Harrison/streamlit handler (#3564)

Co-authored-by: kurupapi <37198601+kurupapi@users.noreply.github.com>

langchain/callbacks/streamlit.py

@@ -10,6 +10,10 @@ from langchain.schema import AgentAction, AgentFinish, LLMResult
 class StreamlitCallbackHandler(BaseCallbackHandler):
     """Callback Handler that logs to streamlit."""
 
+    def __init__(self) -> None:
+        self.tokens_area = st.empty()
+        self.tokens_stream = ""
+
     def on_llm_start(
         self, serialized: Dict[str, Any], prompts: List[str], **kwargs: Any
     ) -> None:
@@ -19,8 +23,9 @@ class StreamlitCallbackHandler(BaseCallbackHandler):
             st.write(prompt)
 
     def on_llm_new_token(self, token: str, **kwargs: Any) -> None:
-        """Do nothing."""
-        pass
+        """Run on new LLM token. Only available when streaming is enabled."""
+        self.tokens_stream += token
+        self.tokens_area.write(self.tokens_stream)
 
     def on_llm_end(self, response: LLMResult, **kwargs: Any) -> None:
         """Do nothing."""