issue_comments: 1023511142

Thanks for raising this suggestion @lanougue .

I expect that a singleton coordinate of a dataset not to be a coordinate of other coordinates present in the dataset

What do you think it should return? When selecting a coordinate there seem to be only 3 options to me:

1) Coordinate variables retain all coordinates from the parent object.

This is what it currently does. It's a bit counter-intuitive, but it prioritises keeping coordinates available for use by users. It also means you can pull out a coordinate variable and then use other coordinates to index into it.

2) Coordinate variables retain no coordinates at all.

We could drop all of them, which makes some sense (why should a coordinate have coordinates?). We can try that out naively by adding a drop_vars call to DataArray._getitem_coord:

```python def _getitem_coord(self, key): from .dataset import _get_virtual_variable

try:
    var = self._coords[key]
    coord_da = self._replace_maybe_drop_dims(var, name=key)
    return coord_da.drop_vars(coord for coord in coord_da.coords)

which makes your example return Coordinates: empty `` but this breaks a fair number of tests - 25 failed just intest_dataarray.py` (although mostly parametrizations of one test):

FAILED xarray/tests/test_dataarray.py::TestDataArray::test_isel_fancy - AssertionError: Left and right DataArray objects are not identical FAILED xarray/tests/test_dataarray.py::TestDataArray::test_sel_dataarray - ValueError: One or more of the specified variables cannot be found in this dataset FAILED xarray/tests/test_dataarray.py::TestDataArray::test_coord_coords - AssertionError: Left and right DataArray objects are not identical FAILED xarray/tests/test_dataarray.py::TestDataArray::test_coords_non_string - AssertionError: Left and right DataArray objects are not identical FAILED xarray/tests/test_dataarray.py::TestDataArray::test_coordinate_diff - AssertionError: Left and right DataArray objects are not equal FAILED xarray/tests/test_dataarray.py::TestReduce1D::test_idxmin[False-x0-5-2-None] - KeyError: "'x' is not a coordinate variable." FAILED xarray/tests/test_dataarray.py::TestReduce1D::test_idxmin[False-x1-5-2-None] - KeyError: "'x' is not a coordinate variable." FAILED xarray/tests/test_dataarray.py::TestReduce1D::test_idxmin[False-x2-5-2-1] - KeyError: "'x' is not a coordinate variable." FAILED xarray/tests/test_dataarray.py::TestReduce1D::test_idxmin[False-x3-5-2-1] - KeyError: "'x' is not a coordinate variable." FAILED xarray/tests/test_dataarray.py::TestReduce1D::test_idxmin[False-x4-nan-nan-0] - KeyError: "'x' is not a coordinate variable." FAILED xarray/tests/test_dataarray.py::TestReduce1D::test_idxmin[False-x5-0-1-None] - KeyError: "'x' is not a coordinate variable." FAILED xarray/tests/test_dataarray.py::TestReduce1D::test_idxmax[False-x0-5-2-None] - KeyError: "'x' is not a coordinate variable." FAILED xarray/tests/test_dataarray.py::TestReduce1D::test_idxmax[False-x1-5-2-None] - KeyError: "'x' is not a coordinate variable." FAILED xarray/tests/test_dataarray.py::TestReduce1D::test_idxmax[False-x2-5-2-1] - KeyError: "'x' is not a coordinate variable." FAILED xarray/tests/test_dataarray.py::TestReduce1D::test_idxmax[False-x3-5-2-1] - KeyError: "'x' is not a coordinate variable." FAILED xarray/tests/test_dataarray.py::TestReduce1D::test_idxmax[False-x4-nan-nan-0] - KeyError: "'x' is not a coordinate variable." FAILED xarray/tests/test_dataarray.py::TestReduce1D::test_idxmax[False-x5-0-1-None] - KeyError: "'x' is not a coordinate variable." FAILED xarray/tests/test_dataarray.py::TestReduce2D::test_idxmin[False-x0-minindex0-maxindex0-nanindex0] - KeyError: "'x' is not a coordinate variable." FAILED xarray/tests/test_dataarray.py::TestReduce2D::test_idxmin[False-x1-minindex1-maxindex1-nanindex1] - KeyError: "'x' is not a coordinate variable." FAILED xarray/tests/test_dataarray.py::TestReduce2D::test_idxmin[False-x2-minindex2-maxindex2-nanindex2] - KeyError: "'x' is not a coordinate variable." FAILED xarray/tests/test_dataarray.py::TestReduce2D::test_idxmin[False-x3-minindex3-maxindex3-nanindex3] - KeyError: "'x' is not a coordinate variable." FAILED xarray/tests/test_dataarray.py::TestReduce2D::test_idxmax[False-x0-minindex0-maxindex0-nanindex0] - KeyError: "'x' is not a coordinate variable." FAILED xarray/tests/test_dataarray.py::TestReduce2D::test_idxmax[False-x1-minindex1-maxindex1-nanindex1] - KeyError: "'x' is not a coordinate variable." FAILED xarray/tests/test_dataarray.py::TestReduce2D::test_idxmax[False-x2-minindex2-maxindex2-nanindex2] - KeyError: "'x' is not a coordinate variable." FAILED xarray/tests/test_dataarray.py::TestReduce2D::test_idxmax[False-x3-minindex3-maxindex3-nanindex3] - KeyError: "'x' is not a coordinate variable."

3) *Coordinate variables retain only coordinates with which they share some dimensions

You might say "well in my example 'y' doesn't depend on dimension 'x', so why is it still there? We should keep the 'x' coord and drop the 'y' one." But I don't really think this option makes sense if you think of all coordinates as labels: the 'y' coord is a label for all values in the DataArray, that just happens to be independent of 'x'. Why drop one and not the other?

---

I agree this is a little counterintuitive, but it is actually causing you problems? Because I think there is a rationale for the current behaviour, and changing it would be a breaking change. In other words, it's not obvious to me that this is a bug rather than an implicit feature.

(One might also argue that if xr.Variable were first-class public API (as in our roadmap), then da.coords[var] should return a Variable, which has no coordinates. At the moment da._coords[key] returns either a Variable or IndexVariable, which then gets wrapped back up into a new DataArray.)

Interested to hear what you think though.