Do not include placeholder in serialize and properly replace placeholder for streams

panel/chat/feed.py

@@ -313,21 +313,15 @@ def _replace_placeholder(self, message: ChatMessage | None = None) -> None:
         if placeholder, otherwise simply append the message.
         Replacing helps lessen the chat log jumping around.
         """
-        index = None
-        if self.placeholder_threshold > 0:
+        with param.parameterized.batch_call_watchers(self):
+            if message is not None:
+                self.append(message)
+
             try:
-                index = self.index(self._placeholder)
+                self.remove(self._placeholder)
             except ValueError:
                 pass
 
-        if index is not None:
-            if message is not None:
-                self[index] = message
-            elif message is None:
-                self.remove(self._placeholder)
-        elif message is not None:
-            self.append(message)
-
     def _build_message(
         self,
         value: dict,
@@ -699,7 +693,7 @@ def undo(self, count: int = 1) -> List[Any]:
             return []
         messages = self._chat_log.objects
         undone_entries = messages[-count:]
-        self[:] = messages[:-count]
+        self._chat_log.objects = messages[:-count]
         return undone_entries
 
     def clear(self) -> List[Any]:
@@ -812,9 +806,11 @@ def serialize(
             exclude_users = ["help"]
         else:
             exclude_users = [user.lower() for user in exclude_users]
+
         messages = [
             message for message in self._chat_log.objects
             if message.user.lower() not in exclude_users
+            and message is not self._placeholder
         ]
 
         if filter_by is not None:

panel/tests/chat/test_feed.py

@@ -820,6 +820,21 @@ async def callback(msg, user, instance):
         time.sleep(1)
         assert chat_feed.objects[-1].object == "A"
 
+    def test_callback_short_time(self, chat_feed):
+        def callback(contents, user, instance):
+            time.sleep(1)
+            message = None
+            string = ""
+            for c in "helloooo":
+                string += c
+                time.sleep(0.001)
+                message = instance.stream(string, message=message, replace=True)
+
+        feed = ChatFeed(callback=callback)
+        feed.send("Message", respond=True)
+        assert feed.objects[-1].object == "helloooo"
+        assert chat_feed._placeholder not in chat_feed._chat_log
+
 
 @pytest.mark.xdist_group("chat")
 class TestChatFeedSerializeForTransformers:
@@ -947,6 +962,21 @@ def say_hi(contents, user, instance):
             {"role": "user", "content": "Hello there!"},
         ]
 
+    def test_serialize_exclude_placeholder(self):
+        def say_hi(contents, user, instance):
+            assert len(instance.serialize()) == 1
+            return f"Hi {user}!"
+
+        chat_feed = ChatFeed(
+            help_text="This chat feed will respond by saying hi!",
+            callback=say_hi
+        )
+
+        chat_feed.send("Hello there!")
+        assert chat_feed.serialize() == [
+            {"role": "user", "content": "Hello there!"},
+            {"role": "assistant", "content": "Hi User!"}
+        ]
 
 @pytest.mark.xdist_group("chat")
 class TestChatFeedSerializeBase: