Better setting of Coordinates in UniqueGrouper

Author: dcherian

xarray/groupers.py

@@ -9,7 +9,7 @@
 import datetime
 from abc import ABC, abstractmethod
 from dataclasses import dataclass, field
-from typing import Any, Literal, cast
+from typing import TYPE_CHECKING, Any, Literal, cast
 
 import numpy as np
 import pandas as pd
@@ -205,21 +205,31 @@ def _factorize_dummy(self) -> EncodedGroups:
         group_indices: GroupIndices = tuple(slice(i, i + 1) for i in range(size))
         size_range = np.arange(size)
         full_index: pd.Index
+        unique_coord: _DummyGroup | Variable
         if isinstance(self.group, _DummyGroup):
             codes = self.group.to_dataarray().copy(data=size_range)
             unique_coord = self.group
             full_index = pd.RangeIndex(self.group.size)
+            coords = Coordinates()
         else:
             codes = self.group.copy(data=size_range)
             unique_coord = self.group.variable.to_base_variable()
             full_index = self.group_as_index
+            if isinstance(full_index, pd.MultiIndex):
+                coords = Coordinates.from_pandas_multiindex(
+                    full_index, dim=self.group.name
+                )
+            else:
+                if TYPE_CHECKING:
+                    assert isinstance(unique_coord, Variable)
+                coords = _coordinates_from_variable(unique_coord)
 
         return EncodedGroups(
             codes=codes,
             group_indices=group_indices,
             full_index=full_index,
             unique_coord=unique_coord,
-            coords=Coordinates(),
+            coords=coords,
         )