Improve error message when a platform doesn't support a widget.

android/src/toga_android/factory.py

@@ -62,3 +62,7 @@ def not_implemented(feature):
     "Paths",
     "dialogs",
 ]
+
+
+def __getattr__(name):  # pragma: no cover
+    raise NotImplementedError(f"Toga's Android backend doesn't implement {name}")

changes/1992.feature.rst

@@ -0,0 +1 @@
+A more informative error message is returned when a platform backend doesn't support a widget.

cocoa/src/toga_cocoa/factory.py

@@ -77,3 +77,7 @@ def not_implemented(feature):
     "WebView",
     "Window",
 ]
+
+
+def __getattr__(name):  # pragma: no cover
+    raise NotImplementedError(f"Toga's Cocoa backend doesn't implement {name}")

gtk/src/toga_gtk/factory.py

@@ -73,3 +73,7 @@ def not_implemented(feature):
     "WebView",
     "Window",
 ]
+
+
+def __getattr__(name):  # pragma: no cover
+    raise NotImplementedError(f"Toga's GTK backend doesn't implement {name}")

iOS/src/toga_iOS/factory.py

@@ -72,3 +72,7 @@ def not_implemented(feature):
     "WebView",
     "Window",
 ]
+
+
+def __getattr__(name):  # pragma: no cover
+    raise NotImplementedError(f"Toga's iOS backend doesn't implement {name}")

web/src/toga_web/factory.py

@@ -75,3 +75,7 @@ def not_implemented(feature):
     # 'WebView',
     # 'Window',
 ]
+
+
+def __getattr__(name):  # pragma: no cover
+    raise NotImplementedError(f"Toga's Web backend doesn't implement {name}")

winforms/src/toga_winforms/factory.py

@@ -72,3 +72,7 @@ def not_implemented(feature):
     "WebView",
     "Window",
 ]
+
+
+def __getattr__(name):  # pragma: no cover
+    raise NotImplementedError(f"Toga's Winforms backend doesn't implement {name}")