issues

• [x] Fixes #6870
• [x] User visible changes (including notable bug fixes) are documented in whats-new.rst

A similar change should solve #6528, but let's get one finished first...

~Posting for discussion, will comment inline~ Ready for merge

```python

import xarray as xr import pandas as pd da = xr.DataArray(pd.Series(1, pd.period_range('2000-1', '2000-12', freq='W')).rename_axis('date'))

da.resample('B', 'date', 'ffill')

TypeError Traceback (most recent call last) <ipython-input-1-eb64a66a8d1f> in <module>() 3 da = xr.DataArray(pd.Series(1, pd.period_range('2000-1', '2000-12', freq='W')).rename_axis('date')) 4 ----> 5 da.resample('B', 'date', 'ffill')

/Users/maximilian/drive/workspace/xarray/xarray/core/common.py in resample(self, freq, dim, how, skipna, closed, label, base, keep_attrs) 577 time_grouper = pd.TimeGrouper(freq=freq, how=how, closed=closed, 578 label=label, base=base) --> 579 gb = self.groupby_cls(self, group, grouper=time_grouper) 580 if isinstance(how, basestring): 581 f = getattr(gb, how)

/Users/maximilian/drive/workspace/xarray/xarray/core/groupby.py in init(self, obj, group, squeeze, grouper, bins, cut_kwargs) 242 raise ValueError('index must be monotonic for resampling') 243 s = pd.Series(np.arange(index.size), index) --> 244 first_items = s.groupby(grouper).first() 245 if first_items.isnull().any(): 246 full_index = first_items.index

/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site-packages/pandas/core/generic.py in groupby(self, by, axis, level, as_index, sort, group_keys, squeeze, kwargs) 3989 return groupby(self, by=by, axis=axis, level=level, as_index=as_index, 3990 sort=sort, group_keys=group_keys, squeeze=squeeze, -> 3991 kwargs) 3992 3993 def asfreq(self, freq, method=None, how=None, normalize=False):

/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site-packages/pandas/core/groupby.py in groupby(obj, by, kwds) 1509 raise TypeError('invalid type: %s' % type(obj)) 1510 -> 1511 return klass(obj, by, kwds) 1512 1513

/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site-packages/pandas/core/groupby.py in init(self, obj, keys, axis, level, grouper, exclusions, selection, as_index, sort, group_keys, squeeze, **kwargs) 368 level=level, 369 sort=sort, --> 370 mutated=self.mutated) 371 372 self.obj = obj

/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site-packages/pandas/core/groupby.py in _get_grouper(obj, key, axis, level, sort, mutated) 2390 # a passed-in Grouper, directly convert 2391 if isinstance(key, Grouper): -> 2392 binner, grouper, obj = key._get_grouper(obj) 2393 if key.key is None: 2394 return grouper, [], obj

/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site-packages/pandas/tseries/resample.py in _get_grouper(self, obj) 1059 def _get_grouper(self, obj): 1060 # create the resampler and return our binner -> 1061 r = self._get_resampler(obj) 1062 r._set_binner() 1063 return r.binner, r.grouper, r.obj

/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site-packages/pandas/tseries/resample.py in _get_resampler(self, obj, kind) 1055 raise TypeError("Only valid with DatetimeIndex, " 1056 "TimedeltaIndex or PeriodIndex, " -> 1057 "but got an instance of %r" % type(ax).name) 1058 1059 def _get_grouper(self, obj):

TypeError: Only valid with DatetimeIndex, TimedeltaIndex or PeriodIndex, but got an instance of 'Index' ```

• [x] Closes #2172 & #2173
• [x] Tests added
• [x] Tests passed
• [x] Fully documented, including whats-new.rst for all changes and api.rst for new API

I don't think this is the most efficient way of doing this, though it does work. Any ideas for a more efficient implementation?

Not sure if we want this?

Probably too strict to enforce on every commit, more permissible to do a one-time update, assuming it doesn't cause merge conflicts.

You can get the same result by running isort -y