BUG: groupby on empty DataArray raises StopIteration

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

Resolves: 3037

Author: HasanAhmadQ7

xarray/core/groupby.py

@@ -232,6 +232,9 @@ def __init__(self, obj, group, squeeze=False, grouper=None, bins=None,
                 raise TypeError('`group` must be an xarray.DataArray or the '
                                 'name of an xarray variable or dimension')
             group = obj[group]
+            if len(group) == 0:
+                raise ValueError("Group must not be empty")
+
             if group.name not in obj.coords and group.name in obj.dims:
                 # DummyGroups should not appear on groupby results
                 group = _DummyGroup(obj, group.name, group.coords)

xarray/tests/test_groupby.py

@@ -104,6 +104,14 @@ def func(arg1, arg2, arg3=0):
     actual = dataset.groupby('x').apply(func, args=(1,), arg3=1)
     assert_identical(expected, actual)
 
+def test_da_groupby_empty():
+
+    empty_array = xr.DataArray([], dims='dim')
+    
+    with pytest.raises(ValueError):
+        empty_array.groupby('dim')
+
+
 
 def test_da_groupby_quantile():