AttributeError: 'NoneType' object has no attribute '_impl'

[UncleGoogle]

toga==0.3.0.dev14 on Windows10 gives me error in title.
Repro:

class MinimalApp(toga.App):

    def button_handler(widget):
        print("hello")

    def startup(self):
        box = toga.Box()

        button = toga.Button('Hello world', on_press=self.button_handler)
        button.style.padding = 50
        button.style.flex = 1
        box.add(button)

        return box


if __name__ == "__main__":

    app = MinimalApp('Name', 'com.hb.er')
    app.main_loop()

         self.interface.main_window._impl.native.Controls.Add(menubar)
AttributeError: 'NoneType' object has no attribute '_impl'

[UncleGoogle]

Ok, after looking into the code https://github.com/beeware/toga/blob/v0.3.0.dev14/src/core/toga/app.py
I see my mistake, I cannot overwrite startup method with the same as toga.App argument startup is. It does more (creates MainWindow). Documentation should warn about ithttps://toga.readthedocs.io/en/latest/reference/api/app.html

[freakboy3742]

Thanks for the report!

That's a good point - I can see how the difference between providing a startup() method as an argument, and overriding startup() on an instance is potentially confusing - definitely worth at least a clarification in the docs, if not a rethink of the naming of either the method or the argument.

[freakboy3742]

The solution here is much the same as the OP discovered. Startup needs to create a main window; your main window is created in the say_hello method, which isn't ever invoked.

You've also got the button handler for the Enviar button pointing back to the same method where the button is defined, which... is going to lead to some interesting recursive behavior.

On Toga's side, catching the "no main window" case would be a useful (and relatively straightforward) error to catch here.

[afmaster]

Thank you @freakboy3742 . I'm sorry for deletion of my post. I've deleted before refreshing the page and seeing your post. Thank you again.

[mattip]

Looking at this issue, it seems startup() is called in the platform-specific invocation of self.interface.startup() which does not expect a return value. One way to solve this would be to add a check that self.main_window is valid after the call. As I understand it, this would need to be done on each platform:

git grep "self.interface.startup()"
android/src/toga_android/app.py:        self.interface.startup()
cocoa/src/toga_cocoa/app.py:        self.interface.startup()
gtk/src/toga_gtk/app.py:        self.interface.startup()
iOS/src/toga_iOS/app.py:        self.interface.startup()
web/src/toga_web/app.py:        self.interface.startup()
winforms/src/toga_winforms/app.py:        self.interface.startup()