issues: 400504690

Code Sample, a copy-pastable example if possible

Currently something like the following raises an error: ``` In [1]: import xarray as xr

In [2]: import pandas as pd

In [3]: import numpy as np

In [4]: times = xr.cftime_range('2000', periods=3)

In [5]: series = pd.Series(np.array([0., np.nan, 1.]), index=times)

In [6]: series Out[6]: 2000-01-01 00:00:00 0.0 2000-01-02 00:00:00 NaN 2000-01-03 00:00:00 1.0 dtype: float64

In [7]: series.dropna()

TypeError Traceback (most recent call last) <ipython-input-7-45eb0c023203> in <module> ----> 1 series.dropna()

~/pandas/pandas/core/series.py in dropna(self, axis, inplace, **kwargs) 4169 4170 if self._can_hold_na: -> 4171 result = remove_na_arraylike(self) 4172 if inplace: 4173 self._update_inplace(result)

~/pandas/pandas/core/dtypes/missing.py in remove_na_arraylike(arr) 539 return arr[notna(arr)] 540 else: --> 541 return arr[notna(lib.values_from_object(arr))]

~/pandas/pandas/core/series.py in getitem(self, key) 801 key = com.apply_if_callable(key, self) 802 try: --> 803 result = self.index.get_value(self, key) 804 805 if not is_scalar(result):

~/xarray-dev/xarray/xarray/coding/cftimeindex.py in get_value(self, series, key) 321 """Adapted from pandas.tseries.index.DatetimeIndex.get_value""" 322 if not isinstance(key, slice): --> 323 return series.iloc[self.get_loc(key)] 324 else: 325 return series.iloc[self.slice_indexer(

~/xarray-dev/xarray/xarray/coding/cftimeindex.py in get_loc(self, key, method, tolerance) 300 else: 301 return pd.Index.get_loc(self, key, method=method, --> 302 tolerance=tolerance) 303 304 def _maybe_cast_slice_bound(self, label, side, kind):

~/pandas/pandas/core/indexes/base.py in get_loc(self, key, method, tolerance) 2595 'backfill or nearest lookups') 2596 try: -> 2597 return self._engine.get_loc(key) 2598 except KeyError: 2599 return self._engine.get_loc(self._maybe_cast_indexer(key))

~/pandas/pandas/_libs/index.pyx in pandas._libs.index.IndexEngine.get_loc()

~/pandas/pandas/_libs/index.pyx in pandas._libs.index.IndexEngine.get_loc()

TypeError: '[ True False True]' is an invalid key ```

Problem description

We currently rely on this in the resampling logic within xarray for a Series indexed by a DatetimeIndex: https://github.com/pydata/xarray/blob/dc87dea52351835af472d131f70a7f7603b3100e/xarray/core/groupby.py#L268 It would be nice if we could do the same with a Series indexed by a CFTimeIndex, e.g. in #2593.

Expected Output

In [7]: series.dropna() Out[7]: 2000-01-01 00:00:00 0.0 2000-01-03 00:00:00 1.0 dtype: float64

Output of xr.show_versions()

INSTALLED VERSIONS ------------------ commit: None python: 3.7.1 | packaged by conda-forge | (default, Nov 13 2018, 09:50:42) [Clang 9.0.0 (clang-900.0.37)] python-bits: 64 OS: Darwin OS-release: 18.2.0 machine: x86_64 processor: i386 byteorder: little LC_ALL: None LANG: en_US.UTF-8 LOCALE: en_US.UTF-8 libhdf5: 1.10.4 libnetcdf: 4.6.2 xarray: 0.10.9+117.g80914e0.dirty pandas: 0.24.0.dev0+1332.g5d134ec numpy: 1.15.4 scipy: 1.1.0 netCDF4: 1.4.2 pydap: None h5netcdf: None h5py: None Nio: None zarr: None cftime: 1.0.3.4 PseudonetCDF: None rasterio: None cfgrib: None iris: None bottleneck: None cyordereddict: None dask: 1.0.0 distributed: 1.25.2 matplotlib: 3.0.2 cartopy: None seaborn: 0.9.0 setuptools: 40.6.3 pip: 18.1 conda: None pytest: 3.10.1 IPython: 7.2.0 sphinx: None