Enable resampling on PeriodIndex

[max-sixty]

This allows resampling with PeriodIndex objects by keeping the group as an index rather than coercing to a DataArray (which coerces any non-native types to objects)

I'm still getting one failure around the name of the IndexVariable still being __resample_dim__ after resample, but wanted to socialize the approach of allowing a name argument to IndexVariable - is this reasonable?

• Closes BUG: Resample on PeriodIndex not working? #1270
• Tests added
• Fully documented, including whats-new.rst for all changes and api.rst for new API

[max-sixty]

Let me know any thoughts / alternative approaches here.

To emphasize - the issue with coercing the labels to a DataArray is that non-native dtypes, such as PeriodIndex get converted to objects, and pandas' resampling machinery (which xarray uses) requires the standard indexes.

[shoyer]

I think we could alternatively fix this by replace dim_coord in the data argument with dim_coord.to_index(), e.g., compare:

ipdb> DataArray(dim_coord, coords=dim_coord.coords, dims=dim_coord.dims, name=RESAMPLE_DIM).to_index()
Index([2000-01-01 00:00, 2000-01-01 06:00, 2000-01-01 12:00, 2000-01-01 18:00,
       2000-01-02 00:00, 2000-01-02 06:00, 2000-01-02 12:00, 2000-01-02 18:00,
       2000-01-03 00:00, 2000-01-03 06:00],
      dtype='object', name='time')
ipdb> DataArray(dim_coord.to_index(), coords=dim_coord.coords, dims=dim_coord.dims, name=RESAMPLE_DIM).to_index()
PeriodIndex(['2000-01-01 00:00', '2000-01-01 06:00', '2000-01-01 12:00',
             '2000-01-01 18:00', '2000-01-02 00:00', '2000-01-02 06:00',
             '2000-01-02 12:00', '2000-01-02 18:00', '2000-01-03 00:00',
             '2000-01-03 06:00'],
            dtype='period[6H]', name='time', freq='6H')

We go to some effort to preserve pandas.Index classes when passed into DataArray/Variable but apparently don't do it consistently everywhere. So there's also probably a lower level fix somewhere (perhaps in xarray.core.variable.as_compatible_data?) to ensure that this happens more consistently.

[max-sixty]

Great! I'll have a look

[shoyer]

I would rather not add an explicit name parameter to IndexVariable.

If anything, I would think about removing the name property. Long term, I'd like to rename IndexVariable -> ImmutableVariable and move the pandas adapter logic into class that we can pass into the data argument of Variable.

[max-sixty]

This is so old that probably it wouldn't even be a good place to start from. Sorry I never pushed it through. I'll close.