Document cftime arithmetic limitations

[larsbuntemeyer]

What is your issue?

Hey all,
i have quick use case where i am unsure if that's related to xarray. I just want to do simple arithmetics with a cftime time axis, e.g.

import xarray as xr
import numpy as np
time = xr.DataArray(xr.cftime_range(start="2000", periods=6, freq="1MS", calendar="noleap"), dims='time')

and then, e.g., compute mid of those timesteps:

time[:-1] + 0.5 * time.diff('time')

which results in

---------------------------------------------------------------------------
UFuncTypeError                            Traceback (most recent call last)
Input In [72], in <cell line: 1>()
----> 1 time[1:] + 0.5 * time.diff('time')

File /work/ch0636/g300046/conda_envs/pyremo-dev/lib/python3.9/site-packages/xarray/core/_typed_ops.py:206, in DataArrayOpsMixin.__add__(self, other)
    205 def __add__(self, other):
--> 206     return self._binary_op(other, operator.add)

File /work/ch0636/g300046/conda_envs/pyremo-dev/lib/python3.9/site-packages/xarray/core/dataarray.py:3530, in DataArray._binary_op(self, other, f, reflexive)
   3526 other_variable = getattr(other, "variable", other)
   3527 other_coords = getattr(other, "coords", None)
   3529 variable = (
-> 3530     f(self.variable, other_variable)
   3531     if not reflexive
   3532     else f(other_variable, self.variable)
   3533 )
   3534 coords, indexes = self.coords._merge_raw(other_coords, reflexive)
   3535 name = self._result_name(other)

File /work/ch0636/g300046/conda_envs/pyremo-dev/lib/python3.9/site-packages/xarray/core/_typed_ops.py:396, in VariableOpsMixin.__add__(self, other)
    395 def __add__(self, other):
--> 396     return self._binary_op(other, operator.add)

File /work/ch0636/g300046/conda_envs/pyremo-dev/lib/python3.9/site-packages/xarray/core/variable.py:2519, in Variable._binary_op(self, other, f, reflexive)
   2516 attrs = self._attrs if keep_attrs else None
   2517 with np.errstate(all="ignore"):
   2518     new_data = (
-> 2519         f(self_data, other_data) if not reflexive else f(other_data, self_data)
   2520     )
   2521 result = Variable(dims, new_data, attrs=attrs)
   2522 return result

UFuncTypeError: ufunc 'add' cannot use operands with types dtype('O') and dtype('<m8[ns]')

The problem is the differentiation results in np.timedelta64 objects:

time.diff('time')

with which the arithmetics are not implemented in cftime. However, differentiation with datetime.timdelta (which is wrapped by np.timedelta64) works in this case since it's implemented in cftime, e.g., when i do np.diff, it works:

time[:-1] + 0.5 * np.diff(time)

i get the expected result. I am not sure what to expect from xarray and if this might be an issue of cftime (e.g., implement differentiation with np.timedelta64). I was wondering why the differentiation with cftime objects results in np.timedelta64 and not datetime.timedelta objects (like in the pure numpy differentation).

[larsbuntemeyer]

Ok, sorry, just found #5402 and #4999 and see that's under discussion. Didn't think of searching the PR's...

[dcherian]

It would be good to document such issues (and summarize the discussion points) at https://docs.xarray.dev/en/stable/user-guide/weather-climate.html#non-standard-calendars-and-dates-outside-the-timestamp-valid-range

PRs welcome!

[larsbuntemeyer]

Slightly related:

• ufuncs on object arrays should call __array_ufunc__ if it exists on the wrapped objects numpy/numpy#15479