Canonicalise alias paths in Scenario.apply

changelog.d/scenario-apply-canonical-paths.fixed.md

@@ -0,0 +1 @@
+`Scenario.apply` on a built simulation now changes the parameter a deprecated `labor_supply_responses` path copies, as constructing the simulation with the scenario does; it used to edit only the alias copy, which formulas do not read, unless the scenario had been summed.

policyengine_uk/tests/test_process_time_reforms.py

@@ -625,3 +625,13 @@ def test_baselines_stay_current_law(change, baseline):
     assert weekly(after, NEW_STATE_PENSION, 2028) == weekly(
         baseline, NEW_STATE_PENSION, 2028
     )
+
+
+@pytest.mark.parametrize("summed", [False, True])
+def test_scenario_apply_changes_the_parameter_an_alias_path_copies(summed):
+    scenario = Scenario(parameter_changes={LSR_LEGACY: 0.3})
+    if summed:
+        scenario = Scenario() + scenario
+    simulation = Simulation(situation=PENSIONER)
+    scenario.apply(simulation)
+    assert value(simulation, LSR, "2028-01-01") == pytest.approx(0.3)

policyengine_uk/utils/scenario.py

@@ -363,7 +363,9 @@ def apply(self, simulation: Simulation) -> None:
         overrides: List[LegacyOverride] = []
         if self.parameter_changes:
             parameters = simulation.tax_benefit_system.parameters
-            for path, value in self.parameter_changes.items():
+            # Keyed by canonical paths, as a sum of scenarios already is, so a
+            # deprecated alias path changes the parameter formulas read.
+            for path, value in _canonical_paths(self.parameter_changes).items():
                 if isinstance(value, dict):
                     # Handle nested parameter changes
                     if not value: