groupby and mean on a MultiIndex level raises ValueError

[jjpr-mit]

What happened?

After using set_index to create a MultiIndex, calling groupby on a MultiIndex level and then mean raises an error.

What did you expect to happen?

Apply mean to groups, no error.

Minimal Complete Verifiable Example

d = DataArray(
    data=[
        [0, 1, 2, 3, 4, 5, 6],
        [7, 8, 9, 10, 11, 12, 13],
        [14, 15, 16, 17, 18, 19, 20]
    ],
    coords={
        "greek": ("a", ['alpha', 'beta', 'gamma']),
        "colors": ("a", ['red', 'green', 'blue']),
        "compass": ("b", ['north', 'south', 'east', 'west', 'northeast', 'southeast', 'southwest']),
        "integer": ("b", [0, 1, 2, 3, 4, 5, 6]),
    },
    dims=("a", "b")
)
d = d.set_index(a=['greek', 'colors'], b=['compass', 'integer'])
g = d.groupby('greek')
m = g.mean(...)

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

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.10/site-packages/xarray/core/_aggregations.py", line 5698, in mean
    return self.reduce(
  File "/usr/local/lib/python3.10/site-packages/xarray/core/groupby.py", line 1201, in reduce
    return self.map(reduce_array, shortcut=shortcut)
  File "/usr/local/lib/python3.10/site-packages/xarray/core/groupby.py", line 1104, in map
    return self._combine(applied, shortcut=shortcut)
  File "/usr/local/lib/python3.10/site-packages/xarray/core/groupby.py", line 1136, in _combine
    index, index_vars = create_default_index_implicit(coord)
  File "/usr/local/lib/python3.10/site-packages/xarray/core/indexes.py", line 1045, in create_default_index_implicit
    index = PandasMultiIndex(array, name)
  File "/usr/local/lib/python3.10/site-packages/xarray/core/indexes.py", line 615, in __init__
    raise ValueError(
ValueError: conflicting multi-index level name 'greek' with dimension 'greek'

Anything else we need to know?

No response

Environment

INSTALLED VERSIONS

commit: None
python: 3.10.7 (main, Sep 13 2022, 14:31:33) [GCC 10.2.1 20210110]
python-bits: 64
OS: Linux
OS-release: 5.15.49-linuxkit
machine: x86_64
processor:
byteorder: little
LC_ALL: None
LANG: C.UTF-8
LOCALE: ('en_US', 'UTF-8')
libhdf5: None
libnetcdf: None

xarray: 2022.11.0
pandas: 1.5.1
numpy: 1.23.4
scipy: None
netCDF4: None
pydap: None
h5netcdf: None
h5py: None
Nio: None
zarr: None
cftime: None
nc_time_axis: None
PseudoNetCDF: None
rasterio: None
cfgrib: None
iris: None
bottleneck: None
dask: None
distributed: None
matplotlib: None
cartopy: None
seaborn: None
numbagg: None
fsspec: None
cupy: None
pint: None
sparse: None
flox: None
numpy_groupies: None
setuptools: 63.2.0
pip: 22.2.2
conda: None
pytest: None
IPython: None
sphinx: None

[mschrimpf]

This error occurs in versions 2022.6 through 2022.11. The code worked fine in previous versions, e.g. in 2022.3.0 it executes as expected.
Related: #6836.

[benbovy]

Thanks @jjpr-mit and @mschrimpf for the report. See #6836 (comment).

[benbovy]

The example above now works (v2023.8.0) without flox:

with xr.set_options(use_flox=False):
    m = g.mean(...)

<xarray.DataArray (greek: 3)>
array([ 3., 10., 17.])
Coordinates:
  * greek    (greek) object 'alpha' 'beta' 'gamma'

But it still fails with flox:

with xr.set_options(use_flox=True):
    m = g.mean(...)

ValueError: Buffer dtype mismatch, expected 'Python object' but got 'long'

cc @dcherian

[dcherian]

This example is working with flox 0.7.2. I'm going to close but feel free to reopen if it doesn't work for you.