Make streamlit import optional (#6510)

langchain/callbacks/streamlit.py

@@ -1,8 +1,6 @@
 """Callback Handler that logs to streamlit."""
 from typing import Any, Dict, List, Optional, Union
 
-import streamlit as st
-
 from langchain.callbacks.base import BaseCallbackHandler
 from langchain.schema import AgentAction, AgentFinish, LLMResult
 
@@ -11,16 +9,25 @@ class StreamlitCallbackHandler(BaseCallbackHandler):
     """Callback Handler that logs to streamlit."""
 
     def __init__(self) -> None:
+        try:
+            import streamlit as st
+        except ImportError as e:
+            raise ImportError(
+                "Could not import streamlit Python package. "
+                "Please install it with `pip install streamlit`."
+            ) from e
+
         self.tokens_area = st.empty()
         self.tokens_stream = ""
+        self.st = st
 
     def on_llm_start(
         self, serialized: Dict[str, Any], prompts: List[str], **kwargs: Any
     ) -> None:
         """Print out the prompts."""
-        st.write("Prompts after formatting:")
+        self.st.write("Prompts after formatting:")
         for prompt in prompts:
-            st.write(prompt)
+            self.st.write(prompt)
 
     def on_llm_new_token(self, token: str, **kwargs: Any) -> None:
         """Run on new LLM token. Only available when streaming is enabled."""
@@ -42,11 +49,11 @@ class StreamlitCallbackHandler(BaseCallbackHandler):
     ) -> None:
         """Print out that we are entering a chain."""
         class_name = serialized["name"]
-        st.write(f"Entering new {class_name} chain...")
+        self.st.write(f"Entering new {class_name} chain...")
 
     def on_chain_end(self, outputs: Dict[str, Any], **kwargs: Any) -> None:
         """Print out that we finished a chain."""
-        st.write("Finished chain.")
+        self.st.write("Finished chain.")
 
     def on_chain_error(
         self, error: Union[Exception, KeyboardInterrupt], **kwargs: Any
@@ -66,7 +73,8 @@ class StreamlitCallbackHandler(BaseCallbackHandler):
     def on_agent_action(self, action: AgentAction, **kwargs: Any) -> Any:
         """Run on agent action."""
         # st.write requires two spaces before a newline to render it
-        st.markdown(action.log.replace("\n", "  \n"))
+
+        self.st.markdown(action.log.replace("\n", "  \n"))
 
     def on_tool_end(
         self,
@@ -76,8 +84,8 @@ class StreamlitCallbackHandler(BaseCallbackHandler):
         **kwargs: Any,
     ) -> None:
         """If not the final action, print out observation."""
-        st.write(f"{observation_prefix}{output}")
+        self.st.write(f"{observation_prefix}{output}")
-        st.write(llm_prefix)
+        self.st.write(llm_prefix)
 
     def on_tool_error(
         self, error: Union[Exception, KeyboardInterrupt], **kwargs: Any
@@ -88,9 +96,9 @@ class StreamlitCallbackHandler(BaseCallbackHandler):
     def on_text(self, text: str, **kwargs: Any) -> None:
         """Run on text."""
         # st.write requires two spaces before a newline to render it
-        st.write(text.replace("\n", "  \n"))
+        self.st.write(text.replace("\n", "  \n"))
 
     def on_agent_finish(self, finish: AgentFinish, **kwargs: Any) -> None:
         """Run on agent end."""
         # st.write requires two spaces before a newline to render it
-        st.write(finish.log.replace("\n", "  \n"))
+        self.st.write(finish.log.replace("\n", "  \n"))