issues

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:

Is your feature request related to a problem? Please describe.

The xarray.cftime_range does not support datetime strings that are the default output from cftime.datetime.strftime() which are the format which cftime_range itself uses internally.

python import cftime import xarray date = cftime.datetime(10,1,1).strftime() print(date) xarray.cftime_range(date, periods=3, freq='Y') outputs ``` 10-01-01 00:00:00

ValueError Traceback (most recent call last) <ipython-input-70-a16c1fcab8d6> in <module> 3 date = cftime.datetime(10,1,1).strftime() 4 print(date) ----> 5 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) 963 964 if start is not None: --> 965 start = to_cftime_datetime(start, calendar) 966 start = _maybe_normalize_date(start, normalize) 967 if end is not None:

/g/data3/hh5/public/apps/miniconda3/envs/analysis3-20.07/lib/python3.7/site-packages/xarray/coding/cftime_offsets.py in to_cftime_datetime(date_str_or_date, calendar) 683 "a calendar type must be provided" 684 ) --> 685 date, _ = _parse_iso8601_with_reso(get_date_type(calendar), date_str_or_date) 686 return date 687 elif isinstance(date_str_or_date, cftime.datetime):

/g/data3/hh5/public/apps/miniconda3/envs/analysis3-20.07/lib/python3.7/site-packages/xarray/coding/cftimeindex.py in _parse_iso8601_with_reso(date_type, timestr) 101 102 default = date_type(1, 1, 1) --> 103 result = parse_iso8601(timestr) 104 replace = {} 105

/g/data3/hh5/public/apps/miniconda3/envs/analysis3-20.07/lib/python3.7/site-packages/xarray/coding/cftimeindex.py in parse_iso8601(datetime_string) 94 if match: 95 return match.groupdict() ---> 96 raise ValueError("no ISO-8601 match for string: %s" % datetime_string) 97 98

ValueError: no ISO-8601 match for string: 10-01-01 00:00:00 ```

Describe the solution you'd like It would be good if xarray.cftime_range supported the default strftime format output from cftime.datetime objects. It is confusing that it uses this format with repr but explicitly does not support it.

Describe alternatives you've considered

Specifying an ISO-8601 compatible format (using T separator) isn't general as it doesn't work for years < 1000 because the year field is not zero padded. python import cftime import xarray date = cftime.datetime(10,1,1).strftime('%Y-%m-%dT%H:%M:%S') print('|{}|'.format(date)) xarray.cftime_range(date, periods=3, freq='Y') produces | 10-01-01T00:00:00| and the error as above.

A work-around is to zero-pad manually python import cftime import xarray date = '{:0>19}'.format(cftime.datetime(10,1,1).strftime('%Y-%m-%dT%H:%M:%S').lstrip()) print(date) xarray.cftime_range(date, periods=3, freq='Y') produces 0010-01-01T00:00:00 CFTimeIndex([0010-12-31 00:00:00, 0011-12-31 00:00:00, 0012-12-31 00:00:00], dtype='object')

Additional context I think this is a relatively small addition to the codebase but would make it easier and less confusing to use the default format that is also used by the the function itself. It is easy to support as it is consistent and uniform.