issue_comments: 512573950

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https://github.com/pydata/xarray/pull/3117#issuecomment-512573950 https://api.github.com/repos/pydata/xarray/issues/3117 512573950 MDEyOklzc3VlQ29tbWVudDUxMjU3Mzk1MA== 1270651 2019-07-17T21:22:47Z 2019-07-17T21:22:47Z CONTRIBUTOR

After writing more tests, turns out sparse-backed xarrays were generating strange coordinates. On closer inspection, this is because sparse.COO objects have their own coords attribute which stores the indices of its nonzeros.

With a serendipitous shape and density of a sparse array, there were the right number of coords such that they got converted into xarray coords:

```

S = sparse.random((100,100)) S.coords
array([[ 0, 0, 3, 3, 4, 5, 6, 7, 7, 7, 9, 10, 11, 14, 14, 16, 17, 18, 19, 19, 21, 21, 21, 21, 22, 23, 23, 24, 24, 25, 26, 28, 29, 31, 35, 35, 36, 36, 38, 39, 41, 42, 42, 43, 44, 46, 46, 47, 48, 48, 49, 49, 49, 49, 50, 52, 53, 54, 55, 56, 57, 57, 58, 60, 60, 61, 61, 62, 64, 64, 68, 70, 72, 73, 76, 77, 78, 79, 79, 80, 81, 83, 84, 85, 88, 89, 90, 90, 90, 92, 92, 93, 93, 94, 94, 96, 97, 98, 98, 99], [14, 28, 58, 67, 66, 37, 50, 9, 66, 67, 6, 22, 44, 51, 64, 26, 53, 91, 45, 81, 13, 25, 42, 86, 47, 22, 67, 77, 81, 22, 96, 18, 23, 96, 34, 57, 6, 96, 91, 86, 75, 86, 88, 91, 30, 6, 65, 47, 61, 74, 14, 73, 82, 83, 32, 42, 53, 68, 21, 38, 48, 50, 87, 8, 89, 22, 57, 60, 92, 94, 19, 79, 38, 53, 32, 95, 69, 22, 46, 17, 17, 86, 36, 7, 71, 35, 9, 58, 79, 22, 68, 10, 47, 48, 54, 72, 24, 47, 63, 86]]) xr.DataArray(S) <xarray.DataArray (dim_0: 100, dim_1: 100)> <COO: shape=(100, 100), dtype=float64, nnz=100, fill_value=0.0> Coordinates: * dim_0 (dim_0) int64 0 0 3 3 4 5 6 7 7 7 ... 92 93 93 94 94 96 97 98 98 99 * dim_1 (dim_1) int64 14 28 58 67 66 37 50 9 66 ... 47 48 54 72 24 47 63 86 ```

A simple fix is to special-case SparseArrays in the DataArray constructor and not extract the sparse coords attribute in that case, but that would require importing sparse.

Would it make sense to just assume that all non-DataArray NEP-18 compliant arrays do not contain an xarray-compliant coords attribute?

 
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