Add addition operator support for Policy and Dynamic classes

changelog_entry.yaml

@@ -0,0 +1,6 @@
+- bump: patch
+  changes:
+    added:
+    - Policy and Dynamic classes now support addition operator (__add__) to combine policies and dynamics.
+    - Parameter values are appended when combining policies/dynamics.
+    - Simulation modifiers are chained in sequence when combining policies/dynamics.

src/policyengine/core/dynamic.py

@@ -15,3 +15,29 @@ class Dynamic(BaseModel):
     simulation_modifier: Callable | None = None
     created_at: datetime = Field(default_factory=datetime.now)
     updated_at: datetime = Field(default_factory=datetime.now)
+
+    def __add__(self, other: "Dynamic") -> "Dynamic":
+        """Combine two dynamics by appending parameter values and chaining simulation modifiers."""
+        if not isinstance(other, Dynamic):
+            return NotImplemented
+
+        # Combine simulation modifiers
+        combined_modifier = None
+        if self.simulation_modifier is not None and other.simulation_modifier is not None:
+
+            def combined_modifier(sim):
+                sim = self.simulation_modifier(sim)
+                sim = other.simulation_modifier(sim)
+                return sim
+
+        elif self.simulation_modifier is not None:
+            combined_modifier = self.simulation_modifier
+        elif other.simulation_modifier is not None:
+            combined_modifier = other.simulation_modifier
+
+        return Dynamic(
+            name=f"{self.name} + {other.name}",
+            description=f"Combined dynamic: {self.name} and {other.name}",
+            parameter_values=self.parameter_values + other.parameter_values,
+            simulation_modifier=combined_modifier,
+        )

src/policyengine/core/policy.py

@@ -15,3 +15,29 @@ class Policy(BaseModel):
     simulation_modifier: Callable | None = None
     created_at: datetime = Field(default_factory=datetime.now)
     updated_at: datetime = Field(default_factory=datetime.now)
+
+    def __add__(self, other: "Policy") -> "Policy":
+        """Combine two policies by appending parameter values and chaining simulation modifiers."""
+        if not isinstance(other, Policy):
+            return NotImplemented
+
+        # Combine simulation modifiers
+        combined_modifier = None
+        if self.simulation_modifier is not None and other.simulation_modifier is not None:
+
+            def combined_modifier(sim):
+                sim = self.simulation_modifier(sim)
+                sim = other.simulation_modifier(sim)
+                return sim
+
+        elif self.simulation_modifier is not None:
+            combined_modifier = self.simulation_modifier
+        elif other.simulation_modifier is not None:
+            combined_modifier = other.simulation_modifier
+
+        return Policy(
+            name=f"{self.name} + {other.name}",
+            description=f"Combined policy: {self.name} and {other.name}",
+            parameter_values=self.parameter_values + other.parameter_values,
+            simulation_modifier=combined_modifier,
+        )