Raise an error if a widget tries to be its own parent

See #1062

Author: davep

src/textual/widget.py

@@ -189,6 +189,9 @@ class Widget(DOMNode):
     hover_style: Reactive[Style] = Reactive(Style, repaint=False)
     highlight_link_id: Reactive[str] = Reactive("")
 
+    class WidgetError(Exception):
+        """Base widget error."""
+
     def __init__(
         self,
         *children: Widget,
@@ -233,6 +236,10 @@ def __init__(
             id=id,
             classes=self.DEFAULT_CLASSES if classes is None else classes,
         )
+
+        if self in children:
+            raise self.WidgetError("A widget can't be its own parent")
+
         self._add_children(*children)
 
     virtual_size = Reactive(Size(0, 0), layout=True)
@@ -374,7 +381,7 @@ def _get_virtual_dom(self) -> Iterable[Widget]:
         if self._scrollbar_corner is not None:
             yield self._scrollbar_corner
 
-    class MountError(Exception):
+    class MountError(WidgetError):
         """Error raised when there was a problem with the mount request."""
 
     def _find_mount_point(self, spot: int | str | "Widget") -> tuple["Widget", int]:

tests/test_widget_mounting.py

@@ -1,14 +1,24 @@
 import pytest
 
 from textual.app import App
+from textual.widget import Widget
 from textual.widgets import Static
 
+class SelfOwn(Widget):
+    """Test a widget that tries to own itself."""
+    def __init__(self) -> None:
+        super().__init__(self)
+
 async def test_mount_via_app() -> None:
     """Perform mount tests via the app."""
 
     # Make a background set of widgets.
     widgets = [Static(id=f"starter-{n}") for n in range( 10 )]
 
+    async with App().run_test() as pilot:
+        with pytest.raises(Widget.WidgetError):
+            await pilot.app.mount(SelfOwn())
+
     async with App().run_test() as pilot:
         # Mount the first one and make sure it's there.
         await pilot.app.mount(widgets[0])