langchain_mistralai[patch]: Invoke callback prior to yielding token (#16986)

- **Description:** Invoke callback prior to yielding token in stream and
astream methods for ChatMistralAI.
- **Issue:** https://github.com/langchain-ai/langchain/issues/16913

libs/partners/mistralai/langchain_mistralai/chat_models.py

@@ -317,9 +317,9 @@ class ChatMistralAI(BaseChatModel):
                 continue
             chunk = _convert_delta_to_message_chunk(delta, default_chunk_class)
             default_chunk_class = chunk.__class__
-            yield ChatGenerationChunk(message=chunk)
             if run_manager:
                 run_manager.on_llm_new_token(token=chunk.content, chunk=chunk)
+            yield ChatGenerationChunk(message=chunk)
 
     async def _astream(
         self,
@@ -342,9 +342,9 @@ class ChatMistralAI(BaseChatModel):
                 continue
             chunk = _convert_delta_to_message_chunk(delta, default_chunk_class)
             default_chunk_class = chunk.__class__
-            yield ChatGenerationChunk(message=chunk)
             if run_manager:
                 await run_manager.on_llm_new_token(token=chunk.content, chunk=chunk)
+            yield ChatGenerationChunk(message=chunk)
 
     async def _agenerate(
         self,

libs/partners/mistralai/tests/unit_tests/test_chat_models.py

@@ -1,7 +1,10 @@
 """Test MistralAI Chat API wrapper."""
 import os
+from typing import Any, AsyncGenerator, Generator
+from unittest.mock import patch
 
 import pytest
+from langchain_core.callbacks.base import BaseCallbackHandler
 from langchain_core.messages import (
     AIMessage,
     BaseMessage,
@@ -12,6 +15,11 @@ from langchain_core.messages import (
 
 # TODO: Remove 'type: ignore' once mistralai has stubs or py.typed marker.
 from mistralai.models.chat_completion import (  # type: ignore[import]
+    ChatCompletionResponseStreamChoice,
+    ChatCompletionStreamResponse,
+    DeltaMessage,
+)
+from mistralai.models.chat_completion import (
     ChatMessage as MistralChatMessage,
 )
 
@@ -63,3 +71,50 @@ def test_convert_message_to_mistral_chat_message(
 ) -> None:
     result = _convert_message_to_mistral_chat_message(message)
     assert result == expected
+
+
+def _make_completion_response_from_token(token: str) -> ChatCompletionStreamResponse:
+    return ChatCompletionStreamResponse(
+        id="abc123",
+        model="fake_model",
+        choices=[
+            ChatCompletionResponseStreamChoice(
+                index=0,
+                delta=DeltaMessage(content=token),
+                finish_reason=None,
+            )
+        ],
+    )
+
+
+def mock_chat_stream(*args: Any, **kwargs: Any) -> Generator:
+    for token in ["Hello", " how", " can", " I", " help", "?"]:
+        yield _make_completion_response_from_token(token)
+
+
+async def mock_chat_astream(*args: Any, **kwargs: Any) -> AsyncGenerator:
+    for token in ["Hello", " how", " can", " I", " help", "?"]:
+        yield _make_completion_response_from_token(token)
+
+
+class MyCustomHandler(BaseCallbackHandler):
+    last_token: str = ""
+
+    def on_llm_new_token(self, token: str, **kwargs: Any) -> None:
+        self.last_token = token
+
+
+@patch("mistralai.client.MistralClient.chat_stream", new=mock_chat_stream)
+def test_stream_with_callback() -> None:
+    callback = MyCustomHandler()
+    chat = ChatMistralAI(callbacks=[callback])
+    for token in chat.stream("Hello"):
+        assert callback.last_token == token.content
+
+
+@patch("mistralai.async_client.MistralAsyncClient.chat_stream", new=mock_chat_astream)
+async def test_astream_with_callback() -> None:
+    callback = MyCustomHandler()
+    chat = ChatMistralAI(callbacks=[callback])
+    async for token in chat.astream("Hello"):
+        assert callback.last_token == token.content