[4.2] Persistent callbacks

.ai/ARCHITECTURE.md

@@ -963,7 +963,7 @@ Renderer                   SharedWorker                 Server
 
 ### Long-Running Callbacks with set_props/get_props
 
-WebSocket callbacks can stream updates to the client during execution using `set_props()` and read current component values using `ctx.get_websocket()`:
+WebSocket callbacks can stream updates to the client during execution using `set_props()` and read current component values using `ctx.websocket`:
 
 ```python
 import asyncio
@@ -975,7 +975,7 @@ from dash import callback, Output, Input, set_props, ctx
     prevent_initial_call=True
 )
 async def long_running_task(n_clicks):
-    ws = ctx.get_websocket()
+    ws = ctx.websocket
     if not ws:
         return "WebSocket not available"
 
@@ -993,7 +993,7 @@ async def long_running_task(n_clicks):
 
 **API:**
 - `set_props(component_id, props_dict)` - Stream prop updates immediately to client
-- `ctx.get_websocket()` - Get WebSocket interface (returns `None` if not in WS context)
+- `ctx.websocket` - Get WebSocket interface (returns `None` if not in WS context)
 - `await ws.get_prop(component_id, prop_name)` - Read current prop value from client
 - `await ws.set_prop(component_id, prop_name, value)` - Set single prop (async version)
 - `await ws.close(code, reason)` - Close the WebSocket connection

dash/_callback.py

@@ -78,6 +78,7 @@ def callback(
     optional: Optional[bool] = False,
     hidden: Optional[bool] = None,
     websocket: Optional[bool] = False,
+    persistent: Optional[bool] = False,
     **_kwargs,
 ) -> Callable[..., Any]:
     """
@@ -172,6 +173,10 @@ def callback(
             The endpoint is relative to the Dash app's base URL.
             Note that the endpoint will not appear in the list of registered
             callbacks in the Dash devtools.
+        :param persistent:
+            If True, this callback will not show the "Updating..." title while
+            running. Useful for persistent WebSocket callbacks that stay active
+            for long periods without requiring a loading indicator.
     """
 
     background_spec: Any = None
@@ -230,6 +235,7 @@ def callback(
         optional=optional,
         hidden=hidden,
         websocket=websocket,
+        persistent=persistent,
     )
 
 
@@ -278,6 +284,7 @@ def insert_callback(
     optional=False,
     hidden=None,
     websocket=False,
+    persistent=False,
 ) -> str:
     if prevent_initial_call is None:
         prevent_initial_call = config_prevent_initial_callbacks
@@ -304,6 +311,7 @@ def insert_callback(
         "optional": optional,
         "hidden": hidden,
         "websocket": websocket,
+        "persistent": persistent,
     }
     if running:
         callback_spec["running"] = running
@@ -658,6 +666,7 @@ def register_callback(
         optional=_kwargs.get("optional", False),
         hidden=_kwargs.get("hidden", None),
         websocket=_kwargs.get("websocket", False),
+        persistent=_kwargs.get("persistent", False),
     )
 
     # pylint: disable=too-many-locals

dash/_callback_context.py

@@ -328,7 +328,7 @@ def custom_data(self):
 
     @property
     @has_context
-    def get_websocket(self) -> typing.Optional[DashWebsocketCallback]:
+    def websocket(self) -> typing.Optional[DashWebsocketCallback]:
         """Get WebSocket interface if running in WebSocket context.
 
         Returns the DashWebsocketCallback instance if the callback is being

dash/_utils.py

@@ -165,6 +165,20 @@ def _concat(x):
 
     if no_output:
         # No output will hash the inputs.
+        # For no-input callbacks, also include the call site to make each unique
+        if not inputs:
+            # Get the call site of the @callback decorator
+            stack = inspect.stack()
+            # Walk up the stack to find the actual callback call site
+            # (skip internal dash package frames)
+            dash_package_path = os.path.dirname(__file__)
+            for frame_info in stack:
+                # Skip frames from within the dash package itself
+                if not frame_info.filename.startswith(dash_package_path):
+                    call_site = f"{frame_info.filename}:{frame_info.lineno}"
+                    return hashlib.sha256(call_site.encode("utf-8")).hexdigest()
+            # Fallback to empty hash if no external frame found
+            return _hash_inputs()
         return _hash_inputs()
 
     if isinstance(output, (list, tuple)):

dash/dash-renderer/src/actions/dependencies.js

@@ -224,14 +224,17 @@ function validateDependencies(parsedDependencies, dispatchError) {
             'In the callback for output(s):\n  ' +
             outputs.map(combineIdAndProp).join('\n  ');
 
-        if (!inputs.length) {
+        if (!inputs.length && dep.prevent_initial_call) {
             dispatchError('A callback is missing Inputs', [
                 head,
