Make components always truthy

CHANGELOG.md

@@ -30,6 +30,7 @@ This project adheres to [Semantic Versioning](https://semver.org/).
 - [#3986](https://github.com/plotly/dash/pull/3986) Adjust `_run_before_hooks` in the `fastapi` backend to honor a response returned by a `before_request` function, matching the `flask` backend's behavior.
 
 ### Fixed
+- [#4078](https://github.com/plotly/dash/pull/4078) Components are now always truthy, so `if component:` and `component or default` no longer treat a component with no children (or no `children` prop, like `DataTable`) as false. With `use_pages=True`, an empty layout such as `html.Div()` still counts as unset and is replaced by `page_container`. Fixes [#2906](https://github.com/plotly/dash/issues/2906).
 - [#4075](https://github.com/plotly/dash/pull/4075) `fastapi` backend: run `before_request` hooks on every route of the Dash app (assets, hook routes, `api_endpoint`, pages, when mounted) and send a non-None hook result as the response, as on Flask. `fastapi` and `quart` also run them on the WebSocket handshake.
 - [#4075](https://github.com/plotly/dash/pull/4075) Background callbacks without `cache_by` no longer share a result between requests with equal inputs (unread Diskcache results now expire after a day, or after `expire` when set, even with `cache_by`), and callbacks with the same source no longer share a cache key. `build_cache_key` takes a new `callback_key` argument.
 - [#4075](https://github.com/plotly/dash/pull/4075) `dcc.Location(refresh=True).href`, `html.Area.href` and `html.Base.href` now block `javascript:` URLs like `html.A`.

dash/dash.py

@@ -42,7 +42,7 @@
     Output,
     State,
 )
-from .development.base_component import ComponentRegistry
+from .development.base_component import Component, ComponentRegistry
 from .exceptions import (
     ProxyError,
     DuplicateCallback,
@@ -247,6 +247,17 @@ async def execute_async_function(func, *args, **kwargs):
     return func(*args, **kwargs)
 
 
+def _layout_is_unset(layout):
+    """True when ``use_pages`` should fill in ``page_container`` for ``layout``.
+
+    Components are always truthy, so an empty component such as
+    ``html.Div()`` is checked by its length to keep it counting as unset.
+    """
+    if isinstance(layout, Component):
+        return len(layout) == 0
+    return not layout
+
+
 # pylint: disable=too-many-instance-attributes
 # pylint: disable=too-many-arguments, too-many-locals
 class Dash(ObsoleteChecker):
@@ -1891,7 +1902,7 @@ def _setup_server(self):
             if self.config.include_assets_files:
                 self._walk_assets_directory()
 
-            if not self.layout and self.use_pages:
+            if self.use_pages and _layout_is_unset(self.layout):
                 self.layout = page_container
 
             _validate.validate_layout(self.layout, self._layout_value())

dash/development/base_component.py

@@ -433,6 +433,11 @@ def __iter__(self):
         for t in self._traverse_ids():
             yield t.id  # type: ignore[reportAttributeAccessIssue]
 
+    def __bool__(self):
+        # Without this, truthiness falls back to __len__, so a component
+        # with no children (or no children prop, like DataTable) is falsy.
+        return True
+
     def __len__(self):
         """Return the number of items in the tree."""
         # TODO - Should we return the number of items that have IDs

tests/integration/multi_page/test_pages_layout.py

@@ -295,3 +295,21 @@ def test_pala007_app_title_discription(dash_duo, clear_pages_state):
     assert dash.page_registry["page1"]["title"] == "Page 1 Title"
     assert dash.page_registry["home"]["description"] == "App Description"
     assert dash.page_registry["page1"]["description"] == "Page 1 Description"
+
+
+def test_pala008_empty_layout_uses_page_container(dash_duo, clear_pages_state):
+    # An empty placeholder layout still counts as unset with use_pages, so
+    # Dash fills in page_container and the registered pages render.
+    # The pages folder has callbacks on ids that only exist in other tests'
+    # layouts, so callback exceptions are suppressed here.
+    app = Dash(__name__, use_pages=True, suppress_callback_exceptions=True)
+    dash.register_page(
+        "empty_layout_home",
+        layout=html.Div("Home for empty layout", id="contents"),
+        path="/",
+    )
+    app.layout = html.Div()
+
+    dash_duo.start_server(app)
+    dash_duo.wait_for_text_to_equal("#contents", "Home for empty layout")
+    assert dash_duo.get_logs() == []

tests/unit/development/test_base_component.py

@@ -6,7 +6,7 @@
 from dash import __version__, Dash
 from dash import html
 from dash.development.base_component import Component
-from dash import dcc, Input, Output
+from dash import dcc, dash_table, Input, Output
 
 Component._prop_names = ("id", "a", "children", "style")
 Component._type = "TestComponent"
@@ -539,3 +539,11 @@ def update2(v):
 def test_debc030_invalid_children_args():
     with pytest.raises(TypeError):
         dcc.Input(children="invalid children")
+
+
+def test_debc031_bool_is_always_true():
+    assert bool(Component())
+    assert bool(Component(children=[]))
+    assert bool(html.Div())
+    assert bool(dash_table.DataTable(data=[{"a": 1}]))
+    assert (html.Div() or None) is not None