issues: 677296128

What happened:

xarray.cftime_range does not accept dates that use base classcftime.datetime objects.

What you expected to happen:

I expected xarray.cftime_range to raise an exception that this is an unsupported cftime.datetime type and for the documentation to reflect this.

Minimal Complete Verifiable Example:

python import cftime import xarray date = cftime.datetime(10,1,1) xarray.cftime_range(date, periods=3, freq='Y')

Anything else we need to know?:

Returns this error: ```python

---

TypeError Traceback (most recent call last) <ipython-input-29-d090ea15e436> in <module> 2 import xarray 3 date = cftime.datetime(10,1,1) ----> 4 xarray.cftime_range(date, periods=3, freq='Y')

/g/data3/hh5/public/apps/miniconda3/envs/analysis3-20.07/lib/python3.7/site-packages/xarray/coding/cftime_offsets.py in cftime_range(start, end, periods, freq, normalize, name, closed, calendar) 973 else: 974 offset = to_offset(freq) --> 975 dates = np.array(list(_generate_range(start, end, periods, offset))) 976 977 left_closed = False

/g/data3/hh5/public/apps/miniconda3/envs/analysis3-20.07/lib/python3.7/site-packages/xarray/coding/cftime_offsets.py in _generate_range(start, end, periods, offset) 744 """ 745 if start: --> 746 start = offset.rollforward(start) 747 748 if end:

/g/data3/hh5/public/apps/miniconda3/envs/analysis3-20.07/lib/python3.7/site-packages/xarray/coding/cftime_offsets.py in rollforward(self, date) 526 def rollforward(self, date): 527 """Roll date forward to nearest end of year""" --> 528 if self.onOffset(date): 529 return date 530 else:

/g/data3/hh5/public/apps/miniconda3/envs/analysis3-20.07/lib/python3.7/site-packages/xarray/coding/cftime_offsets.py in onOffset(self, date) 522 """Check if the given date is in the set of possible dates created 523 using a length-one version of this offset class.""" --> 524 return date.day == _days_in_month(date) and date.month == self.month 525 526 def rollforward(self, date):

/g/data3/hh5/public/apps/miniconda3/envs/analysis3-20.07/lib/python3.7/site-packages/xarray/coding/cftime_offsets.py in _days_in_month(date) 195 else: 196 reference = type(date)(date.year, date.month + 1, 1) --> 197 return (reference - timedelta(days=1)).day 198 199

TypeError: unsupported operand type(s) for -: 'cftime._cftime.datetime' and 'datetime.timedelta' Works if a `datetime` object with a calendar is used: import cftime import xarray date = cftime.DatetimeGregorian(10,1,1) xarray.cftime_range(date, periods=3, freq='Y') Returns:python CFTimeIndex([0010-12-31 00:00:00, 0011-12-31 00:00:00, 0012-12-31 00:00:00], dtype='object') ``` as expected.

The error occurs here

https://github.com/pydata/xarray/blob/master/xarray/coding/cftime_offsets.py#L197

because this operation is not defined for the base class

https://github.com/Unidata/cftime/blob/master/cftime/_cftime.pyx#L1054

The relevant tests all seem to use datetime strings which are by default standard calendar:

https://github.com/pydata/xarray/blob/master/xarray/coding/cftime_offsets.py#L788

Environment:

Output of <tt>xr.show_versions()</tt>

INSTALLED VERSIONS ------------------ commit: None python: 3.7.8 | packaged by conda-forge | (default, Jul 31 2020, 02:25:08) [GCC 7.5.0] python-bits: 64 OS: Linux OS-release: 2.6.32-754.18.2.el6.x86_64 machine: x86_64 processor: x86_64 byteorder: little LC_ALL: en_AU.utf8 LANG: en_AU.UTF-8 LOCALE: en_AU.UTF-8 libhdf5: 1.10.5 libnetcdf: 4.7.4 xarray: 0.16.0 pandas: 1.1.0 numpy: 1.19.1 scipy: 1.5.2 netCDF4: 1.5.3 pydap: installed h5netcdf: 0.8.1 h5py: 2.10.0 Nio: 1.5.5 zarr: 2.4.0 cftime: 1.0.3.4 nc_time_axis: 1.2.0 PseudoNetCDF: None rasterio: 1.1.5 cfgrib: 0.9.8.4 iris: 2.4.0 bottleneck: 1.3.2 dask: 2.22.0 distributed: 2.22.0 matplotlib: 3.3.0 cartopy: 0.18.0 seaborn: 0.10.1 numbagg: None pint: 0.14 setuptools: 49.2.0.post20200712 pip: 20.1.1 conda: installed pytest: 6.0.1 IPython: 7.17.0 sphinx: 3.2.0