issue_comments

@fujiisoup the behavior that you expect has been implemented in #5692 (I still need to fix/update the tests):

```python

ds.sel(x=2) <xarray.Dataset> Dimensions: (y: 2) Coordinates: * y (y) <U1 'a' 'b' x int64 2 Data variables: foo (y) int64 2 5 ```

```python

ds.isel(yx=0) <xarray.Dataset> Dimensions: () Coordinates: yx object ('a', 1) y <U1 'a' x int64 1 Data variables: foo int64 1 ```

Note that for the latter example, the yx coordinate may eventually become irrelevant for multi-indexes (we'll need only the level coordinates).

@gabriel-abrahao feel free to open a new issue for your suggestion.

As a related issue, we could use an option for .sel to keep the selected variable as a singleton coordinate (an array of shape (1,)) instead of making it a scalar.

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(mmh apparently I forgot to post a 2nd comment I wrote)

I'm just wondering if it really makes sense to keep a MultiIndex for a coordinate that is not a dimension anymore like in this case.

And this is just a nonsense :) Sorry it is late!

Yep I agree it would be nice to have scalar coordinates in both cases.

@benbovy Thanks for the comment.

Actually, I expected the following behavior python In [1]: ds.sel(x=2) Out[1]: <xarray.Dataset> Dimensions: (y: 2) Coordinates: * y (y) object 'a' 'b' x int64 2 Data variables: foo (y) int64 2 5 where the selected x is automatically converted to a scalar coordinate.

And also for the second issue, I expected something like python In [2]: ds.isel(yx=0) Out[2]: <xarray.Dataset> Dimensions: () Coordinates: y <U1 'a' x int64 1 Data variables: foo int64 1 both x and y become scalar coordinates.

Back to the first issue, I personally prefer to the automatic renaming because it enables the following chain methods, python ds.sel(x=2).isel(y=1) that might be intuitive especially for the hierarchic dimension structure.

The current behavior is actually dictated by pandas and I think it was initially intentional (before we could access MultiIndex levels as virtual coordinates).

In the first example MultiIndex.get_loc_level() is called, which doesn't return a MutliIndex:

```python In [11]: mdix = ds.yx.to_index()

In [12]: midx.get_loc_level(2, level='x') Out[12]: (array([False, True, False, False, True, False], dtype=bool), Index(['a', 'b'], dtype='object', name='y')) ```

If we want to keep the MultiIndex in this case, then I think we can just ignore new_indexes returned by indexing.remap_label_indexers and remove Dataset._replace_indexes.

But we still have to choose whether automatically renaming the dimension makes sense or not, i.e., choose between

python In [5]: ds.sel(x=2) Out[5]: <xarray.Dataset> Dimensions: (yx: 2) Coordinates: * yx MultiIndex - y (yx) object 'a' 'b' - x (yx) int64 2 2 Data variables: foo (yx) int64 2 5

and

python In [5]: ds.sel(x=2) Out[5]: <xarray.Dataset> Dimensions: (y: 2) Coordinates: * y (y) object 'a' 'b' x (y) int64 2 2 Data variables: foo (y) int64 2 5

The second issue is also due to how MultiIndex works in pandas:

```python In [10]: midx[0] Out[10]: ('a', 1)

In [11]: midx[[0, 1]] Out[11]: MultiIndex(levels=[['a', 'b'], [1, 2, 3]], labels=[[0, 0], [0, 1]], names=['y', 'x']) ```

I'm just wondering if it really makes sense to keep a MultiIndex for a coordinate that is not a dimension anymore like in this case.

@shoyer Thanks for the information. (Sorry for my inaccurate ipython output)

Another related issue is

python In[3]: ds.isel(yx=1) Out[3]: <xarray.Dataset> Dimensions: () Coordinates: yx object ('a', 1) Data variables: foo int64 1 where MultiIndex structure is totally gone. (ds.isel(yx=[0, 1]) works perfectly.)

I am now looking into the code. Do you guys have any idea to fix them? I am thinking to modify Index.remap_label_indexers so that it also returns selected_dims containing names of dims that should be converted to scalar coordinates.

No, I don't think this was intentional/desired, though it might be slightly tricky to fix this with the current code structure.

@benbovy any thoughts?

For reference: ```python In [4]: ds Out[4]: <xarray.Dataset> Dimensions: (yx: 6) Coordinates: * yx (yx) MultiIndex - y (yx) object 'a' 'a' 'a' 'b' 'b' 'b' - x (yx) int64 1 2 3 1 2 3 Data variables: foo (yx) int64 1 2 3 4 5 6

In [5]: ds.sel(x=2) Out[5]: <xarray.Dataset> Dimensions: (y: 2) Coordinates: * y (y) object 'a' 'b' Data variables: foo (y) int64 2 5 ```