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:

The behaviour of reduce appears to have changed in recent versions of xarray such that previous code that worked now throws errors.

MCVE Code Sample

I have repurposed someone else's nice code sample for this, thanks!

```python import pandas as pd import xarray as xr import numpy as np

s_date = '1990-01-01' e_date = '2019-05-01' days = pd.date_range(start=s_date, end=e_date, freq='B', name='time') items = pd.Index([str(i) for i in range(300)], name = 'item') dat = xr.DataArray(np.random.rand(len(days), len(items)), coords=[days, items])

print(dat)

def simplesum(array, axis): print(axis) return np.sum(array, axis)

dat.groupby('time.month').reduce(simplesum) dat.groupby('time.month').reduce(simplesum, axis=0) ```

The reduce appears to insert an axis argument if none is specified. This is the output of the first groupby operations with no axis argument: python 0 0 0 0 0 0 0 0 0 0 0 0 Out[41]: <xarray.DataArray (month: 12, item: 300)> array([[330.18949303, 336.97901528, 337.80472647, ..., 322.37053342, 326.84789948, 342.22782336], [300.3301059 , 307.79967902, 322.53148357, ..., 310.20975273, 291.04344738, 310.56010997], [325.71587689, 337.25153307, 331.35493521, ..., 332.43547569, 328.23330226, 326.43909063], ..., [322.96255713, 321.44723754, 312.59983716, ..., 318.79682437, 315.81592617, 314.27316547], [294.29894222, 291.77253983, 310.85452639, ..., 314.0461447 , 298.99012623, 326.08321702], [323.6778518 , 332.71638634, 324.47244831, ..., 326.82774826, 322.09233181, 327.6385762 ]]) Coordinates: * item (item) object '0' '1' '2' '3' '4' ... '295' '296' '297' '298' '299' * month (month) int64 1 2 3 4 5 6 7 8 9 10 11 12

The second groupby with axis=0 argument throws an error: ```python

ValueError Traceback (most recent call last) <ipython-input-42-381dec6862e6> in <module> ----> 1 dat.groupby('time.month').reduce(simplesum, axis=0)

/g/data3/hh5/public/apps/miniconda3/envs/analysis3-20.01/lib/python3.7/site-packages/xarray/core/groupby.py in reduce(self, func, dim, axis, keep_attrs, shortcut, **kwargs) 836 check_reduce_dims(dim, self.dims) 837 --> 838 return self.map(reduce_array, shortcut=shortcut) 839 840

/g/data3/hh5/public/apps/miniconda3/envs/analysis3-20.01/lib/python3.7/site-packages/xarray/core/groupby.py in map(self, func, shortcut, args, kwargs) 755 grouped = self._iter_grouped() 756 applied = (maybe_wrap_array(arr, func(arr, *args, kwargs)) for arr in grouped) --> 757 return self._combine(applied, shortcut=shortcut) 758 759 def apply(self, func, shortcut=False, args=(), **kwargs):

/g/data3/hh5/public/apps/miniconda3/envs/analysis3-20.01/lib/python3.7/site-packages/xarray/core/groupby.py in _combine(self, applied, restore_coord_dims, shortcut) 774 def _combine(self, applied, restore_coord_dims=False, shortcut=False): 775 """Recombine the applied objects like the original.""" --> 776 applied_example, applied = peek_at(applied) 777 coord, dim, positions = self._infer_concat_args(applied_example) 778 if shortcut:

/g/data3/hh5/public/apps/miniconda3/envs/analysis3-20.01/lib/python3.7/site-packages/xarray/core/utils.py in peek_at(iterable) 180 """ 181 gen = iter(iterable) --> 182 peek = next(gen) 183 return peek, itertools.chain([peek], gen) 184

/g/data3/hh5/public/apps/miniconda3/envs/analysis3-20.01/lib/python3.7/site-packages/xarray/core/groupby.py in <genexpr>(.0) 754 else: 755 grouped = self._iter_grouped() --> 756 applied = (maybe_wrap_array(arr, func(arr, args, *kwargs)) for arr in grouped) 757 return self._combine(applied, shortcut=shortcut) 758

/g/data3/hh5/public/apps/miniconda3/envs/analysis3-20.01/lib/python3.7/site-packages/xarray/core/groupby.py in reduce_array(ar) 832 833 def reduce_array(ar): --> 834 return ar.reduce(func, dim, axis, keep_attrs=keep_attrs, **kwargs) 835 836 check_reduce_dims(dim, self.dims)

/g/data3/hh5/public/apps/miniconda3/envs/analysis3-20.01/lib/python3.7/site-packages/xarray/core/variable.py in reduce(self, func, dim, axis, keep_attrs, keepdims, allow_lazy, **kwargs) 1511 dim = None 1512 if dim is not None and axis is not None: -> 1513 raise ValueError("cannot supply both 'axis' and 'dim' arguments") 1514 1515 if dim is not None:

ValueError: cannot supply both 'axis' and 'dim' arguments ```

Expected Output

I would expect the output of both groupby operations to be the same, though reduce says it should flatten the input if there is no dim or axis argument supplied, it doesn't seem to do this.

The second groupby, with axis=0 argument works with older versions of xarray(0.13.0).

Problem Description

It is impossible to specify a dim argument to reduce. It defaults to axis=0 and when a different axis is specified it throws an error.

Output of xr.show_versions()

Version used and produces error:

The version of xarray does not throw an error when axis argument is supplied: