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isel(multi_index_level_name = MultiIndex.level) corrupts the MultiIndex #8952

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@dcherian

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@dcherian

What happened?

From #8951

if d is a MultiIndex-ed dataset with levels (x, y, z), and m is a dataset with a single coord x
m.isel(x=d.x) builds a dataset with a MultiIndex with levels (y, z). This seems like it should work.

cc @benbovy

What did you expect to happen?

No response

Minimal Complete Verifiable Example

import pandas as pd, xarray as xr, numpy as np

xr.set_options(use_flox=True)

test = pd.DataFrame()
test["x"] = np.arange(100) % 10
test["y"] = np.arange(100)
test["z"] = np.arange(100)
test["v"] = np.arange(100)

d = xr.Dataset.from_dataframe(test)
d = d.set_index(index = ["x", "y", "z"])
print(d)

m = d.groupby("x").mean()
print(m)

print(d.xindexes)
print(m.isel(x=d.x).xindexes)

xr.align(d, m.isel(x=d.x))
#res = d.groupby("x") - m
#print(res)
<xarray.Dataset>
Dimensions:  (index: 100)
Coordinates:
  * index    (index) object MultiIndex
  * x        (index) int64 0 1 2 3 4 5 6 7 8 9 0 1 2 ... 8 9 0 1 2 3 4 5 6 7 8 9
  * y        (index) int64 0 1 2 3 4 5 6 7 8 9 ... 90 91 92 93 94 95 96 97 98 99
  * z        (index) int64 0 1 2 3 4 5 6 7 8 9 ... 90 91 92 93 94 95 96 97 98 99
Data variables:
    v        (index) int64 0 1 2 3 4 5 6 7 8 9 ... 90 91 92 93 94 95 96 97 98 99
<xarray.Dataset>
Dimensions:  (x: 10)
Coordinates:
  * x        (x) int64 0 1 2 3 4 5 6 7 8 9
Data variables:
    v        (x) float64 45.0 46.0 47.0 48.0 49.0 50.0 51.0 52.0 53.0 54.0
Indexes:
  ┌ index    PandasMultiIndex
  │ x
  │ y
  └ z
Indexes:
  ┌ index    PandasMultiIndex
  │ y
  └ z
ValueError...

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.
  • Recent environment — the issue occurs with the latest version of xarray and its dependencies.

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