plotly/dash v4.2.0 on GitHub

[4.2.0] - 2026-06-01 - The Freedom Update

This release marks a major milestone for Dash, bringing unprecedented flexibility to how you build and deploy your applications.

🚀 Multiple Backend Support

Dash is no longer tied to Flask. You can now run your Dash apps on FastAPI or Quart, or even bring your own backend implementation:

# FastAPI backend
app = Dash(__name__, backend="fastapi")

# Quart backend (async-native)
app = Dash(__name__, backend="quart")

# Or use an existing server
from fastapi import FastAPI
server = FastAPI()
app = Dash(__name__, server=server)

Install with pip install dash[fastapi] or pip install dash[quart].

⚡ Websocket Callbacks

Real-time, bidirectional communication is here. Websocket callbacks enable persistent connections for live updates without polling:

@callback(
    Output("live-output", "children"),
    Input("trigger", "n_clicks"),
    websocket=True,
    persistent=True
)
def live_updates(n_clicks):
    ws = ctx.websocket
    while True:
        data = fetch_live_data()
        ws.send(Output("live-output", "children", data))
        time.sleep(1)

🔓 Relaxed Pattern Matching Rules

MATCH wildcards are now allowed in Input and State even when your Output has no wildcards, making dynamic UIs simpler to build:

@callback(
    Output("summary", "children"),  # Fixed ID output
    Input({"type": "item", "index": MATCH}, "value")  # MATCH input - now allowed!
)
def update_summary(value):
    return f"Selected: {value}"

Added

#3783 Add batching/debouncing for websocket set_props messages to reduce lag when updating multiple components in a loop. Configurable via websocket_batch_delay (default 5ms, set to 0 to disable).
#3669 Selection for DataTable cleared with custom action settings.
#3680 Added search_order prop to Dropdown to allow users to preserve original option order during search.
#3745 Added csrf_token_name and csrf_header_name config options to allow configuring the CSRF cookie and header names. Fixes #729.
#3797 Improved websocket callback management with heartbeat configuration and proper reconnection handling.
#3523 Fall back to background callback function names if source cannot be found.
#3771 Add persistent callbacks and no inputs/no outputs callback support.
#3742 Add websocket callbacks to fastapi and quart backends.
#3737 Add displayNotifier to dcc.Graph config props.

Changed

#3746 Improve static typing for dash.callback by preserving wrapped callback signatures, and add callback typing coverage in compliance plus new callback decorator unit and integration tests. Fixes #3691.
Rename ctx.get_websocket to ctx.websocket.
Add threadpool for running websocket callbacks.

Fixed

#3690 Fix dcc.Input when min or max is set to None.
#3723 Fix misaligned dcc.Slider marks when some labels are empty strings.
#3738 Add missing stacklevel=2 to warnings.warn() calls so warnings report the caller's location instead of internal Dash source lines.
#3740 Fix cannot tab into dropdowns in Safari.
#3756 Allow MATCH in Input/State when the callback's Output has no wildcards (fixed-id Output, no Output, or ALL-only wildcard Output). ALLSMALLER still requires a corresponding MATCH in an Output. Fixes #2462.
#3768 Improved Dropdown search performance for large options lists.
#3759 Fix the issue where Patch objects cannot be updated via set_props() in websocket callback. Fix #3742.
#3789 Fixed extra wrapper in DatePickerRange and DatePickerSingle causing styling and layout issues.
#3799 Fixed dropdown crash when filtering large datasets with component-based labels by using stable item keys for virtualized rows.
#3785 Fix patch with dcc.Graph figure. Fix dcc.Graph not sending duplicate clicks because it had the same payload by adding a timestamp in the click event object.
#3798 Fix blueprint registering and double init when using flask run command. Fixes #3787.
#3734 Fix websocket used in the same FastAPI server. Fixes #3636.
#3668 Fix FastAPI url paths order. Fixes #3667.
#3641 Fix dcc.Loading spinner not triggering when callback Output uses the ALL wildcard. Fixes #3619.
#3570 Fix components not remounting when passed from a parent object. Fixes #3330.

plotly/dash v4.2.0 Dash Version 4.2.0 on GitHub