issues

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: