Add type to Generation and sub-classes, handle root validator (#12220)

* Add a type literal for the generation and sub-classes for serialization purposes.
* Fix the root validator of ChatGeneration to return ValueError instead of KeyError or Attribute error if intialized improperly.
* This change is done for langserve to make sure that llm related callbacks can be serialized/deserialized properly.

libs/langchain/langchain/schema/output.py

@@ -1,7 +1,7 @@
 from __future__ import annotations
 
 from copy import deepcopy
-from typing import Any, Dict, List, Optional
+from typing import Any, Dict, List, Literal, Optional
 from uuid import UUID
 
 from langchain.load.serializable import Serializable
@@ -19,6 +19,8 @@ class Generation(Serializable):
     """Raw response from the provider. May include things like the 
         reason for finishing or token log probabilities.
     """
+    type: Literal["Generation"] = "Generation"
+    """Type is used exclusively for serialization purposes."""
     # TODO: add log probs as separate attribute
 
     @classmethod
@@ -54,11 +56,17 @@ class ChatGeneration(Generation):
     """*SHOULD NOT BE SET DIRECTLY* The text contents of the output message."""
     message: BaseMessage
     """The message output by the chat model."""
+    # Override type to be ChatGeneration, ignore mypy error as this is intentional
+    type: Literal["ChatGeneration"] = "ChatGeneration"  # type: ignore[assignment]
+    """Type is used exclusively for serialization purposes."""
 
     @root_validator
     def set_text(cls, values: Dict[str, Any]) -> Dict[str, Any]:
         """Set the text attribute to be the contents of the message."""
-        values["text"] = values["message"].content
+        try:
+            values["text"] = values["message"].content
+        except (KeyError, AttributeError) as e:
+            raise ValueError("Error while initializing ChatGeneration") from e
         return values
 
 
@@ -71,6 +79,9 @@ class ChatGenerationChunk(ChatGeneration):
     """
 
     message: BaseMessageChunk
+    # Override type to be ChatGeneration, ignore mypy error as this is intentional
+    type: Literal["ChatGenerationChunk"] = "ChatGenerationChunk"  # type: ignore[assignment] # noqa: E501
+    """Type is used exclusively for serialization purposes."""
 
     def __add__(self, other: ChatGenerationChunk) -> ChatGenerationChunk:
         if isinstance(other, ChatGenerationChunk):

libs/langchain/tests/unit_tests/schema/runnable/test_runnable.py

@@ -1719,7 +1719,9 @@ async def test_prompt_with_llm(
                 "op": "add",
                 "path": "/logs/FakeListLLM/final_output",
                 "value": {
-                    "generations": [[{"generation_info": None, "text": "foo"}]],
+                    "generations": [
+                        [{"generation_info": None, "text": "foo", "type": "Generation"}]
+                    ],
                     "llm_output": None,
                     "run": None,
                 },

libs/langchain/tests/unit_tests/test_schema.py

@@ -1,12 +1,19 @@
 """Test formatting functionality."""
-
 import unittest
 from typing import Union
 
+import pytest
+
 from langchain.prompts.base import StringPromptValue
 from langchain.prompts.chat import ChatPromptValueConcrete
-from langchain.pydantic_v1 import BaseModel
-from langchain.schema import AgentAction, AgentFinish, Document
+from langchain.pydantic_v1 import BaseModel, ValidationError
+from langchain.schema import (
+    AgentAction,
+    AgentFinish,
+    ChatGeneration,
+    Document,
+    Generation,
+)
 from langchain.schema.agent import AgentActionMessageLog
 from langchain.schema.messages import (
     AIMessage,
@@ -23,6 +30,7 @@ from langchain.schema.messages import (
     messages_from_dict,
     messages_to_dict,
 )
+from langchain.schema.output import ChatGenerationChunk
 
 
 class TestGetBufferString(unittest.TestCase):
@@ -108,6 +116,9 @@ def test_serialization_of_wellknown_objects() -> None:
             AgentFinish,
             AgentAction,
             AgentActionMessageLog,
+            ChatGeneration,
+            Generation,
+            ChatGenerationChunk,
         ]
 
     lc_objects = [
@@ -144,6 +155,16 @@ def test_serialization_of_wellknown_objects() -> None:
             log="",
             message_log=[HumanMessage(content="human")],
         ),
+        Generation(
+            text="hello",
+            generation_info={"info": "info"},
+        ),
+        ChatGeneration(
+            message=HumanMessage(content="human"),
+        ),
+        ChatGenerationChunk(
+            message=HumanMessageChunk(content="cat"),
+        ),
     ]
 
     for lc_object in lc_objects:
@@ -151,3 +172,7 @@ def test_serialization_of_wellknown_objects() -> None:
         assert "type" in d, f"Missing key `type` for {type(lc_object)}"
         obj1 = WellKnownLCObject.parse_obj(d)
         assert type(obj1.__root__) == type(lc_object), f"failed for {type(lc_object)}"
+
+    with pytest.raises(ValidationError):
+        # Make sure that specifically validation error is raised
+        WellKnownLCObject.parse_obj({})