#858 fix test errors

Author: martinjrobins

pybamm/expression_tree/state_vector.py

@@ -242,13 +242,15 @@ def _base_evaluate(self, t=None, y=None, y_dot=None, u=None):
         return out
 
     def diff(self, variable):
+        if variable.id == self.id:
+            return pybamm.Scalar(1)
         if variable.id == pybamm.t.id:
             return StateVectorDot(*self._y_slices, name=self.name + "'",
                                   domain=self.domain,
                                   auxiliary_domains=self.auxiliary_domains,
                                   evaluation_array=self.evaluation_array)
         else:
-            raise NotImplementedError
+            return pybamm.Scalar(0)
 
     def _jac(self, variable):
         if isinstance(variable, pybamm.StateVector):
@@ -307,12 +309,14 @@ def _base_evaluate(self, t=None, y=None, y_dot=None, u=None):
         return out
 
     def diff(self, variable):
-        if variable.id == pybamm.t.id:
+        if variable.id == self.id:
+            return pybamm.Scalar(1)
+        elif variable.id == pybamm.t.id:
             raise pybamm.ModelError(
                 "cannot take second time derivative of a state vector"
             )
         else:
-            raise NotImplementedError
+            return pybamm.Scalar(0)
 
     def _jac(self, variable):
         if isinstance(variable, pybamm.StateVectorDot):

tests/unit/test_expression_tree/test_state_vector.py

@@ -35,14 +35,6 @@ def test_evaluate_list(self):
         y = np.linspace(0, 3, 31)
         np.testing.assert_array_almost_equal(sv.evaluate(y=y), y[:, np.newaxis])
 
-    def test_diff(self):
-        a = pybamm.StateVector(slice(0, 10))
-        with self.assertRaises(NotImplementedError):
-            a.diff(a)
-        b = pybamm.StateVectorDot(slice(0, 10))
-        with self.assertRaises(NotImplementedError):
-            a.diff(b)
-
     def test_name(self):
         sv = pybamm.StateVector(slice(0, 10))
         self.assertEqual(sv.name, "y[0:10]")