operate only on DataArrays could have Dataset inputs run through this. - NB: In many of the computation functions, the logic runs the other way (we define the DataArray function to be the Dataset operation on a single-variable Dataset); having a more consistent way of doing this could make extending functions between DataArrays & Datasets easier. - Maybe jumping ahead — are there functions where the result of func(ds1, ds2) shouldn't be that function mapped over the matching variables?

 
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   xarray.map 714228717
704033519 https://github.com/pydata/xarray/pull/4484#issuecomment-704033519 https://api.github.com/repos/pydata/xarray/issues/4484 MDEyOklzc3VlQ29tbWVudDcwNDAzMzUxOQ== shoyer 1217238 2020-10-06T05:19:44Z 2020-10-06T05:19:44Z MEMBER

I think this could make sense as a generalization of Dataset.map. I would indeed be interested to hear more about the specific use cases that motivated this, though.

 
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   xarray.map 714228717
704031645 https://github.com/pydata/xarray/pull/4484#issuecomment-704031645 https://api.github.com/repos/pydata/xarray/issues/4484 MDEyOklzc3VlQ29tbWVudDcwNDAzMTY0NQ== shoyer 1217238 2020-10-06T05:13:29Z 2020-10-06T05:13:29Z MEMBER

If we're doing to do this, I would suggest that the right signature is xarray.map(func, *datasets, **optional_kwargs), matching Python's builtin map.

 
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   xarray.map 714228717
703962448 https://github.com/pydata/xarray/pull/4484#issuecomment-703962448 https://api.github.com/repos/pydata/xarray/issues/4484 MDEyOklzc3VlQ29tbWVudDcwMzk2MjQ0OA== max-sixty 5635139 2020-10-06T00:37:46Z 2020-10-06T00:37:46Z MEMBER

Hi @kefirbandi , thanks for the PR!

Could I ask what the common use cases for this would be? If I understand correctly, running map(x, y, lambda x: x + y) is equivalent to x + y.

 
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   xarray.map 714228717

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