Test resampling error with monotonic decreasing data.

Author: dcherian

xarray/core/groupby.py

@@ -400,9 +400,11 @@ def __init__(
         unique_coord: DataArray | IndexVariable | _DummyGroup
         if grouper is not None:
             index = safe_cast_to_index(group)
-            if not _unique_and_monotonic(group):
+            if not index.is_montonic_increasing:
                 # TODO: sort instead of raising an error
-                raise ValueError("index must be monotonic for resampling")
+                raise ValueError(
+                    "Index must be monotonic and increasing for resampling."
+                )
             full_index, first_items = self._get_index_and_items(index, grouper)
             sbins = first_items.values.astype(np.int64)
             group_indices = [slice(i, j) for i, j in zip(sbins[:-1], sbins[1:])] + [

xarray/tests/test_groupby.py

@@ -1507,6 +1507,10 @@ def test_resample(self):
         with pytest.raises(ValueError, match=r"index must be monotonic"):
             array[[2, 0, 1]].resample(time="1D")
 
+        reverse = array.isel(time=slice(-1, None, -1))
+        with pytest.raises(ValueError):
+            reverse.resample(time="1D").mean()
+
     def test_da_resample_func_args(self):
         def func(arg1, arg2, arg3=0.0):
             return arg1.mean("time") + arg2 + arg3