`AssertionError` when updating `ListView` in a worker

[kwevin]

Setting loading=True and updating ListView inside of a worker with an empty list will cause it to raise an AssertionError with LoadingIndicator

class MyApp(App[None]):
    def compose(self) -> ComposeResult:
        yield ListView()

    @work
    async def populate_table(self) -> None:
        list_view = self.query_exactly_one(ListView)
        await list_view.clear()
        await list_view.extend([])
        list_view.loading = False

    def on_mount(self) -> None:
        list_view = self.query_exactly_one(ListView)
        list_view.loading = True
        self.populate_table()

app = MyApp()
app.run()

Textual Diagnostics

Versions

Name	Value
Textual	0.79.0
Rich	13.7.1

Python

Name	Value
Version	3.11.1
Implementation	CPython
Compiler	MSC v.1934 64 bit (AMD64)
Executable	C:\Users\Kevin\AppData\Local\pypoetry\Cache\virtualenvs\listentui-qruDwX8r-py3.11\Scripts\python.exe

Operating System

Name	Value
System	Windows
Release	10
Version	10.0.22621

Terminal

Name	Value
Terminal Application	Windows Terminal
TERM	Not set
COLORTERM	Not set
FORCE_COLOR	Not set
NO_COLOR	Not set

Rich Console options

Name	Value
size	width=120, height=30
legacy_windows	False
min_width	1
max_width	120
is_terminal	True
encoding	utf-8
max_height	30
justify	None
overflow	None
no_wrap	False
highlight	None
markup	None
height	None

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[darrenburns]

Yeah, setting loading mounts a child widget which kind of interferes with ListView. Unfortunately the loading attribute is a bit flaky in general at the moment, and this issue is making me think we might need to explore other mechanisms for implementing it (perhaps outwith the DOM, not interfering with children etc).

Anyway, you can still make it work in this case: You want to switch off "loading" which will unmount the LoadingIndicator as a child of ListView. We also need to await this removal to ensure it completes before we try to modify the children of the ListView using extend:

from textual._work_decorator import work
from textual.app import App, ComposeResult
from textual.widgets import ListView

class MyApp(App[None]):
    def compose(self) -> ComposeResult:
        yield ListView()

    def on_mount(self) -> None:
        list_view = self.query_exactly_one(ListView)
        list_view.loading = True
        self.populate_table()

    @work
    async def populate_table(self) -> None:
        list_view = self.query_exactly_one(ListView)
        await list_view.set_loading(False)
        await list_view.clear()
        await list_view.extend([])

app = MyApp()
app.run()

Closing this for now as I've validated the above works, but feel free to let us know if you hit any other problems.

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