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 +34,13 @@ def modifier(simulation):
                 start=start_period,
                 stop=stop_period,
             )
+
+        # Clear caches so calculations use updated parameter values
+        simulation.tax_benefit_system.reset_parameter_caches()
+        for population in simulation.populations.values():
+            for holder in population._holders.values():
+                holder.delete_arrays()
+
         return simulation
 
     return modifier

tests/test_parametric_reforms.py

@@ -0,0 +1,61 @@
+"""Test script for parameter cache clearing fix."""
+
+from datetime import datetime
+from policyengine.tax_benefit_models.uk import uk_latest
+from policyengine.core.policy import ParameterValue as PEParameterValue
+from policyengine.utils.parametric_reforms import simulation_modifier_from_parameter_values
+from policyengine_uk import Microsimulation
+
+
+def test_modifier_clears_cache():
+    """Test that the modifier clears cache so parameter updates take effect."""
+
+    # Create BASELINE simulation (no reform)
+    sim_baseline = Microsimulation(situation={'people': {'person': {'age': {'2026': 30}}}})
+    uc_baseline = sim_baseline.calculate('universal_credit', '2026')[0]
+    param_baseline = sim_baseline.tax_benefit_system.parameters('2026-01').gov.dwp.universal_credit.standard_allowance.amount.SINGLE_OLD
+    print(f'Baseline UC: £{uc_baseline:.2f}')
+    print(f'Baseline SINGLE_OLD: £{param_baseline:.2f}/month')
+
+    # Create REFORM simulation - apply modifier BEFORE any calculations
+    sim_reform = Microsimulation(situation={'people': {'person': {'age': {'2026': 30}}}})
+
+    # Apply modifier immediately (before any calculate calls)
+    param_lookup = {p.name: p for p in uk_latest.parameters}
+    pe_param = param_lookup.get('gov.dwp.universal_credit.standard_allowance.amount.SINGLE_OLD')
+    pv = PEParameterValue(
+        parameter=pe_param,
+        value=533.0,
+        start_date=datetime(2026, 1, 1),
+        end_date=None,
+    )
+    modifier = simulation_modifier_from_parameter_values([pv])
+    modifier(sim_reform)
+
+    # Check param was updated
+    param_reform = sim_reform.tax_benefit_system.parameters('2026-01').gov.dwp.universal_credit.standard_allowance.amount.SINGLE_OLD
+    print(f'Reform SINGLE_OLD: £{param_reform:.2f}/month')
+    assert param_reform == 533.0, f"Parameter not updated: {param_reform}"
+
+    # Now calculate UC on reform sim
+    uc_reform = sim_reform.calculate('universal_credit', '2026')[0]
+    print(f'Reform UC: £{uc_reform:.2f}')
+
+    # Expected values
+    expected_reform_uc = 533.0 * 12  # £6396
+    print(f'\nExpected reform UC: £{expected_reform_uc:.2f}')
+    print(f'Actual reform UC: £{uc_reform:.2f}')
+
+    # Check the increase is correct
+    expected_increase = (533.0 - param_baseline) * 12
+    actual_increase = uc_reform - uc_baseline
+    print(f'\nExpected increase: £{expected_increase:.2f}')
+    print(f'Actual increase: £{actual_increase:.2f}')
+
+    # Assert UC changed correctly
+    assert abs(uc_reform - expected_reform_uc) < 1, f"Reform UC should be ~{expected_reform_uc}, got {uc_reform}"
+    print('\nTest PASSED!')
+
+
+if __name__ == '__main__':
+    test_modifier_clears_cache()