From 51c412082eae8bce864e552ad279c84d426db816 Mon Sep 17 00:00:00 2001
From: Robert Timms <43040151+rtimms@users.noreply.github.com>
Date: Wed, 17 Apr 2024 17:27:01 +0100
Subject: [PATCH] #4018 check indepedent vars in events (#4019)

* #4018 check indepedent vars in events

* #4018 changelog
---
 CHANGELOG.md                                     |  1 +
 pybamm/discretisations/discretisation.py         |  1 +
 .../test_discretisations/test_discretisation.py  | 16 ++++++++++++++++
 3 files changed, 18 insertions(+)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 8322f2a654..ea458b4b60 100644
--- a/CHANGELOG.md
+++ b/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))
diff --git a/pybamm/discretisations/discretisation.py b/pybamm/discretisations/discretisation.py
index 68c2e9f19a..92e9e4e24f 100644
--- a/pybamm/discretisations/discretisation.py
+++ b/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]
diff --git a/tests/unit/test_discretisations/test_discretisation.py b/tests/unit/test_discretisations/test_discretisation.py
index 48795c9318..e9da61e001 100644
--- a/tests/unit/test_discretisations/test_discretisation.py
+++ b/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")