diff of vector expression wrt itself returns a vector rather than a s…

…calar #4087

pybamm/expression_tree/symbol.py

@@ -714,10 +714,10 @@ def diff(self, variable: Symbol):
         """
         if variable == self:
             eval_shape = self.evaluate_for_shape()
-            if isinstance(eval_shape, numbers.Number):
+            if isinstance(eval_shape, numbers.Number) or len(eval_shape) == 1:
                 return pybamm.Scalar(1)
             else:
-                return pybamm.Vector(np.ones_like(eval_shape), domain=self.domains)
+                return pybamm.Vector(np.ones_like(eval_shape))
         elif any(variable == x for x in self.pre_order()):
             return self._diff(variable)
         elif variable == pybamm.t and self.has_symbol_of_classes(
@@ -726,10 +726,10 @@ def diff(self, variable: Symbol):
             return self._diff(variable)
         else:
             eval_shape = self.evaluate_for_shape()
-            if isinstance(eval_shape, numbers.Number):
+            if isinstance(eval_shape, numbers.Number) or len(eval_shape) == 1:
                 return pybamm.Scalar(0)
             else:
-                return pybamm.Vector(np.zeros_like(eval_shape), domain=self.domains)
+                return pybamm.Vector(np.zeros_like(eval_shape))
 
     def _diff(self, variable):
         """

pybamm/solvers/base_solver.py

@@ -1254,7 +1254,7 @@ def _check_events_with_initial_conditions(t_eval, model, y0, inputs):
                 x for x in model.events if x.event_type == pybamm.EventType.TERMINATION
             ]
             idxs = np.where(events_eval < 0)[0]
-            event_names = [termination_events[idx / len(inputs)].name for idx in idxs]
+            event_names = [termination_events[idx // len(inputs)].name for idx in idxs]
             raise pybamm.SolverError(
                 f"Events {event_names} are non-positive at initial conditions"
             )

tests/unit/test_expression_tree/test_symbol.py

@@ -237,14 +237,6 @@ def test_multiple_symbols(self):
         for node, expect in zip(exp.pre_order(), expected_preorder):
             self.assertEqual(node.name, expect)
 
-    def test_symbol_diff(self):
-        a = pybamm.Symbol("a")
-        b = pybamm.Symbol("b")
-        self.assertIsInstance(a.diff(a), pybamm.Scalar)
-        self.assertEqual(a.diff(a).evaluate(), 1)
-        self.assertIsInstance(a.diff(b), pybamm.Scalar)
-        self.assertEqual(a.diff(b).evaluate(), 0)
-
     def test_symbol_evaluation(self):
         a = pybamm.Symbol("a")
         with self.assertRaises(NotImplementedError):