tests: Add tests for very simple derivatives

Author: JDBetteridge

tests/test_derivatives.py

@@ -1250,3 +1250,38 @@ def test_fallback_wrong_custom_size(self):
         exp1 = - .5 * (v._subs(x, x - x.spacing) - v._subs(x, x + x.spacing))/x.spacing
         assert simplify(eq0.rhs - exp0) == 0
         assert simplify(eq1.rhs - exp1) == 0
+
+
+class TestDimension:
+    """
+    Check the few cases where differentiating a dimension is allowed work correctly
+    and errors are raised otherwise.
+    """
+
+    @classmethod
+    def setup_class(cls):
+        cls.grid = Grid(shape=(11, 11), extent=(1, 1))
+        cls.x, cls.y = cls.grid.dimensions
+        u = TimeFunction(name='u', grid=cls.grid, space_order=1)
+        cls.t = u.time_dim
+
+    def test_constant(self):
+        assert Derivative(self.x, self.x) == 1
+
+    def test_null(self):
+        assert Derivative(self.x, (self.x, 2)) == 0
+        assert Derivative(self.x, self.x, self.y) == 0
+        assert Derivative(self.x, self.x, self.y) == 0
+        assert Derivative(self.x, self.x, self.y, self.t) == 0
+        assert Derivative(self.x, self.y, self.t, self.x) == 0
+        assert Derivative(self.x, self.y, self.t, (self.x, 2)) == 0
+
+    def test_error(self):
+        with pytest.raises(ValueError):
+            Derivative(self.x, self.t)
+
+        with pytest.raises(ValueError):
+            Derivative(self.x, self.y, self.t)
+
+        with pytest.raises(ValueError):
+            Derivative(self.x, (self.x, 0))