mount error (#3780)

* mount error

* changelog

CHANGELOG.md

@@ -17,6 +17,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 ### Added
 
 - Added support for Ctrl+Fn and Ctrl+Shift+Fn keys in urxvt https://github.com/Textualize/textual/pull/3737
+- Friendly error messages when trying to mount non-widgets https://github.com/Textualize/textual/pull/3780
 
 ## [0.43.2] - 2023-11-29
 

src/textual/_compose.py

@@ -17,27 +17,51 @@ def compose(node: App | Widget) -> list[Widget]:
         A list of widgets.
     """
     _rich_traceback_omit = True
+    from .widget import MountError, Widget
+
     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())
+    is_generator = hasattr(iter_compose, "throw")
     try:
         while True:
             try:
                 child = next(iter_compose)
             except StopIteration:
                 break
+
+            if not isinstance(child, Widget):
+                mount_error = MountError(
+                    f"Can't mount {type(child)}; expected a Widget instance."
+                )
+                if is_generator:
+                    iter_compose.throw(mount_error)  # type: ignore
+                else:
+                    raise mount_error from None
+
+            try:
+                child.id
+            except AttributeError:
+                mount_error = MountError(
+                    "Widget is missing an 'id' attribute; did you forget to call super().__init__()?"
+                )
+                if is_generator:
+                    iter_compose.throw(mount_error)  # type: ignore
+                else:
+                    raise mount_error from None
+
             if composed:
                 nodes.extend(composed)
                 composed.clear()
             if compose_stack:
                 try:
                     compose_stack[-1].compose_add_child(child)
                 except Exception as error:
-                    if hasattr(iter_compose, "throw"):
+                    if is_generator:
                         # So the error is raised inside the generator
                         # This will generate a more sensible traceback for the dev
                         iter_compose.throw(error)  # type: ignore

src/textual/pilot.py

@@ -328,6 +328,7 @@ async def _post_mouse_events(
             # the driver works and emits a click event.
             widget_at, _ = app.get_widget_at(*offset)
             event = mouse_event_cls(**message_arguments)
+            # Bypass event processing in App.on_event
             app.screen._forward_event(event)
             await self.pause()
 

src/textual/widget.py

@@ -807,7 +807,6 @@ def mount(
             Only one of ``before`` or ``after`` can be provided. If both are
             provided a ``MountError`` will be raised.
         """
-
         # Check for duplicate IDs in the incoming widgets
         ids_to_mount = [widget.id for widget in widgets if widget.id is not None]
         unique_ids = set(ids_to_mount)

tests/test_widget.py

@@ -410,3 +410,30 @@ def compose(self) -> ComposeResult:
     with pytest.raises(NotAContainer):
         async with app.run_test():
             pass
+
+
+async def test_mount_error_not_widget():
+    class NotWidgetApp(App):
+        def compose(self) -> ComposeResult:
+            yield {}
+
+    app = NotWidgetApp()
+    with pytest.raises(MountError):
+        async with app.run_test():
+            pass
+
+
+async def test_mount_error_bad_widget():
+    class DaftWidget(Widget):
+        def __init__(self):
+            # intentionally missing super()
+            pass
+
+    class NotWidgetApp(App):
+        def compose(self) -> ComposeResult:
+            yield DaftWidget()
+
+    app = NotWidgetApp()
+    with pytest.raises(MountError):
+        async with app.run_test():
+            pass