Fix multioutput requiring same number of no_update.

CHANGELOG.md

@@ -4,6 +4,10 @@ This project adheres to [Semantic Versioning](https://semver.org/).
 
 ## [UNRELEASED]
 
+## 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)
+
 ## Deprecated
 
 - [#2985](https://github.com/plotly/dash/pull/2985) Deprecate dynamic component loader.

dash/_callback.py

@@ -528,12 +528,16 @@ def add_context(*args, **kwargs):
                         # list or tuple
                         output_value = list(output_value)
 
-                    # Flatten grouping and validate grouping structure
-                    flat_output_values = flatten_grouping(output_value, output)
+                    if NoUpdate.is_no_update(output_value):
+                        flat_output_values = [output_value]
+                    else:
+                        # Flatten grouping and validate grouping structure
+                        flat_output_values = flatten_grouping(output_value, output)
 
-                _validate.validate_multi_return(
-                    output_spec, flat_output_values, callback_id
-                )
+                if not NoUpdate.is_no_update(output_value):
+                    _validate.validate_multi_return(
+                        output_spec, flat_output_values, callback_id
+                    )
 
                 for val, spec in zip(flat_output_values, output_spec):
                     if NoUpdate.is_no_update(val):

tests/integration/callbacks/test_missing_outputs.py

@@ -1,8 +1,10 @@
+import time
+
 import pytest
 from multiprocessing import Lock, Value
 
 import dash
-from dash import Dash, Input, Output, ALL, MATCH, html, dcc
+from dash import Dash, Input, Output, ALL, MATCH, html, dcc, no_update
 
 from dash.testing.wait import until
 
@@ -336,3 +338,34 @@ def chapter2_assertions():
     dash_duo._wait_for_callbacks()
     chapter2_assertions()
     assert not dash_duo.get_logs()
+
+
+def test_cbmo005_no_update_single_to_multi(dash_duo):
+    # Bugfix for #2986
+    app = dash.Dash(__name__)
+
+    app.layout = html.Div(
+        [
+            dcc.Input(id="input-box", type="text", value=""),
+            html.Button("Submit", id="button"),
+            html.Div(id="output-1", children="Output 1 will be displayed here"),
+            html.Div(id="output-2", children="Output 2 will be displayed here"),
+        ]
+    )
+
+    @app.callback(
+        Output("output-1", "children"),
+        Output("output-2", "children"),
+        Input("button", "n_clicks"),
+        Input("input-box", "value"),
+    )
+    def update_outputs(n_clicks, value):
+        if n_clicks is None:
+            return no_update
+        return "Hello", "world!"
+
+    dash_duo.start_server(app)
+
+    # just wait to make sure the error get logged.
+    time.sleep(1)
+    assert dash_duo.get_logs() == []

tests/integration/renderer/test_dependencies.py

@@ -2,7 +2,10 @@
 
 from dash import Dash, html, dcc, Input, Output
 
+from flaky import flaky
 
+
+@flaky(max_runs=3)
 def test_rddp001_dependencies_on_components_that_dont_exist(dash_duo):
     app = Dash(__name__, suppress_callback_exceptions=True)
     app.layout = html.Div(