Add test suite ensuring key variable reforms produce impacts (#1613)

* Add test suite ensuring key variable reforms produce impacts

Tests 6 common parametric reforms (income tax rate, personal allowance,
NI rate, child benefit, UC standard allowance, fuel duty) to verify
each produces a different result than baseline. Catches silent failures
where reforms run without error but have no effect. Fixes #930.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* Rename test file to avoid collision and fix formatting

Rename test_reform_impacts.py to test_parametric_reform_impacts.py
to avoid name collision with existing microsimulation/test_reform_impacts.py.
Apply ruff formatting.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: vahid-ahmadi

changelog.d/930.md

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+- Add test suite verifying that key parametric reforms produce non-zero impacts

policyengine_uk/tests/test_parametric_reform_impacts.py

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+"""
+Tests that key parametric reforms produce non-zero impacts.
+
+These tests catch silent failures where a reform runs without error
+but produces the same result as the baseline (e.g., due to a
+parameter path being wrong or a formula not picking up the change).
+"""
+
+import pytest
+from policyengine_uk.model_api import Scenario
+from policyengine_uk import Microsimulation
+
+YEAR = 2025
+
+BASE_SITUATION = {
+    "people": {
+        "adult": {
+            "age": {YEAR: 35},
+            "employment_income": {YEAR: 40_000},
+            "hours_worked": {YEAR: 40},
+        },
+        "child": {
+            "age": {YEAR: 5},
+        },
+    },
+    "benunits": {
+        "benunit": {
+            "members": ["adult", "child"],
+        },
+    },
+    "households": {
+        "household": {
+            "members": ["adult", "child"],
+            "region": {YEAR: "LONDON"},
+        },
+    },
+}
+
+
+def _reform_changes_variable(parameter_changes, variable, situation=None):
+    """Check that applying a parameter reform changes a variable's value."""
+    sit = situation or BASE_SITUATION
+    baseline = Microsimulation(situation=sit)
+    scenario = Scenario(parameter_changes=parameter_changes)
+    reformed = Microsimulation(situation=sit, scenario=scenario)
+
+    baseline_val = baseline.calculate(variable, YEAR).values[0]
+    reformed_val = reformed.calculate(variable, YEAR).values[0]
+    return baseline_val != reformed_val
+
+
+class TestReformImpacts:
+    """Verify that common parametric reforms produce non-zero impacts."""
+
+    def test_income_tax_rate_reform(self):
+        """Changing the basic rate of income tax should affect income tax."""
+        assert _reform_changes_variable(
+            {"gov.hmrc.income_tax.rates.uk[0].rate": {str(YEAR): 0.30}},
+            "income_tax",
+        )
+
+    def test_personal_allowance_reform(self):
+        """Changing the personal allowance should affect income tax."""
+        assert _reform_changes_variable(
+            {
+                "gov.hmrc.income_tax.allowances.personal_allowance.amount": {
+                    str(YEAR): 15_000
+                }
+            },
+            "income_tax",
+        )
+
+    def test_ni_rate_reform(self):
+        """Changing the NI employee rate should affect national insurance."""
+        assert _reform_changes_variable(
+            {
+                "gov.hmrc.national_insurance.class_1.rates.employee.main": {
+                    str(YEAR): 0.20
+                }
+            },
+            "national_insurance",
+        )
+
+    def test_child_benefit_reform(self):
+        """Changing the eldest child benefit amount should affect child benefit."""
+        assert _reform_changes_variable(
+            {"gov.hmrc.child_benefit.amount.eldest": {str(YEAR): 30}},
+            "child_benefit",
+        )
+
+    def test_uc_standard_allowance_reform(self):
+        """Changing UC standard allowance should affect universal credit."""
+        low_income_situation = {
+            "people": {
+                "adult": {
+                    "age": {YEAR: 30},
+                    "employment_income": {YEAR: 5_000},
+                    "hours_worked": {YEAR: 10},
+                },
+            },
+            "benunits": {
+                "benunit": {
+                    "members": ["adult"],
+                    "would_claim_uc": {YEAR: True},
+                },
+            },
+            "households": {
+                "household": {
+                    "members": ["adult"],
+                    "region": {YEAR: "LONDON"},
+                },
+            },
+        }
+        assert _reform_changes_variable(
+            {
+                "gov.dwp.universal_credit.standard_allowance.amount.SINGLE_OLD": {
+                    str(YEAR): 500
+                }
+            },
+            "universal_credit",
+            situation=low_income_situation,
+        )
+
+    def test_fuel_duty_reform(self):
+        """Changing fuel duty should affect fuel duty paid."""
+        driver_situation = {
+            "people": {
+                "adult": {
+                    "age": {YEAR: 35},
+                    "employment_income": {YEAR: 30_000},
+                },
+            },
+            "benunits": {
+                "benunit": {
+                    "members": ["adult"],
+                },
+            },
+            "households": {
+                "household": {
+                    "members": ["adult"],
+                    "region": {YEAR: "NORTH_EAST"},
+                    "petrol_spending": {YEAR: 2_000},
+                },
+            },
+        }
+        assert _reform_changes_variable(
+            {"gov.hmrc.fuel_duty.petrol_and_diesel": {str(YEAR): 1.0}},
+            "fuel_duty",
+            situation=driver_situation,
+        )