fix: clear caches after parameter updates in simulation modifier

The simulation modifier now clears both parameter caches and variable
holder caches after updating parameter values. This ensures that
subsequent calculations use the updated parameter values instead of
stale cached values.

Previously, when a simulation was created and then modified with
parameter updates, the cached calculations would still use the old
parameter values, causing reforms to have no effect.

Author: nikhilwoodruff

src/policyengine/utils/parametric_reforms.py

@@ -34,6 +34,18 @@ def modifier(simulation):
                 start=start_period,
                 stop=stop_period,
             )
+
+        # Clear caches so calculations use updated parameter values
+        simulation.tax_benefit_system.reset_parameter_caches()
+
+        # Clear computed variable caches (but preserve input variables)
+        # Variables with formulas are computed; those without are inputs
+        for population in simulation.populations.values():
+            for var_name, holder in population._holders.items():
+                variable = simulation.tax_benefit_system.get_variable(var_name)
+                if variable.formulas:  # Only clear computed variables
+                    holder.delete_arrays()
+
         return simulation
 
     return modifier

tests/test_parametric_reforms.py

@@ -0,0 +1,68 @@
+"""Test that parameter reforms are applied correctly."""
+
+from datetime import datetime
+
+from policyengine.core.policy import ParameterValue as PEParameterValue
+from policyengine.core.policy import Policy as PEPolicy
+from policyengine.tax_benefit_models.uk import uk_latest
+from policyengine.tax_benefit_models.uk.analysis import (
+    UKHouseholdInput,
+    calculate_household_impact,
+)
+
+
+def test_parameter_reform_affects_calculation():
+    """Test that modifying a parameter actually changes the calculation result."""
+    param_lookup = {p.name: p for p in uk_latest.parameters}
+    pe_param = param_lookup["gov.dwp.universal_credit.standard_allowance.amount.SINGLE_OLD"]
+
+    pe_input = UKHouseholdInput(
+        people=[{"age": 30, "employment_income": 0}],
+        benunit={},
+        household={},
+        year=2026,
+    )
+
+    # Baseline
+    baseline = calculate_household_impact(pe_input, policy=None)
+    baseline_uc = baseline.benunit[0]["universal_credit"]
+
+    # Reform - increase standard allowance
+    pv = PEParameterValue(
+        parameter=pe_param,
+        value=533.0,
+        start_date=datetime(2026, 1, 1),
+        end_date=None,
+    )
+    policy = PEPolicy(name="Test", description="Test", parameter_values=[pv])
+    reform = calculate_household_impact(pe_input, policy=policy)
+    reform_uc = reform.benunit[0]["universal_credit"]
+
+    # UC should increase
+    assert reform_uc > baseline_uc
+    assert abs(reform_uc - 533 * 12) < 1
+
+
+def test_parameter_reform_preserves_inputs():
+    """Test that input variables are preserved when applying a reform."""
+    param_lookup = {p.name: p for p in uk_latest.parameters}
+    pe_param = param_lookup["gov.dwp.universal_credit.standard_allowance.amount.SINGLE_OLD"]
+
+    pe_input = UKHouseholdInput(
+        people=[{"age": 30, "employment_income": 5000}],
+        benunit={},
+        household={},
+        year=2026,
+    )
+
+    pv = PEParameterValue(
+        parameter=pe_param,
+        value=533.0,
+        start_date=datetime(2026, 1, 1),
+        end_date=None,
+    )
+    policy = PEPolicy(name="Test", description="Test", parameter_values=[pv])
+    reform = calculate_household_impact(pe_input, policy=policy)
+
+    assert reform.person[0]["employment_income"] == 5000
+    assert reform.person[0]["age"] == 30