support named callback args not wrapped in a list

CHANGELOG.md

@@ -3,14 +3,13 @@ All notable changes to `dash` will be documented in this file.
 This project adheres to [Semantic Versioning](https://semver.org/).
 
 ## [UNRELEASED]
-### Changed
-- [#1368](https://github.com/plotly/dash/pull/1368) Updated pytest to v6.0.1. To avoid deprecation warnings, this also updated pytest-sugar to 0.9.4 and pytest-mock to 3.2.0. The pytest-mock update only effects python >= 3.0. Pytest-mock remains pinned at 2.0.0 for python == 2.7.
-
 ### Added
 - [#1355](https://github.com/plotly/dash/pull/1355) Removed redundant log message and consolidated logger initialization. You can now control the log level - for example suppress informational messages from Dash with `app.logger.setLevel(logging.WARNING)`.
 
 ### Changed
-- [#1180](https://github.com/plotly/dash/pull/1180) `Input`, `Output`, and `State` in callback definitions don't need to be in lists. You still need to provide `Output` items first, then `Input` items, then `State`, and the list form is still supported. In particular, if you want to return a single output item wrapped in a length-1 list, you should still wrap the `Output` in a list. This can be useful for procedurally-generated callbacks.
+- [#1180](https://github.com/plotly/dash/pull/1180) and [#1375](https://github.com/plotly/dash/pull/1375) `Input`, `Output`, and `State` in callback definitions don't need to be in lists. You still need to provide `Output` items first, then `Input` items, then `State`, and the list form is still supported. In particular, if you want to return a single output item wrapped in a length-1 list, you should still wrap the `Output` in a list. This can be useful for procedurally-generated callbacks.
+- [#1368](https://github.com/plotly/dash/pull/1368) Updated pytest to v6.0.1. To avoid deprecation warnings, this also updated pytest-sugar to 0.9.4 and pytest-mock to 3.2.0. The pytest-mock update only effects python >= 3.0. Pytest-mock remains pinned at 2.0.0 for python == 2.7.
+
 
 ## [1.14.0] - 2020-07-27
 ### Added

dash/dependencies.py

@@ -140,7 +140,13 @@ def __repr__(self):
 def extract_callback_args(args, kwargs, name, type_):
     """Extract arguments for callback from a name and type"""
     parameters = kwargs.get(name, [])
-    if not parameters:
+    if parameters:
+        if not isinstance(parameters, (list, tuple)):
+            # accept a single item, not wrapped in a list, for any of the
+            # categories as a named arg (even though previously only output
+            # could be given unwrapped)
+            return [parameters]
+    else:
         while args and isinstance(args[0], type_):
             parameters.append(args.pop(0))
     return parameters
@@ -163,6 +169,9 @@ def handle_callback_args(args, kwargs):
     if len(outputs) == 1:
         out0 = kwargs.get("output", args[0] if args else None)
         if not isinstance(out0, (list, tuple)):
+            # unless it was explicitly provided as a list, a single output
+            # sholuld be unwrapped. That ensures the return value of the
+            # callback is also not expected to be wrapped in a list.
             outputs = outputs[0]
 
     inputs = extract_callback_args(flat_args, kwargs, "inputs", Input)

tests/integration/callbacks/test_validation.py

@@ -144,3 +144,52 @@ def o2(i):
     dash_duo.start_server(app)
     dash_duo.wait_for_text_to_equal("#out1", "1: Hi")
     dash_duo.wait_for_text_to_equal("#out2", "2: Hi")
+
+
+@pytest.mark.parametrize("named_out", [True, False])
+@pytest.mark.parametrize("named_in", [True, False])
+@pytest.mark.parametrize("named_state", [True, False])
+def test_cbva004_named_args(named_out, named_in, named_state, dash_duo):
+    app = Dash(__name__)
+    app.layout = html.Div(
+        [
+            html.Div("Hi", id="in"),
+            html.Div("gh", id="state"),
+            html.Div(id="out1"),
+            html.Div(id="out2"),
+        ]
+    )
+
+    def make_args(*a):
+        args = []
+        kwargs = {}
+        names = ["output", "inputs", "state"]
+        flags = [named_out, named_in, named_state]
+        for ai, name, flag in zip(a, names, flags):
+            if flag:
+                kwargs[name] = ai
+            else:
+                args.append(ai)
+        return args, kwargs
+
+    args, kwargs = make_args(
+        Output("out1", "children"), Input("in", "children"), State("state", "children")
+    )
+
+    @app.callback(*args, **kwargs)
+    def o1(i, s):
+        return "1: " + i + s
+
+    args, kwargs = make_args(
+        [Output("out2", "children")],
+        [Input("in", "children")],
+        [State("state", "children")],
+    )
+
+    @app.callback(*args, **kwargs)
+    def o2(i, s):
+        return ("2: " + i + s,)
+
+    dash_duo.start_server(app)
+    dash_duo.wait_for_text_to_equal("#out1", "1: High")
+    dash_duo.wait_for_text_to_equal("#out2", "2: High")