html_url,issue_url,id,node_id,user,created_at,updated_at,author_association,body,reactions,performed_via_github_app,issue
https://github.com/pydata/xarray/issues/6196#issuecomment-1027064212,https://api.github.com/repos/pydata/xarray/issues/6196,1027064212,IC_kwDOAMm_X849N8GU,35968931,2022-02-01T16:59:51Z,2022-02-01T16:59:51Z,MEMBER,"> For now, I wasn't able to reproduce a minimalist example because the error disappears with minimalist example. I wasn't able to find the fundamental difference between the dataset causing the error and the minimalist one. Printing them are exactly the same.

Can you perhaps use [`ncdump`](https://www.unidata.ucar.edu/software/netcdf/workshops/2011/utilities/Ncdump.html) to show the differences?

> However, I do not understand why it should also be a ""label"" of the other coordinates of the dataset. A singleton dimension should not be ""more important"" than the other (not singleton) dimensions. Why the singleton dimension should become a ""label"" of another dimension while the other dimensions are not. This do not seem logical to me.

I'm not quite sure I understand what you mean - currently aren't all coordinates reattached to any coordinate extracted from a DataArray? (so long as they have compatible dimensions)","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,1115166039
https://github.com/pydata/xarray/issues/6196#issuecomment-1023511142,https://api.github.com/repos/pydata/xarray/issues/6196,1023511142,IC_kwDOAMm_X849AYpm,35968931,2022-01-27T18:16:01Z,2022-01-27T18:41:30Z,MEMBER,"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 in `test_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.","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,1115166039