Enable subtracting a scalar cftime.datetime object from a CFTimeIndex

[spencerkclark]

Code Sample, a copy-pastable example if possible

In [1]: import xarray

In [2]: times = xarray.cftime_range('2000', periods=3)

In [3]: times - times[0]
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-3-97cbca76a8af> in <module>
----> 1 times - times[0]

~/xarray-dev/xarray/xarray/coding/cftimeindex.py in __sub__(self, other)
    417             return CFTimeIndex(np.array(self) - other.to_pytimedelta())
    418         else:
--> 419             return CFTimeIndex(np.array(self) - other)
    420
    421     def _add_delta(self, deltas):

~/xarray-dev/xarray/xarray/coding/cftimeindex.py in __new__(cls, data, name)
    238         result = object.__new__(cls)
    239         result._data = np.array(data, dtype='O')
--> 240         assert_all_valid_date_type(result._data)
    241         result.name = name
    242         return result

~/xarray-dev/xarray/xarray/coding/cftimeindex.py in assert_all_valid_date_type(data)
    194             raise TypeError(
    195                 'CFTimeIndex requires cftime.datetime '
--> 196                 'objects. Got object of {}.'.format(date_type))
    197         if not all(isinstance(value, date_type) for value in data):
    198             raise TypeError(

TypeError: CFTimeIndex requires cftime.datetime objects. Got object of <class 'datetime.timedelta'>.

Problem description

This should result in a pandas.TimedeltaIndex, as is the case for a pandas.DatetimeIndex:

In [4]: import pandas

In [5]: times = pandas.date_range('2000', periods=3)

In [6]: times - times[0]
Out[6]: TimedeltaIndex(['0 days', '1 days', '2 days'], dtype='timedelta64[ns]', freq=None)

Expected Output

In [1]: import xarray

In [2]: times = xarray.cftime_range('2000', periods=3)

In [3]: times - times[0]
Out[3]: TimedeltaIndex(['0 days', '1 days', '2 days'], dtype='timedelta64[ns]', freq=None)

Output of xr.show_versions()

INSTALLED VERSIONS

commit: None
python: 3.6.6 | packaged by conda-forge | (default, Jul 26 2018, 09:55:02)
[GCC 4.2.1 Compatible Apple LLVM 6.1.0 (clang-602.0.53)]
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+127.ga7129d1
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: 1.25.1
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