Copy target coordinates onto regridded result cube (#6844)

docs/src/whatsnew/latest.rst

@@ -36,7 +36,10 @@ This document explains the changes made to Iris for this release
 🐛 Bugs Fixed
 =============
 
-#. N/A
+#. :user:`gaoflow` fixed regridding so that the horizontal coordinates of the
+   result are copies of, rather than references to, the target grid's
+   coordinates. Previously, modifying a result coordinate would also modify the
+   target cube's coordinate (and vice versa). (:issue:`6844`)
 
 
 💣 Incompatible Changes

lib/iris/analysis/_regrid.py

@@ -1040,6 +1040,10 @@ def _create_cube(data, src, src_dims, tgt_coords, num_tgt_dims, regrid_callback)
     if num_tgt_dims == 1:
         grid_dim_x = grid_dim_y = min(src_dims)
     for tgt_coord, dim in zip(tgt_coords, (grid_dim_x, grid_dim_y)):
+        # Copy so that the result does not share coordinate objects with the
+        # target grid (mutating one would otherwise mutate the other). See
+        # #6844.
+        tgt_coord = tgt_coord.copy()
         if len(tgt_coord.shape) == 1:
             if isinstance(tgt_coord, DimCoord) and dim is not None:
                 result.add_dim_coord(tgt_coord, dim)

lib/iris/tests/unit/analysis/area_weighted/test_AreaWeightedRegridder.py

@@ -122,6 +122,22 @@ def test_src_and_target_are_the_same(self):
         result = regridder(src)
         _shared_utils.assert_array_all_close(result.data, target.data)
 
+    def test_result_coords_independent_of_target(self):
+        # The horizontal coordinates of the result must be copies, not the
+        # same objects as the target's: mutating one must not mutate the
+        # other (#6844).
+        src, target = self.grids()
+        regridder = AreaWeightedRegridder(src, target)
+        result = regridder(src)
+        for name in ("latitude", "longitude"):
+            result_coord = result.coord(name)
+            target_coord = target.coord(name)
+            assert result_coord == target_coord
+            assert result_coord is not target_coord
+            original = target_coord.points.copy()
+            result_coord.points = result_coord.points + 100
+            _shared_utils.assert_array_equal(target.coord(name).points, original)
+
     def test_multiple_src_on_same_grid(self):
         coord_names = ["latitude", "longitude"]
         src1 = self.cube(np.linspace(20, 32, 4), np.linspace(10, 22, 4))