#4018 check indepedent vars in events (#4019)

* #4018 check indepedent vars in events

* #4018 changelog

CHANGELOG.md

@@ -14,6 +14,7 @@
 
 ## Bug Fixes
 
+- Fixed a bug where independent variables were removed from models even if they appeared in events ([#4019](https://github.com/pybamm-team/PyBaMM/pull/4019))
 - Fix bug with upwind and downwind schemes producing the wrong discretised system ([#3979](https://github.com/pybamm-team/PyBaMM/pull/3979))
 - Allow evaluation of an `Interpolant` object with a number ([#3932](https://github.com/pybamm-team/PyBaMM/pull/3932))
 - `plot_voltage_components` now works even if the time does not start at 0 ([#3915](https://github.com/pybamm-team/PyBaMM/pull/3915))

pybamm/discretisations/discretisation.py

@@ -1103,6 +1103,7 @@ def remove_independent_variables_from_rhs(self, model):
             # only check children of variables, this will skip the variable itself
             # and catch any other cases
             + [child for var in model.variables.values() for child in var.children]
+            + [event.expression for event in model.events]
         )
         all_vars_in_eqns = unpacker.unpack_list_of_symbols(eqns_to_check)
         all_vars_in_eqns = [var.name for var in all_vars_in_eqns]

tests/unit/test_discretisations/test_discretisation.py

@@ -1241,6 +1241,22 @@ def test_independent_rhs(self):
         disc.process_model(model)
         self.assertEqual(len(model.rhs), 2)
 
+    def test_independent_rhs_with_event(self):
+        a = pybamm.Variable("a")
+        b = pybamm.Variable("b")
+        c = pybamm.Variable("c")
+        model = pybamm.BaseModel()
+        model.rhs = {a: b, b: c, c: -c}
+        model.initial_conditions = {
+            a: pybamm.Scalar(0),
+            b: pybamm.Scalar(1),
+            c: pybamm.Scalar(1),
+        }
+        model.events = [pybamm.Event("a=1", a - 1)]
+        disc = pybamm.Discretisation()
+        disc.process_model(model)
+        self.assertEqual(len(model.rhs), 3)
+
 
 if __name__ == "__main__":
     print("Add -v for more debug output")