Fix grouping by multiindex (#106)

Author: dcherian

flox/xarray.py

@@ -5,6 +5,7 @@
 import numpy as np
 import pandas as pd
 import xarray as xr
+from packaging.version import Version
 
 from .aggregations import Aggregation, _atleast_1d
 from .core import (
@@ -345,12 +346,16 @@ def wrapper(array, *by, func, skipna, **kwargs):
             expect = expect.to_numpy()
         if isinstance(actual, xr.Dataset) and name in actual:
             actual = actual.drop_vars(name)
-        actual[name] = expect
-
-    # if grouping by multi-indexed variable, then restore it
-    for name, index in ds.indexes.items():
-        if name in actual.indexes and isinstance(index, pd.MultiIndex):
-            actual[name] = index
+        # When grouping by MultiIndex, expect is an pd.Index wrapping
+        # an object array of tuples
+        if name in ds.indexes and isinstance(ds.indexes[name], pd.MultiIndex):
+            levelnames = ds.indexes[name].names
+            expect = pd.MultiIndex.from_tuples(expect.values, names=levelnames)
+            actual[name] = expect
+            if Version(xr.__version__) > Version("2022.03.0"):
+                actual = actual.set_coords(levelnames)
+        else:
+            actual[name] = expect
 
     if unindexed_dims:
         actual = actual.drop_vars(unindexed_dims)
@@ -361,7 +366,8 @@ def wrapper(array, *by, func, skipna, **kwargs):
                 template = obj
             else:
                 template = obj[var]
-            actual[var] = _restore_dim_order(actual[var], template, by[0])
+            if actual[var].ndim > 1:
+                actual[var] = _restore_dim_order(actual[var], template, by[0])
 
     if missing_dim:
         for k, v in missing_dim.items():
@@ -370,9 +376,9 @@ def wrapper(array, *by, func, skipna, **kwargs):
             }
             # The expand_dims is for backward compat with xarray's questionable behaviour
             if missing_group_dims:
-                actual[k] = v.expand_dims(missing_group_dims)
+                actual[k] = v.expand_dims(missing_group_dims).variable
             else:
-                actual[k] = v
+                actual[k] = v.variable
 
     if isinstance(obj, xr.DataArray):
         return obj._from_temp_dataset(actual)

tests/test_xarray.py

@@ -321,6 +321,17 @@ def test_multi_index_groupby_sum(engine):
     actual = xarray_reduce(stacked, "space", dim="z", func="sum", engine=engine)
     assert_equal(expected, actual.unstack("space"))
 
+    actual = xarray_reduce(stacked.foo, "space", dim="z", func="sum", engine=engine)
+    assert_equal(expected.foo, actual.unstack("space"))
+
+    ds = xr.Dataset(
+        dict(a=(("z",), np.ones(10))),
+        coords=dict(b=(("z"), np.arange(2).repeat(5)), c=(("z"), np.arange(5).repeat(2))),
+    ).set_index(bc=["b", "c"])
+    expected = ds.groupby("bc").sum()
+    actual = xarray_reduce(ds, "bc", func="sum")
+    assert_equal(expected, actual)
+
 
 @pytest.mark.parametrize("chunks", (None, 2))
 def test_xarray_groupby_bins(chunks, engine):