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 @@
+Verify that ``app.startup`` creates a ``MainWindow``

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):
+        """Since startup can and is overridden by users, verify
+        they did the right thing in their code
+        """
+        self.startup()
+        self._verify_startup()
+
+    def _verify_startup(self):
+        if not isinstance(self.main_window, MainWindow):
+            raise ValueError(
+                "user-defined startup() did not instaniate 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):
+        """The class 'startup()' method has no restrictions"""
+        pass
+
     @property
     def documents(self):
         """Return the list of documents associated with this app.

core/tests/test_app.py

@@ -173,6 +173,20 @@ async def test_handler(sender):
         )
 
 
+class BadAppTests(TestCase):
+    def test_override_startup(self):
+        class BadApp(toga.App):
+            # issue 760. A bogus startup function should raise
+            def startup(self):
+                # This is bogus, it does not create the main window
+                pass
+
+        msg = "did not instaniate"
+        app = BadApp(app_name="pytest", formal_name="pytest", app_id="org.beeware")
+        with self.assertRaisesRegex(ValueError, msg):
+            app.main_loop()
+
+
 class DocumentAppTests(TestCase):
     def setUp(self):
         super().setUp()

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)