stack renames single dimensions instead of creating a MultiIndex

[equaeghe]

Code Sample

>>> da = xr.DataArray([0., 1., 2.])
>>> db = da.stack(z=da.dims)
>>> db
<xarray.DataArray (z: 3)>
array([0., 1., 2.])
Coordinates:
  * z        (z) int64 0 1 2
>>> db.unstack('z')
Traceback (most recent call last):

  File "<ipython-input-127-ec66ebede9c5>", line 1, in <module>
    db.unstack('z')

  File "/usr/lib64/python3.6/site-packages/xarray/core/dataarray.py", line 1248, in unstack
    ds = self._to_temp_dataset().unstack(dim)

  File "/usr/lib64/python3.6/site-packages/xarray/core/dataset.py", line 2315, in unstack
    raise ValueError('cannot unstack a dimension that does not have '

ValueError: cannot unstack a dimension that does not have a MultiIndex

Problem description

stack is not consistently creating a MultiIndex, but just renames a dimension in case there is just the single dimension to be stacked. This breaks code where the (number of) dimensions are (is) not known in advance.

Expected Output

stack should create a MultiIndex in all cases and a stack/unstack sequence should always return the original DataArray.

Output of xr.show_versions()

commit: None
python: 3.6.5.final.0
python-bits: 64
OS: Linux
OS-release: 4.19.23-gentoo
machine: x86_64
processor: Intel(R) Core(TM) i7-2620M CPU @ 2.70GHz
byteorder: little
LC_ALL: None
LANG: nl_BE.UTF-8
LOCALE: nl_BE.UTF-8

xarray: 0.10.8
pandas: 0.19.1
numpy: 1.14.5
scipy: 0.19.1
netCDF4: 1.3.1
h5netcdf: None
h5py: 2.9.0
Nio: None
zarr: None
bottleneck: 1.2.1
cyordereddict: None
dask: None
distributed: None
matplotlib: 2.2.2
cartopy: None
seaborn: None
setuptools: 40.6.3
pip: 9.0.1
conda: None
pytest: 3.10.1
IPython: 5.4.1
sphinx: 1.7.5

[max-sixty]

Thanks for the report @equaeghe

Is this a dupe of #1431 ?

[equaeghe]

Is this a dupe of #1431 ?

Yes. My example and explanation are a bit clearer, so I do not know which would be best to close.

In any case, I hope this is considered for fixing separately from some big refactoring of the MultiIndex machinery, which seems to have stalled. This seems like a corner case that might not be too big, but I may be mistaken.

[keewis]

I think this has already been fixed:

>>> da.stack(z=da.dims)
<xarray.DataArray (z: 3)>
array([0., 1., 2.])
Coordinates:
  * z        (z) MultiIndex
  - dim_0    (z) int64 0 1 2

If you disagree, feel free to reopen.