ENH: Preserve monotonic descending index order when merging

xarray/core/alignment.py

@@ -123,6 +123,14 @@ def align(*objects, join='inner', copy=True, indexes=None, exclude=frozenset(),
                         'indexes along dimension {!r} are not equal'
                         .format(dim))
                 index = joiner(matching_indexes)
+                # Pandas.Index.union gets called for an outer join, and the index
+                # is sorted in ascending order. When all indexes are monotonic
+                # descending, it's desirable to keep maintain descending order.
+                # The ascending sort doesn't happen if only one index is joined.
+                if join == 'outer' and len(matching_indexes) > 1:
+                    if all((matching_index.is_monotonic_decreasing
+                            for matching_index in matching_indexes)):
+                        index = index[::-1]
                 joined_indexes[dim] = index
             else:
                 index = matching_indexes[0]

xarray/tests/test_dataset.py

@@ -1851,13 +1851,13 @@ def test_broadcast_misaligned(self):
                                       coords={'y': [1, 2], 'x': [10, -3]})})
         x2, y2 = broadcast(x, y)
         expected_x2 = Dataset(
-            {'foo': DataArray([[3, 3], [2, 2], [1, 1], [np.nan, np.nan]],
+            {'foo': DataArray([[np.nan, np.nan], [1, 1], [2, 2], [3, 3]],
                               dims=['x', 'y'],
-                              coords={'y': [1, 2], 'x': [-3, -2, -1, 10]})})
+                              coords={'y': [1, 2], 'x': [10, -1, -2, -3]})})
         expected_y2 = Dataset(
             {'bar': DataArray(
-                [[2, 4], [np.nan, np.nan], [np.nan, np.nan], [1, 3]],
-                dims=['x', 'y'], coords={'y': [1, 2], 'x': [-3, -2, -1, 10]})})
+                [[1, 3], [np.nan, np.nan], [np.nan, np.nan], [2, 4]],
+                dims=['x', 'y'], coords={'y': [1, 2], 'x': [10, -1, -2, -3]})})
         assert_identical(expected_x2, x2)
         assert_identical(expected_y2, y2)
 

xarray/tests/test_merge.py

@@ -67,6 +67,13 @@ def test_merge_alignment_error(self):
         with raises_regex(ValueError, 'indexes .* not equal'):
             xr.merge([ds, other], join='exact')
 
+    def test_merge_monotonic_descending_indexes(self):
+        ds = xr.Dataset(coords={'x': [2, 1]})
+        other = xr.Dataset(coords={'x': [3, 2]})
+        actual = xr.merge([ds, other])
+        expected = xr.Dataset(coords={'x': [3, 2, 1]})
+        assert expected.identical(actual)
+
     def test_merge_no_conflicts_single_var(self):
         ds1 = xr.Dataset({'a': ('x', [1, 2]), 'x': [0, 1]})
         ds2 = xr.Dataset({'a': ('x', [2, 3]), 'x': [1, 2]})