Changing App's sub_title fails (regression from 0.4.0)

Since 0.5.0 (also in main), an exception is thrown when setting my App’s self.app.sub_title:

Exception in callback SimulationRunnerUI._handle_task_result(True)(<Task finishe...> result=None>)
handle: <Handle SimulationRunnerUI._handle_task_result(True)(<Task finishe...> result=None>)>
Traceback (most recent call last):
  File "/apps/hosted/spack/opt/spack/linux-rhel7-haswell/gcc-4.8.5/python-3.10.1-ek76ski4ctzhij7smku7sh3bmupopkou/lib/python3.10/asyncio/events.py", line 80, in _run
    self._context.run(self._callback, *self._args)
  File "/projects/Triton/users/brechtm/triton_alpha/main/tools/chipmunk/simulations/runnerui.py", line 92, in _handle_task_result
    self.app.sub_title = "BATCH COMPLETED; EXITING"
  File "/projects/Triton/users/brechtm/triton_alpha/main/.venv/lib/python3.10/site-packages/textual/reactive.py", line 190, in __set__
    self._check_watchers(obj, name, current_value)
  File "/projects/Triton/users/brechtm/triton_alpha/main/.venv/lib/python3.10/site-packages/textual/reactive.py", line 258, in _check_watchers
    require_compute = require_compute or invoke_watcher(
  File "/projects/Triton/users/brechtm/triton_alpha/main/.venv/lib/python3.10/site-packages/textual/reactive.py", line 236, in invoke_watcher
    watch_result = watch_function(value)
  File "/projects/Triton/users/brechtm/triton_alpha/main/.venv/lib/python3.10/site-packages/textual/widgets/_header.py", line 134, in set_sub_title
    self.query_one(HeaderTitle).sub_text = sub_title
  File "/projects/Triton/users/brechtm/triton_alpha/main/.venv/lib/python3.10/site-packages/textual/dom.py", line 728, in query_one
    return query.only_one() if expect_type is None else query.only_one(expect_type)
  File "/projects/Triton/users/brechtm/triton_alpha/main/.venv/lib/python3.10/site-packages/textual/css/query.py", line 245, in only_one
    the_one = self.first(expect_type) if expect_type is not None else self.first()
  File "/projects/Triton/users/brechtm/triton_alpha/main/.venv/lib/python3.10/site-packages/textual/css/query.py", line 217, in first
    raise NoMatches(f"No nodes match {self!r}")
textual.css.query.NoMatches: No nodes match <DOMQuery Header() filter='HeaderTitle'>

[brechtm]

I was able to reproduce is a couple of times with this code, hitting ‘q’ shortly after starting the app:

import asyncio

from textual.app import App, ComposeResult
from textual.widgets import Header, Footer, Button

class HeaderTest( App[ None ] ):

    TITLE = "This is the title"
    SUB_TITLE = "This is the sub-title"

    CSS = """
    Screen {
        align: center middle;
    }
    """
    
    BINDINGS = [
        ("q", "quit", "Quit"),
    ]

    def compose( self ) -> ComposeResult:
        yield Header()
        yield Footer()

    def on_mount(self) -> None:
        self._task = asyncio.create_task(self._relax())
        self._task.add_done_callback(self._relax_callback)
        self.sub_title = "Starting to relax..."

    async def _relax(self):
        await asyncio.sleep(2)

    def _relax_callback(self, task: asyncio.Task) -> None:
        try:
            task.result()
        except asyncio.CancelledError:
            pass
        self.sub_title = "All relaxed now!"
        self.set_timer(2, self.action_quit)

    async def action_quit(self) -> None:
        # self._task.cancel()
        await super().action_quit()


if __name__ == "__main__":
    HeaderTest().run()

Calling cancel() on the task seems to fix the issue.

[brechtm]

I thought I would be able to reproduce it with this, but that’s not the case:

import asyncio

from textual.app import App, ComposeResult
from textual.widgets import Header, Footer, Button

class HeaderTest( App[ None ] ):

    TITLE = "This is the title"
    SUB_TITLE = "This is the sub-title"

    CSS = """
    Screen {
        align: center middle;
    }
    """

    def compose( self ) -> ComposeResult:
        yield Header()
        yield Footer()

    def on_mount(self) -> None:
        self._task = asyncio.create_task(self._relax())
        self._task.add_done_callback(self._relax_callback)
        self.sub_title = "Starting to relax..."

    async def _relax(self):
        await asyncio.sleep(5)

    def _relax_callback(self, task: asyncio.Task) -> None:
        self.sub_title = "All relaxed now!"


if __name__ == "__main__":
    HeaderTest().run()

The exception is raised when I quit the application. I overrode action_quit, which aborts the asyncio tasks. I’ll need to look into that further…