issue_comments: 321115367

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https://github.com/pydata/xarray/pull/1473#issuecomment-321115367 https://api.github.com/repos/pydata/xarray/issues/1473 321115367 MDEyOklzc3VlQ29tbWVudDMyMTExNTM2Nw== 6815844 2017-08-09T00:06:10Z 2017-08-09T00:06:10Z MEMBER

I am wondering how the indexing by DataArray should look like, in particular if the indexing-DataArray has coordinates.

In my current implementation, it bahaves

```python In [1]: import numpy as np ...: import xarray as xr ...: ...: da = xr.DataArray(np.arange(3 * 2).reshape(3, 2), dims=['x', 'y'], ...: coords={'x': [0, 1, 2], 'y': ['a', 'b']}) ...: da ...: Out[1]: <xarray.DataArray (x: 3, y: 2)> array([[0, 1], [2, 3], [4, 5]]) Coordinates: * x (x) int64 0 1 2 * y (y) <U1 'a' 'b'

In [2]: ind = xr.DataArray([2, 1], dims=['a'], ...: coords={'a': [0.1, 0.2], 'time': (('a', ), [10, 20])}) ...: ind # 'a': coordinate, 'time': non-dimension coordinate ...: Out[2]: <xarray.DataArray (a: 2)> array([2, 1]) Coordinates: * a (a) float64 0.1 0.2 time (a) int64 10 20

In [3]: da.isel(x=ind) Out[3]: <xarray.DataArray (a: 2, y: 2)> array([[4, 5], [2, 3]]) Coordinates: x (a) int64 2 1 * y (y) <U1 'a' 'b' * a (a) float64 0.1 0.2 ```

I think we should keep (indexed-version of the) original coordinate (da['x']) even after the indexing. We may need also ind['a'] as a new coordinate. Can we ignore the non-dimensional coordinate in the indexer (ind['time'])?

I am slightly worrying that after repetitive indexing the number of coordinates may unintentionally increase.

 
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