Support for callbacks with no outputs

dash/_callback.py

@@ -62,6 +62,7 @@ def callback(
     cancel=None,
     manager=None,
     cache_args_to_ignore=None,
+    force_no_output=False,
     **_kwargs,
 ):
     """
@@ -183,6 +184,7 @@ def callback(
         *_args,
         **_kwargs,
         long=long_spec,
+        force_no_output=force_no_output,
         manager=manager,
     )
 
@@ -224,6 +226,7 @@ def insert_callback(
     long=None,
     manager=None,
     dynamic_creator=False,
+    force_no_output=False
 ):
     if prevent_initial_call is None:
         prevent_initial_call = config_prevent_initial_callbacks
@@ -246,6 +249,7 @@ def insert_callback(
             "interval": long["interval"],
         },
         "dynamic_creator": dynamic_creator,
+        "force_no_output":force_no_output,
     }
 
     callback_map[callback_id] = {
@@ -286,6 +290,7 @@ def register_callback(  # pylint: disable=R0914
     long = _kwargs.get("long")
     manager = _kwargs.get("manager")
     allow_dynamic_callbacks = _kwargs.get("_allow_dynamic_callbacks")
+    force_no_output = _kwargs.get("force_no_output")
 
     output_indices = make_grouping_by_index(output, list(range(grouping_len(output))))
     callback_id = insert_callback(
@@ -301,6 +306,7 @@ def register_callback(  # pylint: disable=R0914
         long=long,
         manager=manager,
         dynamic_creator=allow_dynamic_callbacks,
+        force_no_output=force_no_output
     )
 
     # pylint: disable=too-many-locals
@@ -454,8 +460,11 @@ def add_context(*args, **kwargs):
                     output_value = list(output_value)
 
                 # Flatten grouping and validate grouping structure
-                flat_output_values = flatten_grouping(output_value, output)
-
+                if len(output):
+                    flat_output_values = flatten_grouping(output_value, output)
+                else:
+                    # for no output callback
+                    flat_output_values = []
             _validate.validate_multi_return(
                 output_spec, flat_output_values, callback_id
             )

dash/_utils.py

@@ -150,6 +150,12 @@ def _concat(x):
         return _id
 
     if isinstance(output, (list, tuple)):
+        if len(output) == 0:
+            if not hashed_inputs:
+                hashed_inputs = hashlib.md5(
+                    ".".join(str(x) for x in inputs).encode("utf-8")
+                ).hexdigest()
+            return ".."+hashed_inputs+".."
         return ".." + "...".join(_concat(x) for x in output) + ".."
 
     return _concat(output)

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

@@ -586,7 +586,7 @@ export function executeCallback(
     dispatch: any,
     getState: any
 ): IExecutingCallback {
-    const {output, inputs, state, clientside_function, long, dynamic_creator} =
+    const {output, inputs, state, clientside_function, long, dynamic_creator, force_no_output} =
         cb.callback;
     try {
         const inVals = fillVals(paths, layout, cb, inputs, 'Input', true);
@@ -636,6 +636,7 @@ export function executeCallback(
                     outputs: isMultiOutputProp(output) ? outputs : outputs[0],
                     inputs: inVals,
                     changedPropIds: keys(cb.changedPropIds),
+                    force_no_output: force_no_output,
                     state: cb.callback.state.length
                         ? fillVals(paths, layout, cb, state, 'State')
                         : undefined

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

@@ -187,14 +187,16 @@ function validateDependencies(parsedDependencies, dispatchError) {
     const outObjs = [];
 
     parsedDependencies.forEach(dep => {
-        const {inputs, outputs, state} = dep;
+        const {inputs, outputs, state, force_no_output} = dep;
         let hasOutputs = true;
         if (outputs.length === 1 && !outputs[0].id && !outputs[0].property) {
             hasOutputs = false;
-            dispatchError('A callback is missing Outputs', [
-                'Please provide an output for this callback:',
-                JSON.stringify(dep, null, 2)
-            ]);
+            if (!force_no_output) {
+                dispatchError('A callback is missing Outputs', [
+                    'Please provide an output for this callback:',
+                    JSON.stringify(dep, null, 2)
+                ]);
+            }
         }
 
         const head =
@@ -234,16 +236,18 @@ function validateDependencies(parsedDependencies, dispatchError) {
                 ]);
             }
             args.forEach((idProp, i) => {
-                validateArg(idProp, head, cls, i, dispatchError);
+                validateArg(idProp, head, cls, i, dispatchError, force_no_output);
             });
         });
 
         findDuplicateOutputs(outputs, head, dispatchError, outStrs, outObjs);
-        findMismatchedWildcards(outputs, inputs, state, head, dispatchError);
+        if (!force_no_output) {
+            findMismatchedWildcards(outputs, inputs, state, head, dispatchError);
+        }
     });
 }
 
