issues: 903055859

I'm not sure if this is a bug or I'm not using xarray correctly, but I used to be able to do this without crashing. The new behavior seems to have been introduced some time between 0.16.2 and 0.18.2.

What happened:

python traceback Traceback (most recent call last): File "scripts/repro.py", line 12, in <module> ds = ds.unstack(['c']) File "/home/darsh/src/notebooks/build/venv/lib/python3.8/site-packages/xarray/core/dataset.py", line 4024, in unstack raise ValueError( ValueError: cannot unstack dimensions that do not have a MultiIndex: ['c']

What you expected to happen:

The code runs without the ValueError exception.

Minimal Complete Verifiable Example:

```python from xarray import DataArray, Dataset

a = DataArray([0], dims=['a']) b = a.stack(b=('a',)).reset_index('b') c = b.stack({'c': ['b']})

ds = Dataset({'d': DataArray(c.data, dims=['c'])}, coords=c.coords) print('\nBefore:') print(ds)

ds = ds.unstack(['c']) print('\nAfter:') print(ds) ```

Anything else we need to know?:

Here's the full output from the example on 0.18.2: ```

Before: <xarray.Dataset> Dimensions: (c: 1) Coordinates: * c (c) MultiIndex - b (c) int64 0 a (c) int64 0 Data variables: d (c) int64 0 Traceback (most recent call last): File "scripts/repro.py", line 12, in <module> ds = ds.unstack(['c']) File "/home/darsh/src/notebooks/build/venv/lib/python3.8/site-packages/xarray/core/dataset.py", line 4024, in unstack raise ValueError( ValueError: cannot unstack dimensions that do not have a MultiIndex: ['c'] `` What confuses me is that thecdimension is shown as aMultiIndex, but it still complains that it doesn't have aMultiIndex. Directly unstackingds.d` rather than the dataset itself also fails with the same exception.

Oddly, it seems to work if I assign the coordinates after constructing the dataset: ```diff diff --git a/scripts/repro.py b/scripts/repro.py index ed2ae7c..d5bd6a3 100644 --- a/scripts/repro.py +++ b/scripts/repro.py @@ -5,7 +5,7 @@ a = DataArray([0], dims=['a']) b = a.stack(b=('a',)).reset_index('b') c = b.stack({'c': ['b']})

-ds = Dataset({'d': DataArray(c.data, dims=['c'])}, coords=c.coords) +ds = Dataset({'d': DataArray(c.data, dims=['c'])}).assign_coords(c.coords) print('\nBefore:') print(ds)

```

With that workaround, or by downgrading to 0.16.2, the example doesn't crash:

```

Before: <xarray.Dataset> Dimensions: (c: 1) Coordinates: * c (c) MultiIndex - b (c) int64 0 a (c) int64 0 Data variables: d (c) int64 0

After: <xarray.Dataset> Dimensions: (b: 1) Coordinates: a (b) int64 0 * b (b) int64 0 Data variables: d (b) int64 0 ```

Environment:

Output of <tt>xr.show_versions()</tt>

INSTALLED VERSIONS ------------------ commit: None python: 3.8.0 (default, Feb 25 2021, 22:10:10) [GCC 8.4.0] python-bits: 64 OS: Linux OS-release: 5.4.0-73-generic machine: x86_64 processor: x86_64 byteorder: little LC_ALL: None LANG: en_US.UTF-8 LOCALE: ('en_US', 'UTF-8') libhdf5: None libnetcdf: None xarray: 0.18.2 pandas: 1.2.4 numpy: 1.20.3 scipy: 1.6.3 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: 2021.05.0 distributed: None matplotlib: 3.4.2 cartopy: None seaborn: None numbagg: None pint: 0.17 setuptools: 39.0.1 pip: 21.1.1 conda: None pytest: 6.2.4 IPython: 7.23.1 sphinx: None None