error if no children allowed (#3758)

* error if no children allowed

* changelog

* changelog

* remove comment

* quote RHS

* annotations

* attempt to fix 3.7

* restore experiment

Author: willmcgugan

CHANGELOG.md

@@ -18,6 +18,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 - Added experimental Canvas class https://github.com/Textualize/textual/pull/3669/
 - Added `keyline` rule https://github.com/Textualize/textual/pull/3669/
+- Widgets can now have an ALLOW_CHILDREN (bool) classvar to disallow adding children to a widget https://github.com/Textualize/textual/pull/3758
 
 ### Changed
 

src/textual/_compose.py

@@ -16,19 +16,33 @@ def compose(node: App | Widget) -> list[Widget]:
     Returns:
         A list of widgets.
     """
+    _rich_traceback_omit = True
     app = node.app
     nodes: list[Widget] = []
     compose_stack: list[Widget] = []
     composed: list[Widget] = []
     app._compose_stacks.append(compose_stack)
     app._composed.append(composed)
+    iter_compose = iter(node.compose())
     try:
-        for child in node.compose():
+        while True:
+            try:
+                child = next(iter_compose)
+            except StopIteration:
+                break
             if composed:
                 nodes.extend(composed)
                 composed.clear()
             if compose_stack:
-                compose_stack[-1].compose_add_child(child)
+                try:
+                    compose_stack[-1].compose_add_child(child)
+                except Exception as error:
+                    if hasattr(iter_compose, "throw"):
+                        # So the error is raised inside the generator
+                        # This will generate a more sensible traceback for the dev
+                        iter_compose.throw(error)  # type: ignore
+                    else:
+                        raise
             else:
                 nodes.append(child)
         if composed:

src/textual/app.py

@@ -2305,6 +2305,7 @@ async def take_screenshot() -> None:
             )
 
     async def _on_compose(self) -> None:
+        _rich_traceback_omit = True
         try:
             widgets = [*self.screen._nodes, *compose(self)]
         except TypeError as error:

src/textual/scroll_view.py

@@ -18,6 +18,8 @@ class ScrollView(ScrollableContainer):
     on the compositor to render children).
     """
 
+    ALLOW_CHILDREN = False
+
     DEFAULT_CSS = """
     ScrollView {
         overflow-y: auto;

src/textual/widget.py

@@ -87,6 +87,10 @@
 }
 
 
+class NotAContainer(Exception):
+    """Exception raised if you attempt to add a child to a widget which doesn't permit child nodes."""
+
+
 _NULL_STYLE = Style()
 
 
@@ -264,6 +268,9 @@ class Widget(DOMNode):
     BORDER_SUBTITLE: ClassVar[str] = ""
     """Initial value for border_subtitle attribute."""
 
+    ALLOW_CHILDREN: ClassVar[bool] = True
+    """Set to `False` to prevent adding children to this widget."""
+
     can_focus: bool = False
     """Widget may receive focus."""
     can_focus_children: bool = True
@@ -488,6 +495,21 @@ def tooltip(self, tooltip: RenderableType | None):
         except NoScreen:
             pass
 
+    def compose_add_child(self, widget: Widget) -> None:
+        """Add a node to children.
+
+        This is used by the compose process when it adds children.
+        There is no need to use it directly, but you may want to override it in a subclass
+        if you want children to be attached to a different node.
+
+        Args:
+            widget: A Widget to add.
+        """
+        _rich_traceback_omit = True
+        if not self.ALLOW_CHILDREN:
+            raise NotAContainer(f"Can't add children to {type(widget)} widgets")
+        self._nodes._append(widget)
+
     def __enter__(self) -> Self:
         """Use as context manager when composing."""
         self.app._compose_stacks[-1].append(self)

src/textual/widgets/_button.py

@@ -153,6 +153,8 @@ class Button(Widget, can_focus=True):
 
     BINDINGS = [Binding("enter", "press", "Press Button", show=False)]
 
+    ALLOW_CHILDREN = False
+
     label: reactive[TextType] = reactive[TextType]("")
     """The text label that appears within the button."""
 

src/textual/widgets/_static.py

@@ -44,6 +44,8 @@ class Static(Widget, inherit_bindings=False):
     }
     """
 
+    ALLOW_CHILDREN = False
+
     _renderable: RenderableType
 
     def __init__(

src/textual/widgets/_toggle_button.py

@@ -51,6 +51,8 @@ class ToggleButton(Static, can_focus=True):
     | `toggle--label` | Targets the text label of the toggle button. |
     """
 
+    ALLOW_CHILDREN = False
+
     DEFAULT_CSS = """
     ToggleButton {
         width: auto;

tests/test_widget.py

@@ -9,8 +9,8 @@
 from textual.css.query import NoMatches
 from textual.geometry import Offset, Size
 from textual.message import Message
-from textual.widget import MountError, PseudoClasses, Widget
-from textual.widgets import Label, LoadingIndicator
+from textual.widget import MountError, NotAContainer, PseudoClasses, Widget
+from textual.widgets import Label, LoadingIndicator, Static
 
 
 @pytest.mark.parametrize(
@@ -394,3 +394,19 @@ class TestWidgetIsMountedApp(App):
         assert widget.is_mounted is False
         await pilot.app.mount(widget)
         assert widget.is_mounted is True
+
+
+async def test_not_allow_children():
+    """Regression test for https://github.com/Textualize/textual/pull/3758"""
+
+    class TestAppExpectFail(App):
+        def compose(self) -> ComposeResult:
+            # Statics don't have children, so this should error
+            with Static():
+                yield Label("foo")
+
+    app = TestAppExpectFail()
+
+    with pytest.raises(NotAContainer):
+        async with app.run_test():
+            pass