feat(python): reject registration of custom strategies that don't qua…

…ck like ducks

python-engine/tests/test_custom_strategy.py

@@ -1,4 +1,5 @@
 from yggdrasil_engine.custom_strategy import CustomStrategyHandler
+import pytest
 
 RAW_STATE = """
 {
@@ -75,3 +76,38 @@ def apply(self, parameters, _context):
     results = handler.evaluate_custom_strategies("Feature.A", {})
     assert results["customStrategy1"] == True
     assert results["customStrategy2"] == False
+
+
+def test_register_custom_strategy_rejects_strategies_without_apply_method():
+    class IncompleteStrategy:
+        pass
+
+    handler = CustomStrategyHandler()
+
+    with pytest.raises(ValueError):
+        handler.register_custom_strategies({"custom": IncompleteStrategy()})
+
+
+def test_register_custom_strategy_rejects_strategies_with_incorrect_number_of_args():
+    class FirstBearStrategy:
+        def apply(self, _parameters):
+            return True
+
+    class SecondBearStrategy:
+        def apply(self, _parameters, _context, _extra):
+            return True
+
+    class ThirdBearStrategy:
+        def apply(self, _parameters, _context):
+            return True
+
+    handler = CustomStrategyHandler()
+
+    with pytest.raises(ValueError):
+        handler.register_custom_strategies({"custom": FirstBearStrategy()})
+
+    with pytest.raises(ValueError):
+        handler.register_custom_strategies({"custom": SecondBearStrategy()})
+
+    ## Goldilocks parameters, we expect this not to raise
+    handler.register_custom_strategies({"custom": ThirdBearStrategy()})

python-engine/yggdrasil_engine/custom_strategy.py

@@ -1,5 +1,6 @@
 import json
 from typing import Dict
+import inspect
 
 _STANDARD_STRATEGIES = [
     "default",
@@ -41,6 +42,14 @@ def update_strategies(self, features_json: str):
     def register_custom_strategies(self, custom_strategies: Dict[str, any]):
         for strategy_name, strategy in custom_strategies.items():
             if hasattr(strategy, "apply"):
+                apply_method = strategy.apply
+                signature = inspect.signature(apply_method)
+                parameters = list(signature.parameters)
+                if len(parameters) != 2:
+                    raise ValueError(
+                        f"Custom strategy '{strategy_name}' does not have an apply method "
+                        f"with exactly two parameters. Found {len(parameters)}."
+                    )
                 self.strategy_implementations[strategy_name] = strategy
             else:
                 raise ValueError(