Add InvalidConversationError (#1565)

llmfoundry/data/finetuning/tasks.py

@@ -76,6 +76,7 @@ def preprocessing_fn(example: Dict) -> Dict[str, str]:
     DatasetTooSmallError,
     IncorrectMessageKeyQuantityError,
     InvalidContentTypeError,
+    InvalidConversationError,
     InvalidExampleTypeError,
     InvalidFileExtensionError,
     InvalidLastChatMessageRoleError,
@@ -270,17 +271,17 @@ def slice_out_last_turn(
         if conversation_through_previous_turn != full_conversation[:len(
             conversation_through_previous_turn,
         )]:
-            raise ValueError(
+            raise InvalidConversationError(
                 f'The full conversation must start with the conversation through the previous turn. {conversation_through_previous_turn=}, {full_conversation=}',
             )
         if conversation_through_previous_turn != prompt_with_history[:len(
             conversation_through_previous_turn,
         )]:
-            raise ValueError(
+            raise InvalidConversationError(
                 f'The prompt_with_history must start with the conversation through the previous turn. {conversation_through_previous_turn=}, {prompt_with_history=}',
             )
         if prompt_with_history != full_conversation[:len(prompt_with_history)]:
-            raise ValueError(
+            raise InvalidConversationError(
                 f'prompt_with_history must be the first part of the full conversation. {prompt_with_history=}, {full_conversation=}',
             )
         prompt = prompt_with_history[len(conversation_through_previous_turn):]

llmfoundry/utils/exceptions.py

@@ -497,3 +497,18 @@ def __init__(self, files_searched: list[str]) -> None:
             message,
             files_searched=files_searched,
         )
+
+
+class InvalidConversationError(UserError):
+    """Error thrown when the conversation is invalid."""
+
+    def __init__(self, message: str) -> None:
+        self.message = message
+        super().__init__(message)
+
+    def __reduce__(self):
+        # Return a tuple of class, a tuple of arguments, and optionally state
+        return (InvalidConversationError, (self.message,))
+
+    def __str__(self):
+        return self.message