Backdate parameters before applying reforms

changelog.d/fix-backdate-before-reforms.fixed.md

@@ -0,0 +1 @@
+Reforms to a parameter first dated after 2015 no longer change years outside the reform's period. Parameters are now backdated to 2015 before the reform is applied (and parameters a reform adds or replaces are backdated afterwards, as before), so a reform starting on or before a parameter's first dated value is no longer copied back to 2015, and a reform ending before that value no longer leaves the years in between undefined, which dropped brackets from scales and made formulas reading the parameter raise ParameterNotFoundError. CountryTaxBenefitSystem and Simulation(reform=...) also now build with any start_instant from 2015 through 2023; they raised ParameterNotFoundError because structural-reform detection read contrib parameters first dated after the start before they were backdated.

policyengine_us/system.py

@@ -125,6 +125,16 @@ def __init__(
         self.parameters = propagate_parameter_metadata(self.parameters)
         add_default_uprating(self)
 
+        # Backdated before any reform is applied. Backdating copies each
+        # parameter's earliest value back to the start of FIRST_MODELED_YEAR,
+        # so on a parameter first dated after that it would copy a reform
+        # value that starts on or before that first date back to that year,
+        # and a reform ending before that date would leave the years between
+        # the two undefined.
+        self.parameters = backdate_parameters(
+            self.parameters, first_instant=f"{FIRST_MODELED_YEAR}-01-01"
+        )
+
         if reform:
             # Applied after the parameter processing pipeline so that values
             # a reform inserts at future dates cannot act as defined values
@@ -134,6 +144,13 @@ def __init__(
             # structural-reform detection, which reads reformed parameter
             # values.
             self.apply_reform_set(reform)
+            # Backdates parameters the reform added or replaced, as before.
+            # Those it only updated already have a value dated by the start of
+            # FIRST_MODELED_YEAR, and ``Parameter.update`` never removes
+            # values dated before the period it sets, so they keep it.
+            self.parameters = backdate_parameters(
+                self.parameters, first_instant=f"{FIRST_MODELED_YEAR}-01-01"
+            )
 
         structural_reform = create_structural_reforms_from_parameters(
             self.parameters, start_instant
@@ -142,10 +159,6 @@ def __init__(
             reform = ()
         reform = (reform, structural_reform)
 
-        self.parameters = backdate_parameters(
-            self.parameters, first_instant=f"{FIRST_MODELED_YEAR}-01-01"
-        )
-
         for parameter in self.parameters.get_descendants():
             parameter.modified = False