Fix regression with grouper object on an IndexVariable (#7920)

* Fix regression with grouper object on an IndexVariable

* Early cast IndexVariable to DataArray to avoid issues with flox

Author: mwtoews

xarray/core/groupby.py

@@ -626,6 +626,7 @@ def _resolve_group(obj: T_Xarray, group: T_Group | Hashable) -> T_Group:
         (group_dim,) = group.dims
         if len(group) != obj.sizes[group_dim]:
             raise ValueError(error_msg)
+        newgroup = DataArray(group)
 
     else:
         if not hashable(group):

xarray/tests/test_groupby.py

@@ -137,6 +137,18 @@ def test_groupby_input_mutation() -> None:
     assert_identical(array, array_copy)  # should not modify inputs
 
 
+@pytest.mark.parametrize("use_flox", [True, False])
+def test_groupby_indexvariable(use_flox: bool) -> None:
+    # regression test for GH7919
+    array = xr.DataArray([1, 2, 3], [("x", [2, 2, 1])])
+    iv = xr.IndexVariable(dims="x", data=pd.Index(array.x.values))
+    with xr.set_options(use_flox=use_flox):
+        actual = array.groupby(iv).sum()
+    actual = array.groupby(iv).sum()
+    expected = xr.DataArray([3, 3], [("x", [1, 2])])
+    assert_identical(expected, actual)
+
+
 @pytest.mark.parametrize(
     "obj",
     [