community: Fix bug with StreamlitChatMessageHistory (#18834)

- **Description:** Fix Streamlit bug which was introduced by
https://github.com/langchain-ai/langchain/pull/18250, update integration
test
- **Issue:** https://github.com/langchain-ai/langchain/issues/18684
- **Dependencies:** None

libs/community/langchain_community/chat_message_histories/streamlit.py

@@ -22,14 +22,13 @@ class StreamlitChatMessageHistory(BaseChatMessageHistory):
 
         if key not in st.session_state:
             st.session_state[key] = []
+        self._messages = st.session_state[key]
         self._key = key
 
     @property
     def messages(self) -> List[BaseMessage]:
         """Retrieve the current list of messages"""
-        import streamlit as st
-
-        return st.session_state[self._key]
+        return self._messages
 
     @messages.setter
     def messages(self, value: List[BaseMessage]) -> None:
@@ -37,6 +36,7 @@ class StreamlitChatMessageHistory(BaseChatMessageHistory):
         import streamlit as st
 
         st.session_state[self._key] = value
+        self._messages = st.session_state[self._key]
 
     def add_message(self, message: BaseMessage) -> None:
         """Add a message to the session memory"""

libs/community/tests/integration_tests/chat_message_histories/test_streamlit.py

@@ -6,7 +6,7 @@ test_script = """
     import streamlit as st
     from langchain.memory import ConversationBufferMemory
     from langchain_community.chat_message_histories import StreamlitChatMessageHistory
-    from langchain_core.messages import message_to_dict
+    from langchain_core.messages import message_to_dict, BaseMessage
 
     message_history = StreamlitChatMessageHistory()
     memory = ConversationBufferMemory(chat_memory=message_history, return_messages=True)
@@ -23,6 +23,15 @@ test_script = """
         st.markdown("Cleared!")
         memory.chat_memory.clear()
 
+    # Use message setter
+    if st.checkbox("Override messages"):
+        memory.chat_memory.messages = [
+            BaseMessage(content="A basic message", type="basic")
+        ]
+        st.session_state["langchain_messages"].append(
+            BaseMessage(content="extra cool message", type="basic")
+        )
+
     # Write the output to st.code as a json blob for inspection
     messages = memory.chat_memory.messages
     messages_json = json.dumps([message_to_dict(msg) for msg in messages])
@@ -33,32 +42,33 @@ test_script = """
 @pytest.mark.requires("streamlit")
 def test_memory_with_message_store() -> None:
     try:
-        from streamlit.testing.script_interactions import InteractiveScriptTests
+        from streamlit.testing.v1 import AppTest
     except ModuleNotFoundError:
         pytest.skip("Incorrect version of Streamlit installed")
 
-    test_handler = InteractiveScriptTests()
-    test_handler.setUp()
-    try:
-        sr = test_handler.script_from_string(test_script).run()
-    except TypeError:
-        # Earlier version expected 2 arguments
-        sr = test_handler.script_from_string("memory_test.py", test_script).run()
+    at = AppTest.from_string(test_script).run(timeout=10)
 
     # Initial run should write two messages
-    messages_json = sr.get("text")[-1].value
+    messages_json = at.get("text")[-1].value
     assert "This is me, the AI" in messages_json
     assert "This is me, the human" in messages_json
 
     # Uncheck the initial write, they should persist in session_state
-    sr = sr.get("checkbox")[0].uncheck().run()
-    assert sr.get("markdown")[0].value == "Skipped add"
-    messages_json = sr.get("text")[-1].value
+    at.get("checkbox")[0].uncheck().run()
+    assert at.get("markdown")[0].value == "Skipped add"
+    messages_json = at.get("text")[-1].value
     assert "This is me, the AI" in messages_json
     assert "This is me, the human" in messages_json
 
     # Clear the message history
-    sr = sr.get("checkbox")[1].check().run()
-    assert sr.get("markdown")[1].value == "Cleared!"
-    messages_json = sr.get("text")[-1].value
+    at.get("checkbox")[1].check().run()
+    assert at.get("markdown")[1].value == "Cleared!"
+    messages_json = at.get("text")[-1].value
     assert messages_json == "[]"
+
+    # Use message setter
+    at.get("checkbox")[1].uncheck()
+    at.get("checkbox")[2].check().run()
+    messages_json = at.get("text")[-1].value
+    assert "A basic message" in messages_json
+    assert "extra cool message" in messages_json