-function validateArg({id, property}, head, cls, i, dispatchError) {
+function validateArg({id, property}, head, cls, i, dispatchError, force_no_output) {
     if (typeof property !== 'string' || !property) {
         dispatchError('Callback property error', [
             head,
@@ -253,7 +257,7 @@ function validateArg({id, property}, head, cls, i, dispatchError) {
     }
 
     if (typeof id === 'object') {
-        if (isEmpty(id)) {
+        if (isEmpty(id) && !force_no_output) {
             dispatchError('Callback item missing ID', [
                 head,
                 `${cls}[${i}].id = {}`,
@@ -290,7 +294,7 @@ function validateArg({id, property}, head, cls, i, dispatchError) {
             }
         }, id);
     } else if (typeof id === 'string') {
-        if (!id) {
+        if (!id && !force_no_output) {
             dispatchError('Callback item missing ID', [
                 head,
                 `${cls}[${i}].id = "${id}"`,
@@ -605,12 +609,16 @@ export function computeGraphs(dependencies, dispatchError) {
 
     const fixIds = map(evolve({id: parseIfWildcard}));
     const parsedDependencies = map(dep => {
-        const {output} = dep;
+        const {output, force_no_output} = dep;
         const out = evolve({inputs: fixIds, state: fixIds}, dep);
-        out.outputs = map(
-            outi => assoc('out', true, splitIdAndProp(outi)),
-            isMultiOutputProp(output) ? parseMultipleOutputs(output) : [output]
-        );
+        if(!force_no_output){
+            out.outputs = map(
+                outi => assoc('out', true, splitIdAndProp(outi)),
+                isMultiOutputProp(output) ? parseMultipleOutputs(output) : [output]
+            );
+        } else {
+            out.outputs = [];
+        }
         return out;
     }, dependencies);
 
@@ -809,7 +817,7 @@ export function computeGraphs(dependencies, dispatchError) {
         // Also collect MATCH keys in the output (all outputs must share these)
         // and ALL keys in the first output (need not be shared but we'll use
         // the first output for calculations) for later convenience.
-        const {matchKeys} = findWildcardKeys(outputs[0].id);
+        const {matchKeys} = findWildcardKeys(outputs.length ? outputs[0].id : undefined);
         const firstSingleOutput = findIndex(o => !isMultiValued(o.id), outputs);
         const finalDependency = mergeRight(
             {matchKeys, firstSingleOutput, outputs},
@@ -1091,7 +1099,7 @@ export function addAllResolvedFromOutputs(resolve, paths, matches) {
             }
         } else {
             const cb = makeResolvedCallback(callback, resolve, '');
-            if (flatten(cb.getOutputs(paths)).length) {
+            if (flatten(cb.getOutputs(paths)).length || callback.force_no_output) {
                 matches.push(cb);
             }
         }

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

@@ -13,6 +13,7 @@ export interface ICallbackDefinition {
     state: ICallbackProperty[];
     long?: LongCallbackInfo;
     dynamic_creator?: boolean;
+    force_no_output: boolean;
 }
 
 export interface ICallbackProperty {
@@ -74,6 +75,7 @@ export interface ICallbackPayload {
     output: string;
     outputs: any[];
     state?: any[] | null;
+    force_no_output: boolean;
 }
 
 export type CallbackResult = {

dash/dash-renderer/webpack.base.config.js

@@ -31,7 +31,7 @@ const defaults = {
             {
                 test: /\.ts(x?)$/,
                 exclude: /node_modules/,
-                use: ['babel-loader', 'ts-loader'],
+                use: ['babel-loader'],
             },
             {
                 test: /\.css$/,

dash/dash.py

@@ -1233,9 +1233,13 @@ def dispatch(self):
             "state", []
         )
         output = body["output"]
-        outputs_list = body.get("outputs") or split_callback_id(output)
-        g.outputs_list = outputs_list  # pylint: disable=assigning-non-slot
-
+
+        if body["force_no_output"]:
+            outputs_list = []
+            g.outputs_list = outputs_list
+        else:
+            outputs_list = body.get("outputs") or split_callback_id(output)
+            g.outputs_list = outputs_list  # pylint: disable=assigning-non-slot
         g.input_values = (  # pylint: disable=assigning-non-slot
             input_values
         ) = inputs_to_dict(inputs)
@@ -1291,15 +1295,18 @@ def dispatch(self):
                 flat_outputs = [outputs_list]
             else:
                 flat_outputs = outputs_list
-
-            outputs_grouping = map_grouping(
-                lambda ind: flat_outputs[ind], outputs_indices
-            )
-            g.outputs_grouping = outputs_grouping  # pylint: disable=assigning-non-slot
-            g.using_outputs_grouping = (  # pylint: disable=assigning-non-slot
-                not isinstance(outputs_indices, int)
-                and outputs_indices != list(range(grouping_len(outputs_indices)))
-            )
+            if len(flat_outputs):
+                outputs_grouping = map_grouping(
+                    lambda ind: flat_outputs[ind], outputs_indices
+                )
+                g.outputs_grouping = outputs_grouping  # pylint: disable=assigning-non-slot
+                g.using_outputs_grouping = (  # pylint: disable=assigning-non-slot
+                    not isinstance(outputs_indices, int)
+                    and outputs_indices != list(range(grouping_len(outputs_indices)))
+                )
+            else:
+                g.outputs_grouping = []
+                g.using_outputs_grouping = []
 
         except KeyError as missing_callback_function:
             msg = f"Callback function not found for output '{output}', perhaps you forgot to prepend the '@'?"