BUG: ensure indexes are reset when coords are modified (#2707)

This was introduced by the recent indexes refactor, but never made it into a
release.

xarray/core/coordinates.py

@@ -234,6 +234,7 @@ def _update_coords(self, coords):
             raise ValueError('cannot add coordinates with new dimensions to '
                              'a DataArray')
         self._data._coords = coords
+        self._data._indexes = None
 
     @property
     def variables(self):

xarray/tests/test_cftimeindex.py

@@ -8,7 +8,7 @@
 from xarray.coding.cftimeindex import (
     CFTimeIndex, _parse_array_of_cftime_strings, _parse_iso8601_with_reso,
     _parsed_string_to_bounds, assert_all_valid_date_type, parse_iso8601)
-from xarray.tests import assert_array_equal, assert_identical
+from xarray.tests import assert_array_equal, assert_allclose, assert_identical
 
 from . import (
     has_cftime, has_cftime_1_0_2_1, has_cftime_or_netCDF4, raises_regex,

xarray/tests/test_dataarray.py

@@ -1220,6 +1220,11 @@ def test_coords_alignment(self):
                              dims='x')
         assert_identical(lhs, expected)
 
+    def test_set_coords_update_index(self):
+        actual = DataArray([1, 2, 3], [('x', [1, 2, 3])])
+        actual.coords['x'] = ['a', 'b', 'c']
+        assert actual.indexes['x'].equals(pd.Index(['a', 'b', 'c']))
+
     def test_coords_replacement_alignment(self):
         # regression test for GH725
         arr = DataArray([0, 1, 2], dims=['abc'])

xarray/tests/test_dataset.py

@@ -587,6 +587,11 @@ def test_coords_modify(self):
         expected = data.merge({'c': 11}).set_coords('c')
         assert_identical(expected, actual)
 
+    def test_update_index(self):
+        actual = Dataset(coords={'x': [1, 2, 3]})
+        actual['x'] = ['a', 'b', 'c']
+        assert actual.indexes['x'].equals(pd.Index(['a', 'b', 'c']))
+
     def test_coords_setitem_with_new_dimension(self):
         actual = Dataset()
         actual.coords['foo'] = ('x', [1, 2, 3])