Extend fiscal year parameter conversion to 2040

changelog_entry.yaml

@@ -0,0 +1,4 @@
+- bump: minor
+  changes:
+    fixed:
+      - Extend fiscal year parameter conversion to cover 2015-2040, fixing issues where policies changing on April 6 (UK fiscal year start) were not reflected in simulations for years 2026+.

policyengine_uk/tests/test_fiscal_year_parameters.py

@@ -0,0 +1,131 @@
+"""
+Tests for UK fiscal year parameter handling.
+
+The UK fiscal year runs April 6 to April 5. PolicyEngine UK converts
+parameters to use fiscal year values by sampling at April 30 of each year.
+
+This test suite verifies that annual period queries (e.g., param("2026"))
+return the correct fiscal year values, especially for policy changes
+that take effect on April 6.
+
+Key policy example:
+- Two-child limit repeal: limit=2 until April 5, 2026, then infinity
+  from April 6, 2026
+"""
+
+import pytest
+from policyengine_uk import CountryTaxBenefitSystem
+
+
+@pytest.fixture(scope="module")
+def uk_system():
+    """Create a UK tax-benefit system for testing."""
+    return CountryTaxBenefitSystem()
+
+
+class TestTwoChildLimitRepeal:
+    """Tests for the two-child limit repeal on April 6, 2026."""
+
+    def test_two_child_limit_fiscal_year_2025(self, uk_system):
+        """Test that the two-child limit is 2 for fiscal year 2025/26."""
+        params = uk_system.get_parameters_at_instant("2025")
+        child_limit = (
+            params.gov.dwp.universal_credit.elements.child.limit.child_count
+        )
+        assert child_limit == 2
+
+    def test_two_child_limit_fiscal_year_2026(self, uk_system):
+        """Test that the two-child limit is infinity for fiscal year 2026/27.
+
+        This is the key test case - the repeal happens on April 6, 2026.
+        When querying for "2026", the system should use April 30, 2026
+        as the reference date, which is AFTER the repeal date.
+        """
+        params = uk_system.get_parameters_at_instant("2026")
+        child_limit = (
+            params.gov.dwp.universal_credit.elements.child.limit.child_count
+        )
+        assert child_limit == float("inf"), (
+            f"Expected infinity for 2026 fiscal year, got {child_limit}. "
+            "The fiscal year conversion may not be covering 2026."
+        )
+
+    def test_annual_query_reflects_fiscal_year(self, uk_system):
+        """
+        Test that annual queries return fiscal year values.
+
+        For year 2026, the fiscal year is April 6, 2026 to April 5, 2027.
+        The two-child limit is repealed on April 6, 2026, so:
+        - 2025 should be 2 (fiscal year 2025/26)
+        - 2026 should be infinity (fiscal year 2026/27)
+        - 2027 should be infinity (fiscal year 2027/28)
+        """
+        params_2025 = uk_system.get_parameters_at_instant("2025")
+        params_2026 = uk_system.get_parameters_at_instant("2026")
+        params_2027 = uk_system.get_parameters_at_instant("2027")
+
+        limit_2025 = (
+            params_2025.gov.dwp.universal_credit.elements.child.limit.child_count
+        )
+        limit_2026 = (
+            params_2026.gov.dwp.universal_credit.elements.child.limit.child_count
+        )
+        limit_2027 = (
+            params_2027.gov.dwp.universal_credit.elements.child.limit.child_count
+        )
+
+        assert limit_2025 == 2
+        assert limit_2026 == float("inf")
+        assert limit_2027 == float("inf")
+
+
+class TestFiscalYearBoundary:
+    """Tests for fiscal year boundary behavior.
+
+    Note: After fiscal year conversion, the entire year is set to the
+    April 30 value. This means January-March dates return the fiscal
+    year value for that calendar year, not the previous fiscal year.
+    """
+
+    def test_january_2026_returns_fiscal_year_2026_value(self, uk_system):
+        """January 2026 should return fiscal year 2026/27 value.
+
+        After fiscal year conversion, the year 2026 is set to the
+        April 30, 2026 value (infinity, post-repeal).
+        """
+        params = uk_system.get_parameters_at_instant("2026-01-15")
+        child_limit = (
+            params.gov.dwp.universal_credit.elements.child.limit.child_count
+        )
+        # This returns inf because the whole year 2026 is set to fiscal year value
+        assert child_limit == float("inf")
+
+    def test_december_2025_returns_fiscal_year_2025_value(self, uk_system):
+        """December 2025 should return fiscal year 2025/26 value."""
+        params = uk_system.get_parameters_at_instant("2025-12-15")
+        child_limit = (
+            params.gov.dwp.universal_credit.elements.child.limit.child_count
+        )
+        # The year 2025 is set to April 30, 2025 value (2, pre-repeal)
+        assert child_limit == 2
+
+
+class TestFiscalYearCoverage:
+    """Tests to verify fiscal year conversion covers all needed years."""
+
+    @pytest.mark.parametrize(
+        "year", [2025, 2026, 2027, 2028, 2029, 2030, 2040]
+    )
+    def test_year_in_conversion_range(self, uk_system, year):
+        """
+        Test that years from 2025-2040 can be queried.
+
+        This verifies that the fiscal year conversion range covers
+        all years needed for long-term projections.
+        """
+        params = uk_system.get_parameters_at_instant(str(year))
+        assert params is not None
+        # Verify we can access a parameter
+        pa = params.gov.hmrc.income_tax.allowances.personal_allowance.amount
+        assert pa is not None
+        assert pa > 0

policyengine_uk/utils/parameters.py

@@ -26,7 +26,18 @@ def backdate_parameters(
 
 
 def convert_to_fiscal_year_parameters(parameters):
-    YEARS = list(range(2015, 2026))
+    """
+    Convert parameters to use UK fiscal year values.
+
+    The UK fiscal year runs April 6 to April 5. When querying a parameter
+    for a year (e.g., param("2026")), we want the value at April 30 of
+    that year (which represents the fiscal year starting April 6).
+
+    This function samples each parameter at April 30 of each year and
+    sets that as the value for the entire year period.
+    """
+    # Cover years from 2015 through 2040 for long-term projections
+    YEARS = list(range(2015, 2041))
     for param in parameters.get_descendants():
         if isinstance(param, Parameter):
             for year in YEARS: