Document cftime arithmetic limitations #7381
Description
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).