issues: 274591962

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id node_id number title user state locked assignee milestone comments created_at updated_at closed_at author_association active_lock_reason draft pull_request body reactions performed_via_github_app state_reason repo type
274591962 MDU6SXNzdWUyNzQ1OTE5NjI= 1722 Change in behavior of .set_index() from pandas 0.20.3 to 0.21.0 4160723 closed 0     1 2017-11-16T17:05:20Z 2017-11-17T00:54:51Z 2017-11-17T00:54:51Z MEMBER      

I use xarray 0.9.6 for both examples below.

With pandas 0.20.3, Dataset.set_index gives me what I expect (i.e., the grid__x data variable becomes a coordinate x):

```python In [1]: import xarray as xr

In [2]: import pandas as pd

In [3]: pd.version Out[3]: '0.20.3'

In [4]: ds = xr.Dataset({'grid__x': ('x', [1, 2, 3])})

In [5]: ds.set_index(x='grid__x') Out[5]: <xarray.Dataset> Dimensions: (x: 3) Coordinates: * x (x) int64 1 2 3 Data variables: empty ```

With pandas 0.21.0, it creates a MultiIndex, which is not what I expect here when setting an index with only one data variable:

```python In [1]: import xarray as xr

In [2]: import pandas as pd

In [3]: pd.version Out[3]: '0.21.0'

In [4]: ds = xr.Dataset({'grid__x': ('x', [1, 2, 3])})

In [5]: ds.set_index(x='grid__x') Out[5]: <xarray.Dataset> Dimensions: (x: 3) Coordinates: * x (x) MultiIndex - grid__x (x) int64 1 2 3 Data variables: empty ```

 
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