groupby on empty DataArray raises StopIteration

[lenaqr]

This seems similar to #1764 and #2240 so apologies if it is a duplicate, but I have a minimal example where it happens on an empty DataArray:

Code Sample

import xarray as xr
xr.DataArray([], dims='dim').groupby('dim').mean()  # raises StopIteration

Problem Description

Using groupby on an empty DataArray or Dataset raises StopIteration. It should raise a more meaningful error.

In particular, I had this issue in a function I was calling inside of a generator, so the StopIteration just broke out of the generator and it took some digging to figure out what was going wrong in my code.

Output of xr.show_versions()

INSTALLED VERSIONS

commit: None
python: 3.6.5.final.0
python-bits: 64
OS: Linux
OS-release: 3.10.0-862.14.4.el7.x86_64
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.utf8
LOCALE: en_US.UTF-8

xarray: 0.10.7
pandas: 0.23.0
numpy: 1.14.3
scipy: 1.1.0
netCDF4: 1.4.0
h5netcdf: 0.6.1
h5py: 2.7.1
Nio: None
zarr: None
bottleneck: 1.2.1
cyordereddict: None
dask: 0.17.5
distributed: 1.21.8
matplotlib: 2.2.2
cartopy: None
seaborn: 0.8.1
setuptools: 39.1.0
pip: 10.0.1
conda: None
pytest: 3.5.1
IPython: 6.4.0
sphinx: 1.7.4

[shoyer]

Agreed, this is definitely a bug. I'm not sure if we can define meaningful behavior for this case, but at the least we should raise a better error.

[HasanAhmadQ7]

@shoyer I would like to work on this issue (hopefully my first contribution).

I believe the error should indicate that we cannot groupby an empty group. This would be consistent with the documentation:

DataArray.groupby(group, squeeze: bool = True, restore_coord_dims: Optional[bool] = None)
Parameters
groupstr, DataArray or IndexVariable
Array whose unique values should be used to group this array. If a string, must be the name of a variable contained in this dataset.

For the groupby class, we can raise an error in case the groupby object instantiation does not specify values to be grouped by. In the case above:

ValueError: variable to groupby must not be empty

If you find such a solution acceptable, I would create a pull request.

Thank you in advance