# Copyright (c) Streamlit Inc. (2018-2022) Snowflake Inc. (2022-2025) # # Licensed under the Apache License, Version 2.0 (the "License"); # you may not use this file except in compliance with the License. # You may obtain a copy of the License at # # http://www.apache.org/licenses/LICENSE-2.0 # # Unless required by applicable law or agreed to in writing, software # distributed under the License is distributed on an "AS IS" BASIS, # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. # See the License for the specific language governing permissions and # limitations under the License. from __future__ import annotations from typing import TYPE_CHECKING, Any, Callable from streamlit.proto.ForwardMsg_pb2 import ForwardMsg if TYPE_CHECKING: from streamlit.proto.Delta_pb2 import Delta class ForwardMsgQueue: """Accumulates a session's outgoing ForwardMsgs. Each AppSession adds messages to its queue, and the Server periodically flushes all session queues and delivers their messages to the appropriate clients. ForwardMsgQueue is not thread-safe - a queue should only be used from a single thread. """ _before_enqueue_msg: Callable[[ForwardMsg], None] | None = None @staticmethod def on_before_enqueue_msg( before_enqueue_msg: Callable[[ForwardMsg], None] | None, ) -> None: """Set a callback to be called before a message is enqueued. Used in static streamlit app generation.""" ForwardMsgQueue._before_enqueue_msg = before_enqueue_msg def __init__(self): self._queue: list[ForwardMsg] = [] # A mapping of (delta_path -> _queue.indexof(msg)) for each # Delta message in the queue. We use this for coalescing # redundant outgoing Deltas (where a newer Delta supersedes # an older Delta, with the same delta_path, that's still in the # queue). self._delta_index_map: dict[tuple[int, ...], int] = {} def get_debug(self) -> dict[str, Any]: from google.protobuf.json_format import MessageToDict return { "queue": [MessageToDict(m) for m in self._queue], "ids": list(self._delta_index_map.keys()), } def is_empty(self) -> bool: return len(self._queue) == 0 def enqueue(self, msg: ForwardMsg) -> None: """Add message into queue, possibly composing it with another message.""" if ForwardMsgQueue._before_enqueue_msg: ForwardMsgQueue._before_enqueue_msg(msg) if not _is_composable_message(msg): self._queue.append(msg) return # If there's a Delta message with the same delta_path already in # the queue - meaning that it refers to the same location in # the app - we attempt to combine this new Delta into the old # one. This is an optimization that prevents redundant Deltas # from being sent to the frontend. delta_key = tuple(msg.metadata.delta_path) if delta_key in self._delta_index_map: index = self._delta_index_map[delta_key] old_msg = self._queue[index] composed_delta = _maybe_compose_deltas(old_msg.delta, msg.delta) if composed_delta is not None: new_msg = ForwardMsg() new_msg.delta.CopyFrom(composed_delta) new_msg.metadata.CopyFrom(msg.metadata) self._queue[index] = new_msg return # No composition occurred. Append this message to the queue, and # store its index for potential future composition. self._delta_index_map[delta_key] = len(self._queue) self._queue.append(msg) def clear( self, retain_lifecycle_msgs: bool = False, fragment_ids_this_run: list[str] | None = None, ) -> None: """Clear the queue, potentially retaining lifecycle messages. The retain_lifecycle_msgs argument exists because in some cases (in particular when a currently running script is interrupted by a new BackMsg), we don't want to remove certain messages from the queue as doing so may cause the client to not hear about important script lifecycle events (such as the script being stopped early in order to be rerun). If fragment_ids_this_run is provided, delta messages not belonging to any fragment or belonging to a fragment not in fragment_ids_this_run will be preserved to prevent clearing messages unrelated to the running fragments. """ if not retain_lifecycle_msgs: self._queue = [] else: self._queue = [ _update_script_finished_message(msg, fragment_ids_this_run is not None) for msg in self._queue if msg.WhichOneof("type") in { "new_session", "script_finished", "session_status_changed", "parent_message", } or ( # preserve all messages if this is a fragment rerun and... fragment_ids_this_run is not None and ( # the message is not a delta message # (not associated with a fragment) or... msg.delta is None or ( # it is a delta but not associated with any of the passed # fragments msg.delta is not None and ( msg.delta.fragment_id is None or msg.delta.fragment_id not in fragment_ids_this_run ) ) ) ) ] self._delta_index_map = {} def flush(self) -> list[ForwardMsg]: """Clear the queue and return a list of the messages it contained before being cleared. """ queue = self._queue self.clear() return queue def __len__(self) -> int: return len(self._queue) def _is_composable_message(msg: ForwardMsg) -> bool: """True if the ForwardMsg is potentially composable with other ForwardMsgs.""" if not msg.HasField("delta"): # Non-delta messages are never composable. return False # We never compose add_rows messages in Python, because the add_rows # operation can raise errors, and we don't have a good way of handling # those errors in the message queue. delta_type = msg.delta.WhichOneof("type") return delta_type != "add_rows" and delta_type != "arrow_add_rows" def _maybe_compose_deltas(old_delta: Delta, new_delta: Delta) -> Delta | None: """Combines new_delta onto old_delta if possible. If the combination takes place, the function returns a new Delta that should replace old_delta in the queue. If the new_delta is incompatible with old_delta, the function returns None. In this case, the new_delta should just be appended to the queue as normal. """ old_delta_type = old_delta.WhichOneof("type") if old_delta_type == "add_block": # We never replace add_block deltas, because blocks can have # other dependent deltas later in the queue. For example: # # placeholder = st.empty() # placeholder.columns(1) # placeholder.empty() # # The call to "placeholder.columns(1)" creates two blocks, a parent # container with delta_path (0, 0), and a column child with # delta_path (0, 0, 0). If the final "placeholder.empty()" Delta # is composed with the parent container Delta, the frontend will # throw an error when it tries to add that column child to what is # now just an element, and not a block. return None new_delta_type = new_delta.WhichOneof("type") if new_delta_type == "new_element": return new_delta if new_delta_type == "add_block": return new_delta return None def _update_script_finished_message( msg: ForwardMsg, is_fragment_run: bool ) -> ForwardMsg: """ When we are here, the message queue is cleared from non-lifecycle messages before they were flushed to the browser. If there were no non-lifecycle messages in the queue, changing the type here should not matter for the frontend anyways, so we optimistically change the `script_finished` message to `FINISHED_EARLY_FOR_RERUN`. This indicates to the frontend that the previous run was interrupted by a new script start. Otherwise, a `FINISHED_SUCCESSFULLY` message might trigger a reset of widget states on the frontend. """ if msg.WhichOneof("type") == "script_finished" and ( # If this is not a fragment run (= full app run), its okay to change the # script_finished type to FINISHED_EARLY_FOR_RERUN because another full app run # is about to start. # If this is a fragment run, it is allowed to change the state of # all script_finished states except for FINISHED_SUCCESSFULLY, which we use to # indicate that a full app run has finished successfully (in other words, a # fragment should not modify the finished status of a full app run, because # the fragment finished state is different and the frontend might not trigger # cleanups etc. correctly). is_fragment_run is False or msg.script_finished != ForwardMsg.ScriptFinishedStatus.FINISHED_SUCCESSFULLY ): msg.script_finished = ForwardMsg.ScriptFinishedStatus.FINISHED_EARLY_FOR_RERUN return msg