diff --git a/src/sim_explorer/assertion.py b/src/sim_explorer/assertion.py
index d957420..d953ead 100644
--- a/src/sim_explorer/assertion.py
+++ b/src/sim_explorer/assertion.py
@@ -58,12 +58,12 @@ def info(self, sym: str, typ: str = "instance") -> str | int:
                     assert len(self._cases_variables[var]["instances"]) == 1, f"Non-unique instance for variable {var}"
                     instance = self._cases_variables[var]["instances"][0]  # use the unique instance
                 else:
-                    instance = parts[0] + "".join("_" + x for x in parts[1:])
+                    instance = parts[0] + "".join(f"_{x}" for x in parts[1:])
                     assert instance in self._cases_variables[var]["instances"], f"No instance {instance} of {var}"
                 break
             if not len(parts):
                 raise KeyError(f"The symbol {sym} does not seem to represent a registered variable") from None
-            var = parts.pop() + "_" + var
+            var = f"{parts.pop()}_{var}"
         if typ == "instance":  # get the instance
             return instance
         if typ == "variable":  # get the generic variable name
@@ -107,11 +107,11 @@ def expr(self, key: str, ex: str | None = None) -> str | CodeType:
 
         def make_func(name: str, args: dict, body: str) -> str:
             """Make a python function from the body."""
-            code = "def _" + name + "("
+            code = f"def _{name}("
             for a in args:
-                code += a + ", "
+                code += f"{a}, "
             code += "):\n"
-            code += "    return " + body + "\n"
+            code += f"    return {body}" + "\n"
             return code
 
         if ex is None:  # getter
@@ -146,7 +146,7 @@ def syms(self, key: str) -> list[str]:
         else:
             return syms
 
-    def expr_get_symbols_functions(self, expr: str) -> tuple[list[str], list[str]]:
+    def expr_get_symbols_functions(self, expr: str) -> tuple[list[str], list[str]]:  # noqa: C901
         """Get the symbols used in the expression.
 
         1. Symbol as listed in expression and function body. In general <instant>_<variable>[<index>]
@@ -255,7 +255,7 @@ def register_vars(self, variables: dict[str, dict[str, Any]]) -> None:
             for inst in info["instances"]:
                 if len(info["instances"]) == 1:  # the instance is unique
                     _ = self.symbol(key, len(info["names"]))  # we allow to use the 'short name' if unique
-                _ = self.symbol(inst + "_" + key, len(info["names"]))  # fully qualified name can always be used
+                _ = self.symbol(f"{inst}_{key}", len(info["names"]))
 
     def make_locals(self, loc: dict[str, Any]) -> dict[str, Any]:
         """Adapt the locals with 'allowed' functions."""
@@ -307,9 +307,9 @@ def eval_single(self, key: str, kvargs: dict[str, Any] | list[Any] | tuple[Any,
         loc = self.make_locals(locals())
         exec(self._compiled[key], loc, loc)  # noqa: S102
         # print("kvargs", kvargs, self._syms[key], self.expr_get_symbols_functions(key))  # noqa: ERA001
-        return self._eval(locals()["_" + key], kvargs)
+        return self._eval(locals()[f"_{key}"], kvargs)
 
-    def eval_series(
+    def eval_series(  # noqa: C901, PLR0912
         self,
         key: str,
         data: list[list[int | float | bool]],
@@ -346,7 +346,7 @@ def eval_series(
         argnames = self._syms[key]
         loc = self.make_locals(locals())
         exec(self._compiled[key], loc, loc)  # the function is then available as _<key> among locals()  # noqa: S102
-        func = locals()["_" + key]  # scalar function of all used arguments
+        func = locals()[f"_{key}"]
         _temp = self._temporal[key]["type"] if ret is None else Temporal.UNDEFINED
 
         for _row in data:
diff --git a/src/sim_explorer/case.py b/src/sim_explorer/case.py
index c962a95..a89929e 100644
--- a/src/sim_explorer/case.py
+++ b/src/sim_explorer/case.py
@@ -854,8 +854,14 @@ class Results:
             If "" default file name is used, if None, results are not stored.
     """
 
-    def __init__(self, case: Case | str | Path | None = None, file: str | Path | None = None):
+    def __init__(
+        self,
+        case: Case | str | Path | None = None,
+        file: str | Path | None = None,
+    ) -> None:
         self.file: Path | None  # None denotes that results are not automatically saved
+        self.case: Case | None = None
+        self.res: Json5
         if (case is None or isinstance(case, (str, Path))) and file is not None:
             self._init_from_existing(file)  # instantiating from existing results file (work with data)
         elif isinstance(case, Case):  # instantiating from cases file (for data collection)
@@ -863,7 +869,7 @@ def __init__(self, case: Case | str | Path | None = None, file: str | Path | Non
         else:
             raise ValueError(f"Inconsistent init arguments case:{case}, file:{file}")
 
-    def _init_from_existing(self, file: str | Path):
+    def _init_from_existing(self, file: str | Path) -> None:
         self.file = Path(file)
         assert self.file.exists(), f"File {file} is expected to exist."
         self.res = Json5(self.file)
@@ -872,13 +878,17 @@ def _init_from_existing(self, file: str | Path):
             cases = Cases(Path(case))
         except ValueError:
             raise CaseInitError(f"Cases {Path(case)} instantiation error") from ValueError
-        self.case: Case | None = cases.case_by_name(name=self.res.jspath(path="$.header.case", typ=str, errorMsg=True))
+        self.case = cases.case_by_name(name=self.res.jspath(path="$.header.case", typ=str, errorMsg=True))
         assert isinstance(self.case, Case), f"Case {self.res.jspath('$.header.case', str, True)} not found"
         assert isinstance(self.case.cases, Cases), "Cases object not defined"
-        self._header_transform(False)
+        self._header_transform(tostring=False)
         self.case.add_results_object(self)  # make Results object known to self.case
 
-    def _init_new(self, case: Case, file: str | Path | None = ""):
+    def _init_new(
+        self,
+        case: Case,
+        file: str | Path | None = "",
+    ) -> None:
         assert isinstance(case, Case), f"Case object expected as 'case' in Results. Found {type(case)}"
         self.case = case
         if file is not None:  # use that for storing results data as Json5
diff --git a/src/sim_explorer/system_interface.py b/src/sim_explorer/system_interface.py
index 50f51c2..317fcb4 100644
--- a/src/sim_explorer/system_interface.py
+++ b/src/sim_explorer/system_interface.py
@@ -55,7 +55,7 @@ def __init__(
         description: str = "",
         log_level: str = "fatal",
         **kwargs: Any,  # noqa: ANN401
-    ):
+    ) -> None:
         self.structure_file = Path(structure_file)
         self.name = name  # overwrite if the system includes that
         self.description = description  # overwrite if the system includes that
@@ -76,6 +76,7 @@ def path(self) -> Path:
     @staticmethod
     def read_system_structure(
         file: Path,
+        *,
         fmus_exist: bool = True,
     ) -> dict[str, Any]:
         """Read the systemStructure file and perform checks.
@@ -338,7 +339,7 @@ def default_initial(
             return "calculated" if only_default else ("calculated", "exact", "approx")
         return init if only_default else (init,)
 
-    def allowed_action(
+    def allowed_action(  # noqa: C901
         self,
         action: str,
         comp: int | str,
@@ -513,7 +514,14 @@ def _add_set(
         # new set action
         actions[time].append((cvar, comp, refs, values))
 
-    def _add_get(self, actions: dict, time: float, cvar: str, comp: str, cvar_info: dict):
+    def _add_get(
+        self,
+        actions: dict[float, list[tuple[str, str, Any]]],
+        time: float,
+        cvar: str,
+        comp: str,
+        cvar_info: dict[str, Any],
+    ) -> None:
         """Perform final processing and add the get action to the list (if appropriate).
 
         Properties of get actions:
@@ -534,17 +542,17 @@ def _add_get(self, actions: dict, time: float, cvar: str, comp: str, cvar_info:
                 return  # the get action is already registered
         actions[time].append((cvar, comp, cvar_info["refs"]))
 
-    def add_actions(
+    def add_actions(  # noqa: PLR0913
         self,
-        actions: dict,
+        actions: dict[float, list[tuple[str, str, Any]]],
         act_type: str,
         cvar: str,
-        cvar_info: dict,
-        values: tuple | None,
+        cvar_info: dict[str, Any],
+        values: tuple[TValue] | None,
         at_time: float,
         stoptime: float,
         rng: tuple[int, ...] | None = None,
-    ):
+    ) -> None:
         """Add specified actions to the provided action dict.
         The action list is simulator-agnostic and need 'compilation' before they are used in a simulation.
 
@@ -566,15 +574,27 @@ def add_actions(
             where value-list and rng are only present for set actions
             at-time=-1 for get actions denote step actions
         """
-        assert isinstance(at_time, (float, int)), f"Actions require a defined time as float. Found {at_time}"
+        assert isinstance(at_time, float | int), f"Actions require a defined time as float. Found {at_time}"
         if at_time not in actions:
-            actions.update({at_time: []})  # make sure that there is a suitable slot
+            actions[at_time] = []  # make sure that there is a suitable slot
         for comp in cvar_info["instances"]:
             if act_type == "get" or (act_type == "step" and at_time == -1):  # normal get or step without time spec
-                self._add_get(actions, at_time, cvar, comp, cvar_info)
+                self._add_get(
+                    actions=actions,
+                    time=at_time,
+                    cvar=cvar,
+                    comp=comp,
+                    cvar_info=cvar_info,
+                )
             elif act_type == "step" and at_time >= 0:  # step actions with specified interval
                 for time in np.arange(start=at_time, stop=stoptime, step=at_time):
-                    self._add_get(actions, time, cvar, comp, cvar_info)
+                    self._add_get(
+                        actions=actions,
+                        time=time,
+                        cvar=cvar,
+                        comp=comp,
+                        cvar_info=cvar_info,
+                    )
 
             elif act_type == "set":
                 assert values is not None, f"Variable {cvar}: Value needed for 'set' actions."
@@ -584,7 +604,7 @@ def add_actions(
                     cvar=cvar,
                     comp=comp,
                     cvar_info=cvar_info,
-                    values=tuple([cvar_info["type"](x) for x in values]),
+                    values=tuple(cvar_info["type"](x) for x in values),
                     rng=rng,
                 )
             else:
@@ -600,14 +620,12 @@ def action_step(self, act_info: tuple[Any, ...], typ: type) -> Callable[..., Any
         """
         raise NotImplementedError("The method 'action_step()' cannot be used in SystemInterface") from None
 
