TextArea seems to crash under special circumstances

[UlrichGoebel]

Hi,

to start with: I have found a workaround for the following problem - see below. So it might be helpful for others or just interesting to explain the reason for it. I can't do the latter.

In my application I need to define my own __init__ in the Main(App) class. So first I make my stuff and then I call the __init__ from the super class.

All that works fine for Input widgets, but crashes using TextArea widgets. I didn't test it for other widgets as Select or RadioSet.

Here a minimal example. In the real world the buildWidgets does much more of course.

from textual.app import App
from textual.widgets import Input, TextArea

class Main(App):
  
    def __init__(self, driver_class=None, css_path=None, watch_css=False):
        self.buildWidgets()
        super().__init__(driver_class, css_path, watch_css)
            
    def buildWidgets(self):
        self.MyWidget = Input()                  # works
        # self.MyWidget = TextArea()            # doesn' work
            
    def compose(self):
        yield self.MyWidget
    
if __name__ == "__main__":
    app = Main()
    app.run()

Running this with Input() in buildWidgets works, using TextArea() doesn't. It ends with a really long Error Message:

Traceback (most recent call last):
  File "/home/ulrich/PythonHobby/CLUI/Textual/.venv/lib/python3.10/site-packages/textual/message_pump.py", line 209, in app
    return active_app.get()
LookupError: <ContextVar name='active_app' at 0x7f3a88919120>

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/ulrich/PythonHobby/CLUI/Textual/./Test.py", line 20, in <module>
    app = Main()
  File "/home/ulrich/PythonHobby/CLUI/Textual/./Test.py", line 9, in __init__
    self.buildWidgets()
  File "/home/ulrich/PythonHobby/CLUI/Textual/./Test.py", line 14, in buildWidgets
    self.MyWidget = TextArea()
  File "/home/ulrich/PythonHobby/CLUI/Textual/.venv/lib/python3.10/site-packages/textual/widgets/_text_area.py", line 437, in __init__
    self._set_document(text, language)
  File "/home/ulrich/PythonHobby/CLUI/Textual/.venv/lib/python3.10/site-packages/textual/widgets/_text_area.py", line 853, in _set_document
    self.move_cursor((0, 0))
  File "/home/ulrich/PythonHobby/CLUI/Textual/.venv/lib/python3.10/site-packages/textual/widgets/_text_area.py", line 1586, in move_cursor
    self.selection = Selection.cursor(location)
  File "/home/ulrich/PythonHobby/CLUI/Textual/.venv/lib/python3.10/site-packages/textual/reactive.py", line 276, in __set__
    self._check_watchers(obj, name, current_value)
  File "/home/ulrich/PythonHobby/CLUI/Textual/.venv/lib/python3.10/site-packages/textual/reactive.py", line 301, in _check_watchers
    invoke_watcher(obj, private_watch_function, old_value, value)
  File "/home/ulrich/PythonHobby/CLUI/Textual/.venv/lib/python3.10/site-packages/textual/reactive.py", line 77, in invoke_watcher
    watch_result = cast(WatchCallbackBothValuesType, watch_function)(
  File "/home/ulrich/PythonHobby/CLUI/Textual/.venv/lib/python3.10/site-packages/textual/widgets/_text_area.py", line 583, in _watch_selection
    self.app.cursor_position = self.cursor_screen_offset
  File "/home/ulrich/PythonHobby/CLUI/Textual/.venv/lib/python3.10/site-packages/textual/message_pump.py", line 216, in app
    raise NoActiveAppError()
textual._context.NoActiveAppError

It seems clear that building the TextArea makes problems.

Found Workaround?

The last line textual._context.NoActiveAppError gave me the idea to change the order in the __init__: first the super().__init__ then my stuff. Then it works, even for TextArea. No idea why, so what... The order doesn't matter to me as long as I don't manipulate the parameters driver_class, css_path and watch_css. I can live with that.

Best regards
Ulrich

[davep]

Building widgets before your application has had the chance to initialise itself isn't generally going to be a good idea. At the very least you'd want to be making the super call before building the widgets.

[UlrichGoebel]

Thanks for the quick reply!
Is there a better place to build widgets? May be at the beginning of the compose, so it would come to something like that:

class Main(App):
  
    def __init__(self, driver_class=None, css_path=None, watch_css=False):
        super().__init__(driver_class, css_path, watch_css)
        self.buildWidgets()
            
    def buildWidgets(self):
        self.MyWidget = TextArea()
            
    def compose(self):
        self.buildWidgets()
        yield self.MyWidget
    
if __name__ == "__main__":
    app = Main()
    app.run()

Or is there a suitable event? (Load?) So it would come to something like

class Main():

    ...

    def on_something(self):
        self.buildWidgets()

[davep]

Within compose would be more common. In most code you'll find that a compose method simply yields the widgets into the DOM, and then they are queried back when they're needed. See this dialog in one of my own applications, for example.

Note by the way in your example code that you're calling buildWidgets twice, which means you're going to be building one set and then never using them, instead using the subsequently-built set.

You should find that the tutorial introduces these ideas, and the guide goes on to build upon them. In particular the guide to App basics, and especially it's introduction of widgets and how to compose them should contain some useful background.

[UlrichGoebel]

Thanks for the links. What I intend to do is create UIs on an existing PostgreSQL databases with about 300 widgets, organized in about 25 forms. These forms should be instances of a class Form which I have to build. So perheps I will open a new topic about "organizing large numbers of textual widgets in forms". Maybe there are hints or even interest in such a project.