How to initialize cursor position for Input widget?

[learnbyexample]

I want to initialize an Input widget with some text and set a particular cursor position. Here's what I tried (Textual version 0.10.0):

from textual.app import App
from textual.binding import Binding
from textual.widgets import Input, Footer

class Test(App):
    BINDINGS = [Binding('ctrl+u', 'cursor', 'Cursor', show=True)]

    def __init__(self):
        super().__init__()
        self.ip = Input(value="apple")

    def compose(self):
        yield self.ip
        yield Footer()

    def on_mount(self):
        self.ip.focus()
        self.ip.cursor_position = 3

    def action_cursor(self):
        self.ip.cursor_position = 3

if __name__ == '__main__':
    app = Test()
    app.run()

When I run the above code, the cursor position is always at the end. When I press Ctrl+u it changes as expected.

[davep]

The issue here is that Input has an on_focus handler which always sets the cursor position to the end of the input (funnily enough I was wondering about the design choice there just this morning). While you are setting the cursor position after calling focus, I would imagine that due to the async nature of Textual the on_focus of the Input is happening after your on_mount has finished (see the code for Widget.focus if you want to dive deeper).

Unfortunately it seems that, right at the moment, there's no easy way to override that behaviour -- the obvious thing of making a custom input widget from Input and just overriding on_focus doesn't quite work out. It's a little bit hacky, but here's what I came up with as a minimal approach right now:

from textual.app import App
from textual.binding import Binding
from textual.widgets import Input, Footer

class InputAt(Input):

    def __init__(self, position, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self._on_focus_position = position

    def _on_focus_cursor_position(self):
        self.cursor_position = self._on_focus_position

    def on_focus(self):
        self.call_after_refresh(self._on_focus_cursor_position)

class Test(App):
    BINDINGS = [Binding('ctrl+u', 'cursor', 'Cursor', show=True)]

    def __init__(self):
        super().__init__()
        self.ip = InputAt(3, value="apple")

    def compose(self):
        yield self.ip
        yield Footer()

    def on_mount(self):
        self.ip.focus()

    def action_cursor(self):
        self.ip.cursor_position = 3

if __name__ == '__main__':
    app = Test()
    app.run()

Simply put, it sets things up so that once focus has been set, it sets up some code to be run once all other refresh work has taken place to finally settle the cursor position at the given location.

[learnbyexample]

Thanks for the explanation and the workaround!

Will be great if something like this is added to the Input widget too.