No error checking on function-based app startup

[tleeuwenburg]

This was originally reported as an app startup error; see this comment below for the diagnosis and proposed fix for the problem

---

Describe the bug

I am experiencing a null pointer error, stack trace below.

I'm a toga newbie, so I'm not sure how to create a simple reproduction example for this error. I'm not sure how to build toga for myself in order to get in and debug what's going on either. I am presuming the issue is most likely in my own code, but the error message isn't very helpful in identifying which part of my code is at fault. I don't know what information is helpful to the team in tracking this one down.

Exception ignored on calling ctypes callback function: <function CFTimerHandle._cf_timer_callback.<locals>.cf_timer_callback at 0x1139d58a0>
Traceback (most recent call last):
  File "/<path>/env/name/lib/python3.11/site-packages/rubicon/objc/eventloop.py", line 183, in cf_timer_callback
    callback(*args)
  File "<path>/env/name/lib/python3.11/site-packages/rubicon/objc/eventloop.py", line 334, in _callback
    context.run(callback, *args)
  File "<path>/env/name/lib/python3.11/site-packages/toga/app.py", line 630, in _startup
    self.startup()
  File "<path>/env/name/lib/python3.11/site-packages/toga/app.py", line 672, in startup
    self.main_window.content = self._startup_method(self)
    ^^^^^^^^^^^^^^^^^^^^^^^^
  File "<path>/dev/env/name/lib/python3.11/site-packages/toga/window.py", line 407, in content
    widget.app = self.app
    ^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'app'

Steps to reproduce

I'm just running a fairly simple GUI app. This message appears in the terminal when I do so. Actual GUI functionality seems fine, but I'm just at an early prototyping stage at the moment. My code probably contains some bugs.

Expected behavior

The error message should make more sense.

Screenshots

No response

Environment

• Operating System: macos
• Python version: 3.11.11
• Software versions:
  • Briefcase: n/a
  • Toga: 0.5.0
  • ...

[freakboy3742]

Thanks for the report - the most useful debugging detail at this point is a reproduction case. You say a "simple app"... but how simple? Are you seeing this on an empty app? What do you need to add to a Hello World app to trigger the error?

[tleeuwenburg]

import toga
from toga.style.pack import COLUMN, ROW, Pack

class GUI:
    def __init__(self):
        self.app = toga.App("Show issue", "noname.ShowIssue", startup=self.startup)
        self.main_window = None
    def main_loop(self):
        self.app.main_loop()

    def startup(self, app):
        self.main_window = toga.MainWindow()

        # Create the three panels
        panel1 = self.create_panel1()
        panel2 = self.create_panel2()

        # Combine them into a main box with margin
        main_box = toga.Box(
            children=[panel1, panel2],
            style=Pack(direction=ROW, margin=10)
        )

        self.main_window.content = main_box
        self.main_window.show()

    def create_panel1(self):
        panel = toga.Box(style=Pack(direction=COLUMN, margin=5))

        button1 = toga.Button('Button 1', on_press=lambda widget: self.handle_btn(1))
        button2 = toga.Button('Button 2', on_press=lambda widget: self.handle_btn(2))

        panel.add(button1)
        panel.add(button2)

        return panel

    def create_panel2(self):
        tile_grid = toga.Box(style=Pack(direction=COLUMN, margin=5))

        for row in range(12):  # Create a 12x12 grid
            row_box = toga.Box(style=Pack(direction=ROW))
            for col in range(12):
                button = toga.Button(f'?', on_press=lambda widget, r=row, c=col: self.handle_tile_click(r, c))
                row_box.add(button)
            tile_grid.add(row_box)

        return tile_grid

    def handle_btn(self, whichbtn):
        info = {'whichbtn': str(whichbtn)}
        print(f"A button was clicked: {info}")

    def handle_tile_click(self, row, col):
        print(f"Tile clicked at row: {row}, column: {col}")


if __name__ == "__main__":

    gui = GUI()
    gui.main_loop()

[tleeuwenburg]

That will reproduce it. I'm guessing I have misunderstood some of the startup / initialisation processes, I just didn't find the error message helped me understand what had gone wrong.

[freakboy3742]

Ok - Yes, there's a problem with your app; but there's also some less than ideal error handling on Toga's side.

Toga has 2 ways to define an app - a class-based approach, and a function-based approach. You're doing a sort of hybrid of both. You're creating a GUI class, which then has an instance of the Toga App as a member; you're then initialising the Toga app using function-based startup mechanism. However, in the process, you're not meeting the requirements of either the class-based app or the function based app startup approach.

To make the app a class based app, you don't need an additional class - GUI is your app. Delete the __init__ and main_loop methods completely, and modify the startup method to begin:

class GUI(toga.App):
    def startup(self):
        self.main_window = toga.MainWindow()
        ...

and then, when you start the app in the main method:

    gui = GUI("Show issue", "noname.ShowIssue")

When you run gui.main_loop(), that will invoke the startup method of the app subclass. The expectation is that by the end of your startup method, it will assign a value to the main_window attribute of the app; it's up to your app whether the app actually shows that main window, though (in your case, it will).

Alternatively, to use the function-based approach - the startup() method you pass in to the App needs to return the content for the main window. Replace the startup method with:

    def startup(self, app):
        # Create the three panels
        panel1 = self.create_panel1()
        panel2 = self.create_panel2()

        # Combine them into a main box with margin
        main_box = toga.Box(
            children=[panel1, panel2], style=Pack(direction=ROW, margin=10)
        )

        return main_box

and remove the references to main_window in __init__. The default toga.App initalization mechanism will create the main window, and populate it with the main_box content that is returned by the startup method.

In general, we advise using the class-based approach. The function-based startup is useful for starting out and educational purposes where you don't want to have to introduce the concept of classes - but you hit limitations pretty quickly. If you start a new project with briefcase, you'll get a stub app that class-based.

Functionally, the bug you're seeing has been caused because while you're creating an object called a main window, and you're populating it, it's not actually your app's main window (from Toga's perspective, anyway) - but you're using a function-based app startup, and returning "no content" - so Toga is crashing trying to set the content of the (default) main window with an empty widget (because your startup function isn't returning a widget object). That's definitely an error case we should be handling - the startup() method in Toga Core should be checking that the content returned by _startup_method() (which will be the user-provided startup definition) is non-None, and returning a "your startup method doesn't return any content" error message.

That's a relatively straightforward piece of error handling; I'll modify the ticket title and tag this as a good first-time contribution - if you don't get to it first, someone at the PyCon sprints next week should be able to take a swing at it.

[tleeuwenburg]

Thank you for the very helpful and detailed reply. It will help me fix up my own code. If I can figure out how to contribute a fix I may do so, but I think I need to learn a bit more first...

[brianchin-sudo]

I'm on it.