issue_comments

Okay, a couple changes to the original PR: we now perform a shallow copy of the returned index in DataWithCoords.get_index and Indexes.__getitem__. The latter is needed because both Dataset.indexes and DataArray.indexes returns an Indexes object. Finally, I added a bunch of tests for various index accessors.

This seems pretty robust:

```python

import pandas as pd import xarray as xr index = pd.Index(list('abcd'), name='foo') series = pd.Series(range(4), index) array = xr.DataArray.from_series(series) array.get_index('foo').name = 'bar' array.get_index('foo').name 'foo' array['foo'].to_index().name = 'bar' array['foo'].to_index().name 'foo' array.to_index().name = 'bar' array.to_index().name 'foo' array.indexes['foo'].name = 'bar' array.indexes['foo'].name 'foo' ```

Since a Dataset is just a collection of DataArray objects, the behavior propagates nicely, e.g.

```python

dataset = xr.Dataset({'myvar': array}) dataset['myvar'].to_index().name = 'bar' dataset['myvar'].to_index().name 'foo' dataset.indexes['foo'].name = 'bar' dataset.indexes['foo'].name 'foo' ```

Locally, I've moved my changes from DataArray.to_series to DataWithCoords.get_index, and my tests still pass. After work, I'll write some more tests, e.g. for for DataArray.to_index, and push.

@shoyer, it seems like this issue happens anywhere the user can modify a Pandas index. For example, the Dataset bug in my comment yesterday. Do we want all views of indexes to be immutable or do we trust the user to not do stuff like this:

```python

dataset.indexes['mutable?'].name = 'yes' dataset.indexes['mutable?'].name 'yes' ```

@max-sixty, name is just a mutable property on a Pandas index:

```python

dates = pd.date_range('01-Jan-2019', '31-Jan-2019', name='mutable?') series = pd.Series(np.random.randn(dates.size), dates) series.index.name 'mutable?' series.index.name = 'yes' series.index.name 'yes' ```

But you're right that to_series is the wrong place. It looks like to_series calls to_index, which calls get_index, which is inherited by other classes such as Dataset. For example, the bug persists with Dataset despite my fix:

```python

dates = pd.date_range('01-Jan-2019', '31-Jan-2019', name='mutable?') series = pd.Series(np.random.randn(dates.size), dates) dataset = xr.Dataset({'foo': series}) dataset.indexes['mutable?'].name = 'yes' dataset.indexes['mutable?'].name 'yes' ```

Does anyone know if get_index is the best place for this? Maybe the tests should be somewhere more generic as well.