Merge pull request #5588 from Textualize/preflight

Preflight

CHANGELOG.md

@@ -5,6 +5,12 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](http://keepachangelog.com/)
 and this project adheres to [Semantic Versioning](http://semver.org/).
 
+## Unreleased
+
+### Added
+
+- Added Widget.preflight_checks to perform some debug checks after a widget is instantiated, to catch common errors. https://github.com/Textualize/textual/pull/5588
+
 ## [2.1.2] - 2025-02-26
 
 ### Fixed

src/textual/app.py

@@ -1026,7 +1026,7 @@ def is_attached(self) -> bool:
     @property
     def debug(self) -> bool:
         """Is debug mode enabled?"""
-        return "debug" in self.features
+        return "debug" in self.features or constants.DEBUG
 
     @property
     def is_headless(self) -> bool:

src/textual/widget.py

@@ -651,6 +651,22 @@ def text_selection(self) -> Selection | None:
         """Text selection information, or `None` if no text is selected in this widget."""
         return self.screen.selections.get(self, None)
 
+    def preflight_checks(self) -> None:
+        """Called in debug mode to do preflight checks.
+
+        This is used by Textual to log some common errors, but you could implement this
+        in custom widgets to perform additional checks.
+
+        """
+
+        if hasattr(self, "CSS"):
+            from textual.screen import Screen
+
+            if not isinstance(self, Screen):
+                self.log.warning(
+                    f"'{self.__class__.__name__}.CSS' will be ignored (use 'DEFAULT_CSS' class variable for widgets)"
+                )
+
     def _cover(self, widget: Widget) -> None:
         """Set a widget used to replace the visuals of this widget (used for loading indicator).
 
@@ -1475,6 +1491,8 @@ def _post_register(self, app: App) -> None:
                 tie_breaker=tie_breaker,
                 scope=scope,
             )
+        if app.debug:
+            app.call_next(self.preflight_checks)
 
     def _get_box_model(
         self,