task: added warning when none is called in intervention handler (#4149)

* task: added warning when none is called in intervention handler

* add leading underscore to indicate private to _warn_if_none method in intervention.py

* address comment of returning Any for result in intervention.py

* Update intervention.py to remove redundant name change

* Format and lint

---------

Co-authored-by: Jack Gerrits <[email protected]>
Co-authored-by: Jack Gerrits <[email protected]>

python/packages/autogen-core/src/autogen_core/base/intervention.py

@@ -1,3 +1,4 @@
+import warnings
 from typing import Any, Awaitable, Callable, Protocol, final
 
 from autogen_core.base import AgentId
@@ -14,6 +15,23 @@
 class DropMessage: ...
 
 
+def _warn_if_none(value: Any, handler_name: str) -> None:
+    """
+    Utility function to check if the intervention handler returned None and issue a warning.
+
+    Args:
+        value: The return value to check
+        handler_name: Name of the intervention handler method for the warning message
+    """
+    if value is None:
+        warnings.warn(
+            f"Intervention handler {handler_name} returned None. This might be unintentional. "
+            "Consider returning the original message or DropMessage explicitly.",
+            RuntimeWarning,
+            stacklevel=2,
+        )
+
+
 InterventionFunction = Callable[[Any], Any | Awaitable[type[DropMessage]]]
 
 
@@ -27,10 +45,13 @@ async def on_response(
 
 class DefaultInterventionHandler(InterventionHandler):
     async def on_send(self, message: Any, *, sender: AgentId | None, recipient: AgentId) -> Any | type[DropMessage]:
+        _warn_if_none(message, "on_send")
         return message
 
     async def on_publish(self, message: Any, *, sender: AgentId | None) -> Any | type[DropMessage]:
+        _warn_if_none(message, "on_publish")
         return message
 
     async def on_response(self, message: Any, *, sender: AgentId, recipient: AgentId | None) -> Any | type[DropMessage]:
+        _warn_if_none(message, "on_response")
         return message