test, fix invalid user-provided startup() method

android/src/toga_android/app.py

@@ -187,7 +187,7 @@ def create(self):
         # the app's `.native` is the listener's native Java class.
         self._listener = TogaApp(self)
         # Call user code to populate the main window
-        self.interface.startup()
+        self.interface._startup()
 
     def open_document(self, fileURL):
         print("Can't open document %s (yet)" % fileURL)

changes/2047.feature.rst

@@ -0,0 +1 @@
+Applications now verify that a main window has been created as part of the ``startup()`` method.

cocoa/src/toga_cocoa/app.py

@@ -255,7 +255,7 @@ def create(self):
         self._create_app_commands()
 
         # Call user code to populate the main window
-        self.interface.startup()
+        self.interface._startup()
 
         # Create the lookup table of menu items,
         # then force the creation of the menus.

core/src/toga/app.py

@@ -553,6 +553,19 @@ def startup(self):
 
         self.main_window.show()
 
+    def _startup(self):
+        # This is a wrapper around the user's startup method that performs any
+        # post-setup validation.
+        self.startup()
+        self._verify_startup()
+
+    def _verify_startup(self):
+        if not isinstance(self.main_window, MainWindow):
+            raise ValueError(
+                "Application does not have a main window. "
+                "Does your startup() method assign a value to self.main_window?"
+            )
+
     def about(self):
         """Display the About dialog for the app.
 
@@ -683,6 +696,10 @@ def __init__(
     def _create_impl(self):
         return self.factory.DocumentApp(interface=self)
 
+    def _verify_startup(self):
+        # No post-startup validation required for DocumentApps
+        pass
+
     @property
     def documents(self):
         """Return the list of documents associated with this app.

core/tests/test_app.py

@@ -172,6 +172,21 @@ async def test_handler(sender):
             args=(None,),
         )
 
+    def test_override_startup(self):
+        class BadApp(toga.App):
+            "A startup method that doesn't assign main window raises an error (#760)"
+
+            def startup(self):
+                # Override startup but don't create a main window
+                pass
+
+        app = BadApp(app_name="bad_app", formal_name="Bad Aoo", app_id="org.beeware")
+        with self.assertRaisesRegex(
+            ValueError,
+            r"Application does not have a main window.",
+        ):
+            app.main_loop()
+
 
 class DocumentAppTests(TestCase):
     def setUp(self):
@@ -191,3 +206,15 @@ def test_app_documents(self):
         doc = MagicMock()
         self.app._documents.append(doc)
         self.assertEqual(self.app.documents, [doc])
+
+    def test_override_startup(self):
+        mock = MagicMock()
+
+        class DocApp(toga.DocumentApp):
+            def startup(self):
+                # A document app doesn't have to provide a Main Window.
+                mock()
+
+        app = DocApp(app_name="docapp", formal_name="Doc App", app_id="org.beeware")
+        app.main_loop()
+        mock.assert_called_once()

dummy/src/toga_dummy/app.py

@@ -25,13 +25,15 @@ def __init__(self, interface):
 
     def create(self):
         self._action("create")
+        self.interface._startup()
 
     @not_required_on("mobile")
     def create_menus(self):
         self._action("create menus")
 
     def main_loop(self):
         self._action("main loop")
+        self.create()
 
     def set_main_window(self, window):
         self._set_value("main_window", window)

gtk/src/toga_gtk/app.py

@@ -95,7 +95,7 @@ def gtk_startup(self, data=None):
         )
         self._create_app_commands()
 
-        self.interface.startup()
+        self.interface._startup()
 
         # Create the lookup table of menu items,
         # then force the creation of the menus.

iOS/src/toga_iOS/app.py

@@ -65,7 +65,7 @@ def __init__(self, interface):
 
     def create(self):
         """Calls the startup method on the interface."""
-        self.interface.startup()
+        self.interface._startup()
 
     def open_document(self, fileURL):
         """Add a new document to this app."""

web/src/toga_web/app.py

@@ -37,7 +37,7 @@ def create(self):
         self.create_menus()
 
         # Call user code to populate the main window
-        self.interface.startup()
+        self.interface._startup()
 
     def _create_submenu(self, group, items):
         submenu = create_element(

winforms/src/toga_winforms/app.py

@@ -129,7 +129,7 @@ def create(self):
         self._create_app_commands()
 
         # Call user code to populate the main window
-        self.interface.startup()
+        self.interface._startup()
         self.create_menus()
         self.interface.main_window._impl.set_app(self)