-    def init_simulator(self):
+    def init_simulator(self) -> bool:
         """Instantiate and initialize the simulator, so that simulations can be run.
         Perforemd separately from __init__ so that it can be repeated before simulation runs.
         """
         raise NotImplementedError("The method 'init_simulator()' cannot be used in SystemInterface") from None
-        return False
 
-    def run_until(self, time: int | float):
+    def run_until(self, time: int | float) -> bool:
         """Instruct the simulator to simulate until the given time."""
         raise NotImplementedError("The method 'run_until()' cannot be used in SystemInterface") from None
-        return False
diff --git a/src/sim_explorer/system_interface_osp.py b/src/sim_explorer/system_interface_osp.py
index 951f98a..44ac560 100644
--- a/src/sim_explorer/system_interface_osp.py
+++ b/src/sim_explorer/system_interface_osp.py
@@ -4,9 +4,9 @@
 from typing import Any
 
 from libcosimpy.CosimExecution import CosimExecution
-from libcosimpy.CosimLogging import CosimLogLevel, log_output_level  # type: ignore
-from libcosimpy.CosimManipulator import CosimManipulator  # type: ignore
-from libcosimpy.CosimObserver import CosimObserver  # type: ignore
+from libcosimpy.CosimLogging import CosimLogLevel, log_output_level
+from libcosimpy.CosimManipulator import CosimManipulator
+from libcosimpy.CosimObserver import CosimObserver
 
 from sim_explorer.system_interface import SystemInterface
 
@@ -31,19 +31,19 @@ def __init__(
         name: str | None = None,
         description: str = "",
         log_level: str = "fatal",
-        **kwargs,
-    ):
+        **kwargs: Any,  # noqa: ANN401, ARG002
+    ) -> None:
         super().__init__(structure_file, name, description, log_level)
         self.full_simulator_available = True  # system and components specification + simulation capabilities
         # Note: The initialization of the OSP simulator itself is performed in init_simulator()
         #      Since this needs to be repeated before every simulation
 
-    def init_simulator(self):
+    def init_simulator(self) -> bool:
         """Instantiate and initialize the simulator, so that simulations can be run.
         Perforemd separately from __init__ so that it can be repeated before simulation runs.
         """
         log_output_level(CosimLogLevel[self.log_level.upper()])
-        # ck, msg = self._check_system_structure(self.sysconfig)
+        # ck, msg = self._check_system_structure(self.sysconfig)  # noqa: ERA001
         # assert ck, msg
         assert self.structure_file.exists(), "Simulator initialization requires the structure file."
         self.simulator = CosimExecution.from_osp_config_file(str(self.structure_file))
@@ -60,7 +60,7 @@ def init_simulator(self):
         assert self.simulator.status().current_time == 0
         return not self.simulator.status().error_code
 
-    def _action_func(self, act_type: int, var_type: type):
+    def _action_func(self, act_type: int, var_type: type) -> Callable[..., Any]:
         """Determine the correct action function and return it."""
         if act_type == 0:  # initial settings
             return {
@@ -86,15 +86,12 @@ def _action_func(self, act_type: int, var_type: type):
 
     def do_action(self, time: int | float, act_info: tuple[Any, ...], typ: type) -> bool:
         """Do the action described by the tuple using OSP functions."""
-        if len(act_info) == 4:  # set action
+        if len(act_info) == 4:  # set action  # noqa: PLR2004
             cvar, comp, refs, values = act_info
             _comp = self.component_id_from_name(comp)
             if time <= 0:  # initial setting
                 func = self._action_func(0, typ)
-                for r, v in zip(refs, values, strict=False):
-                    if not func(_comp, r, v):
-                        return False
-                return True
+                return all(func(_comp, r, v) for r, v in zip(refs, values, strict=False))
 
             return self._action_func(1, typ)(_comp, refs, values)
         # get action
@@ -112,6 +109,6 @@ def action_step(self, act_info: tuple[Any, ...], typ: type) -> Callable[..., Any
         _comp = self.component_id_from_name(comp)
         return partial(self._action_func(act_type=2, var_type=typ), _comp, refs)
 
-    def run_until(self, time: int | float):
+    def run_until(self, time: int | float) -> bool:
         """Instruct the simulator to simulate until the given time."""
         return self.simulator.simulate_until(time)