Add laziness tests for calculus 'delta_cube' and 'differentiate'.

Author: pp-mo

lib/iris/tests/test_analysis_calculus.py

@@ -44,6 +44,24 @@ def test_delta_coord_lookup(self):
         assert delta_coord == delta.coord(coord)
         assert coord == cube.coord(delta_coord)
 
+    @pytest.mark.parametrize("is_lazy", [True, False], ids=["is_lazy", "not_lazy"])
+    def test_delta_lazypreserving(self, is_lazy):
+        cube = iris.cube.Cube(np.arange(10), standard_name="air_temperature")
+        # Add a coordinate with a lot of metadata.
+        coord = iris.coords.DimCoord(
+            np.arange(10),
+            long_name="projection_x_coordinate",
+            var_name="foo",
+            attributes={"source": "testing"},
+            units="m",
+            coord_system=iris.coord_systems.OSGB(),
+        )
+        cube.add_dim_coord(coord, 0)
+        if is_lazy:
+            cube.data = cube.lazy_data()
+        delta = iris.analysis.calculus.cube_delta(cube, "projection_x_coordinate")
+        assert delta.has_lazy_data() == is_lazy
+
 
 class TestDeltaAndMidpoint:
     def _simple_filename(self, suffix):
@@ -303,6 +321,13 @@ def test_diff_wrt_lat(self):
             approx_data=True,
         )
 
+    @pytest.mark.parametrize("is_lazy", [True, False], ids=["is_lazy", "not_lazy"])
+    def test_diff_lazypreserving(self, is_lazy):
+        if is_lazy:
+            self.cube.data = self.cube.lazy_data()
+        t = iris.analysis.calculus.differentiate(self.cube, "longitude")
+        assert t.has_lazy_data() == is_lazy
+
 
 class TestCalculusSimple2:
     @pytest.fixture(autouse=True)