how to group cftime coordinate by week?

[aaronspring]

Suppose you have to work with cftime coordinates, how do you group by week?
The only way I found working was converting to datetime calendar with convert_calendar.

Is there a cftime solution? Is there a reason why CFTimeIndex doesnt implement week?

import xarray as xr
xr.__version__ # '2022.3.0'

import cftime
cftime.__version__ # '1.6.0'

ds = xr.tutorial.load_dataset('air_temperature', use_cftime=True)

ds.convert_calendar(calendar='standard').groupby('time.week').mean() # works

ds.groupby('time.week').mean() # AttributeError: 'CFTimeIndex' object has no attribute 'isocalendar'

ds.groupby(ds.time.dt.isocalendar().week).mean() # AttributeError: 'CFTimeIndex' object has no attribute 'isocalendar'


ds = xr.tutorial.load_dataset('air_temperature', use_cftime=False)

ds.groupby('time.week').mean() # FutureWarning: dt.weekofyear and dt.week have been deprecated. Please use dt.isocalendar().week instead.

ds.groupby(ds.time.dt.isocalendar().week).mean() # works

[spencerkclark]

The primary reason is that cftime.datetime objects do not implement an isocalendar method (see datetime.date.isocalendar for more details). You could consider raising an issue in the cftime repo. If they did, it would make it pretty easy to enable in xarray.