Better error message on no output callbacks exceptions

Author: T4rk1n

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

@@ -302,12 +302,19 @@ const observer: IStoreObserverDefinition<IStoreState> = {
             }
 
             if (error !== undefined) {
-                const outputs = payload
-                    ? map(combineIdAndProp, flatten([payload.outputs])).join(
-                          ', '
-                      )
-                    : output;
-                let message = `Callback error updating ${outputs}`;
+                let message;
+                if (cb.callback.no_output) {
+                    const inpts = keys(cb.changedPropIds).join(', ');
+                    message = `Callback error with no output from input ${inpts}`;
+                } else {
+                    const outputs = payload
+                        ? map(
+                              combineIdAndProp,
+                              flatten([payload.outputs])
+                          ).join(', ')
+                        : output;
+                    message = `Callback error updating ${outputs}`;
+                }
                 if (clientside_function) {
                     const {namespace: ns, function_name: fn} =
                         clientside_function;

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

@@ -14,6 +14,7 @@ export interface ICallbackDefinition {
     long?: LongCallbackInfo;
     dynamic_creator?: boolean;
     running: any;
+    no_output?: boolean;
 }
 
 export interface ICallbackProperty {

tests/integration/callbacks/test_arbitrary_callbacks.py

@@ -141,3 +141,27 @@ def on_click(n_clicks):
 
     dash_duo.wait_for_element("#click").click()
     dash_duo.wait_for_text_to_equal("#output", "Clicked 1 times")
+
+
+def test_arb005_no_output_error(dash_duo):
+    app = Dash()
+
+    app.layout = html.Div([html.Button("start", id="start")])
+
+    @app.callback(Input("start", "n_clicks"), prevent_initial_call=True)
+    def on_click(clicked):
+        return f"clicked {clicked}"
+
+    dash_duo.start_server(
+        app,
+        debug=True,
+        use_reloader=False,
+        use_debugger=True,
+        dev_tools_hot_reload=False,
+    )
+
+    dash_duo.wait_for_element("#start").click()
+    dash_duo.wait_for_text_to_equal(
+        ".dash-fe-error__title",
+        "Callback error with no output from input start.n_clicks",
+    )