issues: 906175200

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id node_id number title user state locked assignee milestone comments created_at updated_at closed_at author_association active_lock_reason draft pull_request body reactions performed_via_github_app state_reason repo type
906175200 MDExOlB1bGxSZXF1ZXN0NjU3MjA1NTM2 5402 `dt.to_pytimedelta` to allow arithmetic with cftime objects 20629530 open 0     1 2021-05-28T22:48:50Z 2022-06-09T14:50:16Z   CONTRIBUTOR   0 pydata/xarray/pulls/5402
  • [ ] Closes #xxxx
  • [x] Tests added
  • [x] Passes pre-commit run --all-files
  • [ ] User visible changes (including notable bug fixes) are documented in whats-new.rst
  • [ ] New functions/methods are listed in api.rst

When playing with cftime objects a problem I encountered many times is that I can sub two arrays and them add it back to another. Subtracting to cftime datetime arrays result in an array of np.timedelta64. And when trying to add it back to another cftime array, we get a UFuncTypeError because the two arrays have incompatible dtypes : '<m8[ns]' and 'O'.

Example: ```python import xarray as xr da = xr.DataArray(xr.cftime_range('1900-01-01', freq='D', periods=10), dims=('time',))

An array of timedelta64[ns]

dt = da - da[0]

da[-1] + dt # Fails ```

However, if the two arrays were of 'O' dtype, then the subtraction would be made by cftime which supports datetime.timedelta objects.

This solution here adds a to_pytimedelta to the TimedeltaAccessor, mirroring the name of the similar function on pd.Series.dt. It uses a monkeypatching workaround to prevent xarray to case the array back into numpy objects.

The user still has to check if the data is in cftime or numpy to adapt the operation (calling dt.to_pytimedelta or not), but custom workaround were always overly complicated for such a simple problem, this helps.

Also, this doesn't work with dask arrays because loading a dask array triggers the variable constructor and thus recasts the array of datetime.timedelta to numpy.timedelta[64].

I realize I maybe should have opened an issue before, but I had this idea and it all rushed along.

 
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