MultiIndex coordinates do not exist updating v2022.3 to v2022.12

[lkugler]

What happened?

Hi, thank you all for this amazing tool and its capabilities.

I noticed my code breaking from an update of xarray of v2022.3 to v2022.12
I wanted to select the latitude inside the MultiIndex position=(lat, lon) but in the new version it doesn't seem to exist when I overwrite an existing index. It works though, when the DataArray is created with the MultiIndex in the first place.

See the code example below. In the meantime I will use xarray v2022.3
Thank you!

What did you expect to happen?

In v2022.3 the output would be (selecting latitudes)

>>> da.lat
<xarray.DataArray 'lat' (position: 4)>
array([0.1, 0.2, 0.7, 0.9])
Coordinates:
  * position  (position) MultiIndex
  - lat       (position) float64 0.1 0.2 0.7 0.9
  - lon       (position) float64 0.1 0.2 0.7 0.9

Minimal Complete Verifiable Example

import xarray as xr
import numpy as np
import pandas as pd
print(xr.__version__)

def test_xarray_multiindex():
    mda = xr.DataArray(np.random.rand(4), 
                       coords={"position": pd.Index([0.1, 0.2, 0.7, 0.9], name='position')}, 
                       dims="position")
    midx = pd.MultiIndex.from_arrays([[0.1, 0.2, 0.7, 0.9], [0.1, 0.2, 0.7, 0.9]], 
                                     names=("lat", "lon"))
    mda['position'] = midx
    print(mda.lat)  # <-- raises Error in xarray 2022.12

test_xarray_multiindex()

MVCE confirmation

• Minimal example — the example is as focused as reasonably possible to demonstrate the underlying issue in xarray.
• Complete example — the example is self-contained, including all data and the text of any traceback.
• Verifiable example — the example copy & pastes into an IPython prompt or Binder notebook, returning the result.
• New issue — a search of GitHub Issues suggests this is not a duplicate.

Relevant log output

AttributeError                            Traceback (most recent call last)
Cell In[1], line 16
     13     print(mda)
     14     print(mda.lat)  # <-- raises Error in some xr versions
---> 16 test_xarray_multiindex()

Cell In[1], line 14, in test_xarray_multiindex()
     12 mda['position'] = midx
     13 print(mda)
---> 14 print(mda.lat)

File /home/swd/manual/nwp/2023.1/lib/python3.10/site-packages/xarray/core/common.py:268, in AttrAccessMixin.__getattr__(self, name)
    266         with suppress(KeyError):
    267             return source[name]
--> 268 raise AttributeError(
    269     f"{type(self).__name__!r} object has no attribute {name!r}"
    270 )

AttributeError: 'DataArray' object has no attribute 'lat'

Anything else we need to know?

No response

Environment

INSTALLED VERSIONS ------------------ commit: None python: 3.10.9 | packaged by conda-forge | (main, Feb 2 2023, 20:20:04) [GCC 11.3.0] python-bits: 64 OS: Linux OS-release: 4.18.0-372.19.1.el8_6.x86_64 machine: x86_64 processor: x86_64 byteorder: little LC_ALL: None LANG: en_US.UTF-8 LOCALE: ('en_US', 'UTF-8') libhdf5: 1.12.2 libnetcdf: 4.8.1

xarray: 2023.2.0
pandas: 1.5.3
numpy: 1.23.5
scipy: 1.10.0
netCDF4: 1.6.2
pydap: None
h5netcdf: 1.1.0
h5py: 3.8.0
Nio: None
zarr: None
cftime: 1.6.2
nc_time_axis: None
PseudoNetCDF: None
rasterio: None
cfgrib: None
iris: None
bottleneck: None
dask: 2023.2.0
distributed: 2023.2.0
matplotlib: 3.7.0
cartopy: 0.21.1
seaborn: None
numbagg: None
fsspec: 2023.1.0
cupy: None
pint: 0.20.1
sparse: None
flox: None
numpy_groupies: None
setuptools: 67.3.2
pip: 23.0
conda: None
pytest: None
mypy: None
IPython: 8.10.0
sphinx: None

[benbovy]

Thanks for the report @lkugler !

Directly assigning a multi-index like mda['position'] = midx is now ambiguous because all levels of the multi-index are now exposed as actual coordinates. We should provide a temporary fix or at least issue a warning. A proper way to assign a pandas multi-index is implemented in #7368. In the meantime, the workaround below should work for your example (it might stop working in the future, though):

mda.coords.update(xr.Dataset(coords={"position": midx}))