Enable parameter adds/subtracts

changelog_entry.yaml

@@ -0,0 +1,4 @@
+- bump: minor
+  changes:
+    added:
+    - Adds/subtracts option for parameter names.

policyengine_core/simulations/simulation.py

@@ -662,15 +662,41 @@ def _run_formula(
         if formula is None:
             values = None
             if variable.adds is not None:
+                if isinstance(variable.adds, str):
+                    try:
+                        adds_parameter = get_parameter(
+                            self.tax_benefit_system.parameters,
+                            variable.adds,
+                        )
+                    except:
+                        raise ValueError(
+                            f"In the variable '{variable.name}', the 'adds' attribute is a string '{variable.adds}' that does not match any parameter."
+                        )
+                    adds_list = adds_parameter(period.start)
+                else:
+                    adds_list = variable.adds
                 values = 0
-                for added_variable in variable.adds:
+                for added_variable in adds_list:
                     values = values + self.calculate(
                         added_variable, period, map_to=variable.entity.key
                     )
             if variable.subtracts is not None:
+                if isinstance(variable.subtracts, str):
+                    try:
+                        subtracts_parameter = get_parameter(
+                            self.tax_benefit_system.parameters,
+                            variable.subtracts,
+                        )
+                    except:
+                        raise ValueError(
+                            f"In the variable '{variable.name}', the 'subtracts' attribute is a string '{variable.subtracts}' that does not match any parameter."
+                        )
+                    subtracts_list = subtracts_parameter(period.start)
+                else:
+                    subtracts_list = variable.subtracts
                 if values is None:
                     values = 0
-                for subtracted_variable in variable.subtracts:
+                for subtracted_variable in subtracts_list:
                     values = values - self.calculate(
                         subtracted_variable, period, map_to=variable.entity.key
                     )

policyengine_core/variables/variable.py

@@ -110,10 +110,10 @@ class Variable:
     """Index of the variable in the module it is defined in."""
 
     adds: List[str] = None
-    """List of variables that are added to the variable."""
+    """List of variables that are added to the variable. Alternatively, can be a parameter name."""
 
     subtracts: List[str] = None
-    """List of variables that are subtracted from the variable."""
+    """List of variables that are subtracted from the variable. Alternatively, can be a parameter name."""
 
     uprating: str = None
     """Name of a parameter used to uprate the variable."""
@@ -236,7 +236,7 @@ def __init__(self, baseline_variable=None):
             default=None,
         )
 
-        self.adds = self.set(attr, "adds", allowed_type=list)
+        self.adds = self.set(attr, "adds")
         self.subtracts = self.set(attr, "subtracts", allowed_type=list)
         self.uprating = self.set(attr, "uprating", allowed_type=str)
         self.hidden_input = self.set(