Fix error handler and grouped outputs

CHANGELOG.md

@@ -7,6 +7,7 @@ This project adheres to [Semantic Versioning](https://semver.org/).
 ## Fixed
 
 - [#2987](https://github.com/plotly/dash/pull/2987) Fix multioutput requiring same number of no_update. Fixes [#2986](https://github.com/plotly/dash/issues/2986)
+- [2988](https://github.com/plotly/dash/pull/2988) Fix error handler and grouped outputs. Fixes [#2983](https://github.com/plotly/dash/issues/2983)
 
 ## Deprecated
 

dash/_callback.py

@@ -509,10 +509,7 @@ def add_context(*args, **kwargs):
 
                         # If the error returns nothing, automatically puts NoUpdate for response.
                         if output_value is None:
-                            if not multi:
-                                output_value = NoUpdate()
-                            else:
-                                output_value = [NoUpdate() for _ in output_spec]
+                            output_value = NoUpdate()
                     else:
                         raise err
 

tests/integration/callbacks/test_callback_error.py

@@ -10,9 +10,12 @@ def global_callback_error_handler(err):
     app.layout = [
         html.Button("start", id="start-local"),
         html.Button("start-global", id="start-global"),
+        html.Button("start-grouped", id="start-grouped"),
         html.Div(id="output"),
         html.Div(id="output-global"),
         html.Div(id="error-message"),
+        # test for #2983
+        html.Div("default-value", id="grouped-output"),
     ]
 
     def on_callback_error(err):
@@ -36,6 +39,14 @@ def on_start(_):
     def on_start_global(_):
         raise Exception("global error")
 
+    @app.callback(
+        output=dict(test=Output("grouped-output", "children")),
+        inputs=dict(start=Input("start-grouped", "n_clicks")),
+        prevent_initial_call=True,
+    )
+    def on_start_grouped(start=0):
+        raise Exception("grouped error")
+
     dash_duo.start_server(app)
     dash_duo.find_element("#start-local").click()
 
@@ -44,3 +55,9 @@ def on_start_global(_):
 
     dash_duo.find_element("#start-global").click()
     dash_duo.wait_for_text_to_equal("#output-global", "global: global error")
+
+    dash_duo.find_element("#start-grouped").click()
+    dash_duo.wait_for_text_to_equal("#output-global", "global: grouped error")
+    dash_duo.wait_for_text_to_equal("#grouped-output", "default-value")
+
+    assert dash_duo.get_logs() == []