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  • stack().unstack() not the same as original for datavars dependent on single coordinate of multi_index · 3 ✖

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id html_url issue_url node_id user created_at updated_at ▲ author_association body reactions performed_via_github_app issue
1324753837 https://github.com/pydata/xarray/issues/7297#issuecomment-1324753837 https://api.github.com/repos/pydata/xarray/issues/7297 IC_kwDOAMm_X85O9iOt benbovy 4160723 2022-11-23T09:17:33Z 2022-11-23T09:17:33Z MEMBER

But does this still work properly with broadcasting? For example, let's say there is another data variable b (midx) and an operation is done like ds_stacked['c'] = ds_stacked.a + ds_stacked.b. Then it should be that c (midx) and a (x) should be "repeated" to midx.x

I think it would keep things much simpler if we consider "x" and "midx" as two separate dimensions in the stacked Dataset, i.e., ds_stacked['c'] would result in a 2-d array (x, midx). There's no such thing like a "midx.x" dimension in Xarray.

 
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   stack().unstack() not the same as original for datavars dependent on single coordinate of multi_index 1454832041
1323849354 https://github.com/pydata/xarray/issues/7297#issuecomment-1323849354 https://api.github.com/repos/pydata/xarray/issues/7297 IC_kwDOAMm_X85O6FaK benbovy 4160723 2022-11-22T15:24:53Z 2022-11-22T15:36:46Z MEMBER

The last example in your comment is probably the most meaningful one:

```

<xarray.Dataset>

Dimensions: (x: 2, midx: 4)

Coordinates:

* midx (midx) object MultiIndex

* x (midx) int32 1 1 2 2

* y (midx) int32 3 4 3 4

Data variables:

a (x) int32 6 7

```

To avoid name conflicts, we could just discard the original dimension coordinates x and y. Like here above, "x" becomes a dimension without coordinate. In that example, when unstacking we would retrieve the "x" dimension coordinate like in the original dataset.

(note: I think it is now possible to have a dimension "x" and a coordinate "x" with different dimensions, but I haven't checked).

 
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   stack().unstack() not the same as original for datavars dependent on single coordinate of multi_index 1454832041
1323478134 https://github.com/pydata/xarray/issues/7297#issuecomment-1323478134 https://api.github.com/repos/pydata/xarray/issues/7297 IC_kwDOAMm_X85O4qx2 benbovy 4160723 2022-11-22T10:50:01Z 2022-11-22T10:50:01Z MEMBER

Interesting! I don't think that when adding stack / unstack we were thinking that variables with only a subset of the stacked dimensions would be a common use case.

I guess it would be possible to add some option to stack only the variables that have all the dimensions to be stacked, and leave the other variables unchanged? However, one problem with keeping the original dimension coordinates is that we would have name conflicts between the single index coordinates and the multi-index coordinates.

In your expected example, the "x" coordinate is part of the multi-index but it doesn't have the same dimension "midx"? I find it rather confusing.

 
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   stack().unstack() not the same as original for datavars dependent on single coordinate of multi_index 1454832041

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