Fix ruff errors

pybamm/expression_tree/operations/serialise.py

@@ -7,6 +7,8 @@
 import numpy as np
 import re
 
+from typing import Optional
+
 
 class Serialise:
     """
@@ -78,9 +80,9 @@ class _EmptyDict(dict):
     def save_model(
         self,
         model: pybamm.BaseModel,
-        mesh: pybamm.Mesh = None,
-        variables: pybamm.FuzzyDict = None,
-        filename: str = None,
+        mesh: Optional[pybamm.Mesh] = None,
+        variables: Optional[pybamm.FuzzyDict] = None,
+        filename: Optional[str] = None,
     ):
         """Saves a discretised model to a JSON file.
 
@@ -142,7 +144,7 @@ def save_model(
             json.dump(model_json, f)
 
     def load_model(
-        self, filename: str, battery_model: pybamm.BaseModel = None
+        self, filename: str, battery_model: Optional[pybamm.BaseModel] = None
     ) -> pybamm.BaseModel:
         """
         Loads a discretised, ready to solve model into PyBaMM.

pybamm/simulation.py

@@ -10,6 +10,7 @@
 from functools import lru_cache
 from datetime import timedelta
 from pybamm.util import have_optional_dependency
+from typing import Optional
 
 from pybamm.expression_tree.operations.serialise import Serialise
 
@@ -795,7 +796,9 @@ def solve(
                     # Hacky patch to allow correct processing of end_time and next_starting time
                     # For efficiency purposes, op_conds treats identical steps as the same object
                     # regardless of the initial time. Should be refactored as part of #3176
-                    op_conds_unproc = self.experiment.operating_conditions_steps_unprocessed[idx]
+                    op_conds_unproc = (
+                        self.experiment.operating_conditions_steps_unprocessed[idx]
+                    )
 
                     start_time = current_solution.t[-1]
 
@@ -1192,7 +1195,7 @@ def save(self, filename):
 
     def save_model(
         self,
-        filename: str = None,
+        filename: Optional[str] = None,
         mesh: bool = False,
         variables: bool = False,
     ):