                 'there are no `Input` elements.',
                 'Without `Input` elements, it will never get called.',
                 '',
                 'Subscribing to `Input` components will cause the',
-                'callback to be called whenever their values change.'
+                'callback to be called whenever their values change.',
+                '',
+                'If you want a callback without inputs that fires on initial load,',
+                'set prevent_initial_call=False.'
             ]);
         }
 

dash/dash-renderer/src/actions/dependencies_ts.ts

@@ -352,12 +352,18 @@ export const getLayoutCallbacks = (
 
 export const getUniqueIdentifier = ({
     anyVals,
-    callback: {inputs, outputs, state}
-}: ICallback): string =>
-    concat(
-        map(combineIdAndProp, [...inputs, ...outputs, ...state]),
+    callback: {inputs, outputs, state, output}
+}: ICallback): string => {
+    const idParts = map(combineIdAndProp, [...inputs, ...outputs, ...state]);
+    // For no-output callbacks, include the output hash to ensure uniqueness
+    if (outputs.length === 0 && output) {
+        idParts.push(output);
+    }
+    return concat(
+        idParts,
         Array.isArray(anyVals) ? anyVals : anyVals === '' ? [] : [anyVals]
     ).join(',');
+};
 
 export function includeObservers(
     id: any,

dash/dash-renderer/src/actions/index.js

@@ -5,7 +5,12 @@ import {getAppState} from '../reducers/constants';
 import {getAction} from './constants';
 import * as cookie from 'cookie';
 import {validateCallbacksToLayout} from './dependencies';
-import {includeObservers, getLayoutCallbacks} from './dependencies_ts';
+import {
+    includeObservers,
+    getLayoutCallbacks,
+    makeResolvedCallback,
+    resolveDeps
+} from './dependencies_ts';
 import {computePaths, getPath} from './paths';
 import {recordUiEdit} from '../persistence';
 
@@ -95,12 +100,62 @@ function triggerDefaultState(dispatch, getState) {
         );
     }
 
-    dispatch(
-        addRequestedCallbacks(
-            getLayoutCallbacks(graphs, paths, layout.components, {
-                outputsOnly: true
-            })
+    const layoutCallbacks = getLayoutCallbacks(
+        graphs,
+        paths,
+        layout.components,
+        {
+            outputsOnly: true
+        }
+    );
+
+    // Also include no-output callbacks whose inputs are in the layout (or have no inputs)
+    const noOutputCallbacks = (graphs.callbacks || [])
+        .filter(cb => cb.noOutput && !cb.prevent_initial_call)
+        .map(cb => {
+            const resolved = makeResolvedCallback(cb, resolveDeps(), '');
+            resolved.initialCall = true;
+            return resolved;
+        })
+        .filter(cb => {
+            // If no inputs, always include (fires once on initial load)
+            if (cb.callback.inputs.length === 0) {
+                return true;
+            }
+            // Check if any input is in the layout
+            const inputs = cb.getInputs(paths);
+            return inputs.some(inp =>
+                Array.isArray(inp) ? inp.length > 0 : inp
+            );
+        });
+
+    // Also include no-input callbacks (with outputs) that should fire on initial load
+    const noInputCallbacks = (graphs.callbacks || [])
+        .filter(
+            cb =>
+                !cb.noOutput &&
+                cb.inputs.length === 0 &&
+                !cb.prevent_initial_call
         )
+        .map(cb => {
+            const resolved = makeResolvedCallback(cb, resolveDeps(), '');
+            resolved.initialCall = true;
+            return resolved;
+        })
+        .filter(cb => {
+            // Check if any output is in the layout
+            const outputs = cb.getOutputs(paths);
+            return outputs.some(out =>
+                Array.isArray(out) ? out.length > 0 : out
+            );
+        });
+
+    dispatch(
+        addRequestedCallbacks([
+            ...layoutCallbacks,
+            ...noOutputCallbacks,
+            ...noInputCallbacks
+        ])
     );
 }
 

dash/dash-renderer/src/observers/isLoading.ts

@@ -9,7 +9,12 @@ const observer: IStoreObserverDefinition<IStoreState> = {
 
         const pendingCallbacks = getPendingCallbacks(callbacks);
 
-        const next = Boolean(pendingCallbacks.length);
+        // Filter out persistent callbacks - they shouldn't trigger the loading indicator
+        const nonPersistentCallbacks = pendingCallbacks.filter(
+            cb => !cb.callback.persistent
+        );
+
+        const next = Boolean(nonPersistentCallbacks.length);
 
         if (isLoading !== next) {
             dispatch(setIsLoading(next));

dash/dash-renderer/src/types/callbacks.ts

@@ -16,6 +16,7 @@ export interface ICallbackDefinition {
     running: any;
     no_output?: boolean;
     websocket?: boolean;
+    persistent?: boolean;
 }
 
 export interface ICallbackProperty {