Fix Scenario backwards compatibility (#1317)

* Fix Scenario backwards compatibility

Enhances the Scenario class to support all Reform object types including instances and callable objects. This maintains backwards compatibility with existing code that uses various Reform representations.

* Fix bug

* Remove duplicate VAT entry

Author: nikhilwoodruff

changelog_entry.yaml

@@ -0,0 +1,4 @@
+- bump: patch
+  changes:
+    fixed:
+    - Scenario class now supports all Reform object types and maintains backwards compatibility.

policyengine_uk/utils/scenario.py

@@ -2,6 +2,7 @@
 from typing import Optional, Callable, Dict, Type, Union
 from policyengine_core.simulations import Simulation
 from policyengine_core.reforms import Reform
+from policyengine_core.periods import period, instant
 
 
 class Scenario(BaseModel):
@@ -111,8 +112,42 @@ def modifier(simulation: Simulation) -> None:
 
         elif isinstance(reform, dict):
             # Dictionary of parameter changes
-            return cls(
-                parameter_changes=reform,
+            # Make sure to capture YYYY-MM-DD.YYYY-MM-DD.
+
+            def modifier(sim: Simulation):
+                for parameter in reform:
+                    if isinstance(reform[parameter], dict):
+                        for period_str, value in reform[parameter].items():
+                            if "." in period_str:
+                                start = instant(period_str.split(".")[0])
+                                stop = instant(period_str.split(".")[1])
+                                period_ = None
+                            else:
+                                period_ = period(period_str)
+                            sim.tax_benefit_system.parameters.get_child(
+                                parameter
+                            ).update(
+                                start=start,
+                                stop=stop,
+                                period=period_,
+                                value=value,
+                            )
+                    else:
+                        start = instant("2023-01-01")
+                        stop = None
+                        period_ = None
+
+                        sim.tax_benefit_system.parameters.get_child(
+                            parameter
+                        ).update(
+                            start=start,
+                            stop=stop,
+                            period=period_,
+                            value=reform[parameter],
+                        )
+
+            return Scenario(
+                simulation_modifier=modifier,
             )
 
         elif isinstance(reform, tuple):

policyengine_uk/variables/gov/gov_tax.py

@@ -26,7 +26,6 @@ class gov_tax(Variable):
         "national_insurance",
         "LVT",
         "carbon_tax",
-        "vat_change",
         "capital_gains_tax",
         "private_school_vat",
         "corporate_incident_tax_revenue_change",