Dash Version 4.4.0

Added

• #3826 WebSocket callback dispatch no longer lets long-lived callbacks limit the number of concurrent users. Async callbacks (including session-persistent ones) run directly on the connection event loop instead of occupying a worker thread, and synchronous callbacks run on a shared ThreadPoolExecutor whose size is configurable via the new websocket_max_workers argument to Dash (default 4). A synchronous persistent (no-output) callback now warns at registration since it would tie up a worker thread.

Fixed

• #3861 Fix synchronous WebSocket callbacks not inheriting ContextVar values bound by ASGI middleware. copy_context() is now captured on the event-loop thread before the callback is submitted to the thread pool, instead of inside the worker thread where only default values were visible.
• #3779 Fix dash.testing Browser.get_logs() returning None on non-Chrome webdrivers, which broke assertions like assert dash_duo.get_logs() == []. It now returns [], matching the Chrome code path.
• #3822 Fix UnboundLocalError for user_callback_output in async background callbacks (Celery and Diskcache managers) when the callback raises PreventUpdate or another exception before the variable is assigned.
• #3819 Fix RuntimeError: No active request in context when a non-Dash path falls through to the FastAPI catch-all route. Fixes #3812.
• #3838 Replace mcp dependency with inline types.
• #3824 Fix dash.testing ThreadedRunner.stop() hanging at teardown for Quart apps. Fixes #3823.
• #3425 dcc.DatePicker: Fix updatemode="bothdates" not always respected

Changed

• Drop support for Python 3.8 (end-of-life since October 2024). The minimum supported version is now Python 3.9.