Convert not_implemented to use a warning subclass.

android/src/toga_android/factory.py

@@ -1,3 +1,5 @@
+from toga import NotImplementedWarning
+
 from . import dialogs
 from .app import App, MainWindow
 from .command import Command
@@ -31,7 +33,7 @@ from .window import Window
 
 
 def not_implemented(feature):
-    print(f"[Android] Not implemented: {feature}")  # pragma: nocover
+    NotImplementedWarning.warn("Android", feature)  # pragma: nocover
 
 
 __all__ = [

cocoa/src/toga_cocoa/factory.py

@@ -1,3 +1,5 @@
+from toga import NotImplementedWarning
+
 from . import dialogs
 from .app import App, DocumentApp, MainWindow
 from .command import Command
@@ -35,7 +37,7 @@ from .window import Window
 
 
 def not_implemented(feature):
-    print(f"[Cocoa] Not implemented: {feature}")  # pragma: nocover
+    NotImplementedWarning.warn("Cocoa", feature)  # pragma: nocover
 
 
 __all__ = [

core/src/toga/__init__.py

@@ -1,3 +1,5 @@
+import warnings
+
 from .app import App, DocumentApp, DocumentMainWindow, MainWindow
 
 # Resources
@@ -37,7 +39,19 @@ from .widgets.tree import Tree
 from .widgets.webview import WebView
 from .window import Window
 
+
+class NotImplementedWarning(RuntimeWarning):
+    # pytest.warns() requires that Warning() subclasses are constructed by passing a
+    # single argument (the warning message). Use a factory method to avoid reproducing
+    # the message format and the warn invocation.
+    @classmethod
+    def warn(self, platform, feature):
+        """Raise a warning that a feature isn't implemented on a platform."""
+        warnings.warn(NotImplementedWarning(f"[{platform}] Not implemented: {feature}"))
+
+
 __all__ = [
+    "NotImplementedWarning",
     # Applications
     "App",
     "DocumentApp",

dummy/src/toga_dummy/factory.py

@@ -1,3 +1,5 @@
+from toga import NotImplementedWarning
+
 from . import dialogs
 from .app import App, DocumentApp, MainWindow
 from .command import Command
@@ -36,7 +38,7 @@ from .window import Window
 
 
 def not_implemented(feature):
-    raise NotImplementedError()
+    NotImplementedWarning.warn("Dummy", feature)  # pragma: nocover
 
 
 __all__ = [

gtk/src/toga_gtk/factory.py

@@ -1,3 +1,5 @@
+from toga import NotImplementedWarning
+
 from . import dialogs
 from .app import App, DocumentApp, MainWindow
 from .command import Command
@@ -32,7 +34,7 @@ from .window import Window
 
 
 def not_implemented(feature):
-    print(f"[GTK+] Not implemented: {feature}")  # pragma: nocover
+    NotImplementedWarning.warn("GTK", feature)  # pragma: nocover
 
 
 __all__ = [

iOS/src/toga_iOS/factory.py

@@ -1,3 +1,5 @@
+from toga import NotImplementedWarning
+
 from . import dialogs
 from .app import App, MainWindow
 from .colors import native_color
@@ -36,7 +38,7 @@ from .window import Window
 
 
 def not_implemented(feature):
-    print(f"[iOS] Not implemented: {feature}")  # pragma: nocover
+    NotImplementedWarning.warn("iOS", feature)  # pragma: nocover
 
 
 __all__ = [

textual/src/toga_textual/factory.py

@@ -1,3 +1,5 @@
+from toga import NotImplementedWarning
+
 from . import dialogs
 from .app import App, DocumentApp, MainWindow
 
@@ -41,7 +43,7 @@ from .window import Window
 
 
 def not_implemented(feature):
-    print(f"[Textual] Not implemented: {feature}")  # pragma: nocover
+    NotImplementedWarning.warn("Textual", feature)  # pragma: nocover
 
 
 __all__ = [

web/src/toga_web/factory.py

@@ -1,3 +1,5 @@
+from toga import NotImplementedWarning
+
 from . import dialogs
 from .app import App, MainWindow  # DocumentApp
 from .command import Command
@@ -39,7 +41,7 @@ from .widgets.textinput import TextInput
 
 
 def not_implemented(feature):
-    print(f"[Web] Not implemented: {feature}")  # pragma: nocover
+    NotImplementedWarning.warn("Web", feature)  # pragma: nocover
 
 
 __all__ = [

winforms/src/toga_winforms/factory.py

@@ -1,3 +1,5 @@
+from toga import NotImplementedWarning
+
 from . import dialogs
 from .app import App, MainWindow
 from .command import Command
@@ -31,7 +33,7 @@ from .window import Window
 
 
 def not_implemented(feature):
-    print(f"[Winforms] Not implemented: {feature}")  # pragma: nocover
+    NotImplementedWarning.warn("Winforms", feature)  # pragma: nocover
 
 
 __all__ = [