Fix indexes created by Dataset.swap_dims (#2845)

Fixes GH2842

Author: shoyer

doc/whats-new.rst

@@ -25,6 +25,9 @@ Enhancements
 Bug fixes
 ~~~~~~~~~
 
+- ``swap_dims`` would create incorrect ``indexes`` (:issue:`2842`).
+  By `Stephan Hoyer <https://github.com/shoyer>`_.
+
 
 .. _whats-new.0.12.0:
 

xarray/core/dataset.py

@@ -2307,24 +2307,20 @@ def swap_dims(self, dims_dict, inplace=None):
         coord_names.update(dims_dict.values())
 
         variables = OrderedDict()
+        indexes = OrderedDict()
         for k, v in self.variables.items():
             dims = tuple(dims_dict.get(dim, dim) for dim in v.dims)
             if k in result_dims:
                 var = v.to_index_variable()
+                if k in self.indexes:
+                    indexes[k] = self.indexes[k]
+                else:
+                    indexes[k] = var.to_index()
             else:
                 var = v.to_base_variable()
             var.dims = dims
             variables[k] = var
 
-        indexes = OrderedDict()
-        for k, v in self.indexes.items():
-            if k in dims_dict:
-                new_name = dims_dict[k]
-                new_index = variables[k].to_index()
-                indexes[new_name] = new_index
-            else:
-                indexes[k] = v
-
         return self._replace_with_new_dims(variables, coord_names,
                                            indexes=indexes, inplace=inplace)
 

xarray/tests/test_dataset.py

@@ -2002,14 +2002,11 @@ def test_swap_dims(self):
         assert_identical(expected, actual)
         assert isinstance(actual.variables['y'], IndexVariable)
         assert isinstance(actual.variables['x'], Variable)
+        assert actual.indexes['y'].equals(pd.Index(list('abc')))
 
         roundtripped = actual.swap_dims({'y': 'x'})
         assert_identical(original.set_coords('y'), roundtripped)
 
-        actual = original.copy()
-        actual = actual.swap_dims({'x': 'y'})
-        assert_identical(expected, actual)
-
         with raises_regex(ValueError, 'cannot swap'):
             original.swap_dims({'y': 'x'})
         with raises_regex(ValueError, 'replacement dimension'):