Ordered Groupby Keys

[jhamman]

The current behavior of the xarray's Groupby.groups property provides a standard (unordered) dictionary. This is fine for most cases but leads to odd orderings in use cases like this one where I am using xarray's FacetGrid plotting:

plot_kwargs = dict(col='season', vmin=15, vmax=35, levels=12, extend='both')

da_obs = ds_obs.SALT.isel(depth=0).groupby('time.season').mean('time')
da_obs.plot(**plot_kwargs)

Note that MAM and JJA are out of order.

I think this could be easily fixed by using an OrderedDict in xarray.core.Groupby.groups.

[shoyer]

I agree this is annoying, but I don't think your diagnosis is correct here. The groups property isn't used by any internal routines AFAICT. The issue is that groups are sorted, but as text rather than ordered categorical -- notice that the labels are ordered alphabetically.

[jhamman]

Hmmm, a mystery. I'll look into this a bit more.

[shoyer]

For what it's worth, I don't think we have any good solutions short of
adding our own array type do to Categorical in xarray. We could set
sort=False in some cases when we call pd.factorize but that's not a great
alternative.

[tdihp]

Ahh, so it's sorted, instead of keeping the original order.

I was expecting DataArray.groupby().reduce would work like np.apply_along_axis, and used the data of the result directly.

[jbusecke]

Just stumbled across this issue. Is there a recommended workaround?

I am usually doing this (specific to seasons):

import xarray as xr
ds = xr.tutorial.open_dataset('air_temperature')
airtemp_seasonal = ds.groupby('time.season').mean('time').sortby(xr.DataArray(['DJF','MAM','JJA', 'SON'],dims=['season']))

Thought this might help some folks who need to solve this problem.

[dcherian]

I use reindex instead of sortby