.sel does not keep selected coordinate value in case with MultiIndex

.sel method usually keeps selected coordinate value as a scalar coordinate

In[4] ds1 = xr.Dataset({'foo': (('x',), [1, 2, 3])}, {'x': [1, 2, 3], 'y': 'a'})

Out[4]: 
<xarray.Dataset>
Dimensions:  ()
Coordinates:
    y        <U1 'a'
    x        int64 1
Data variables:
    foo      int64 1

But in MultiIndex case, does not.

In[5] ds2 = xr.Dataset({'foo': (('x',), [4, 5, 6])}, {'x': [1, 2, 3], 'y': 'b'})
        ds = xr.concat([ds1, ds2], dim='y').stack(yx=['y', 'x'])
        ds.sel(x=2)
Out[5] 
<xarray.Dataset>
Dimensions:  (y: 2)
Coordinates:
  * y        (y) object 'a' 'b'
Data variables:
    foo      (y) int64 2 5

x is gone. Is it a desired behavior?

[benbovy]

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

>>> 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

>>> 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.

[gabriel-abrahao]

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.