"Can't mount widget(s) before Vertical() is mounted" when reactive recompose

[comzyh]

Similar to #4570.

Reproduce: using the following code and press the "t" key repeatedly.

import asyncio

from textual.app import App, ComposeResult
from textual.containers import Vertical
from textual.reactive import reactive
from textual.widget import Widget
from textual.widgets import Footer, Header, Label


class FooWidget(Widget):

    num_lines = reactive(0, recompose=True)

    def compose(self) -> ComposeResult:
        with Vertical():
            for i in range(self.num_lines):
                yield Label(f"Test line {i}")

    async def add_widgets(self) -> None:
        for i in range(32):
            self.num_lines += 1
            # v = self.query_one(Vertical)
            # v.mount(Label(f"Test line {i}"))
            await asyncio.sleep(0.01)


class TestApp(App):
    BINDINGS = [
        ("t", "trigger", "Test"),
        ("q", "quit", "Quit"),
    ]

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

        with Vertical(id="vertical_01"):
            yield FooWidget()
        yield Footer()

    async def action_trigger(self) -> None:

        v = self.query_one("#vertical_01", Vertical)
        f = v.query_one(FooWidget)
        f.workers.cancel_all()
        await f.remove()
        new_f = FooWidget(id="new_f")
        await_mount = v.mount(new_f)
        await await_mount
        new_f.run_worker(new_f.add_widgets())


if __name__ == "__main__":
    app = TestApp()
    app.run()

exception:

Details

```
$ textual run  --dev mount_bug_reproduce.py
╭────────────────────────────────────────────────── Traceback (most recent call last) ───────────────────────────────────────────────────╮
│ /home/comzyh/.virtualenvs/textual/lib/python3.11/site-packages/textual/widget.py:3735 in _compose                                      │
│                                                                                                                                        │
│   3732 │   │   │   self.app._handle_exception(error)                                                                                   │
│   3733 │   │   else:                                                                                                                   │
│   3734 │   │   │   self._extend_compose(widgets)                                                                                       │
│ ❱ 3735 │   │   │   await self.mount_composed_widgets(widgets)                                                                          │
│   3736 │                                                                                                                               │
│   3737 │   async def mount_composed_widgets(self, widgets: list[Widget]) -> None:                                                      │
│   3738 │   │   """Called by Textual to mount widgets after compose.                                                                    │
│                                                                                                                                        │
│ ╭────────────────────────────────────── locals ──────────────────────────────────────╮                                                 │
│ │    self = Vertical()                                                               │                                                 │
│ │ widgets = [Label(), Label(), Label(), Label(), Label(), Label(), Label(), Label()] │                                                 │
│ ╰────────────────────────────────────────────────────────────────────────────────────╯                                                 │
│                                                                                                                                        │
│ /home/comzyh/.virtualenvs/textual/lib/python3.11/site-packages/textual/widget.py:3748 in mount_composed_widgets                        │
│                                                                                                                                        │
│   3745 │   │   │   widgets: A list of child widgets.                                                                                   │
│   3746 │   │   """                                                                                                                     │
│   3747 │   │   if widgets:                                                                                                             │
│ ❱ 3748 │   │   │   await self.mount_all(widgets)                                                                                       │
│   3749 │                                                                                                                               │
│   3750 │   def _extend_compose(self, widgets: list[Widget]) -> None:                                                                   │
│   3751 │   │   """Hook to extend composed widgets.                                                                                     │
│                                                                                                                                        │
│ ╭────────────────────────────────────── locals ──────────────────────────────────────╮                                                 │
│ │    self = Vertical()                                                               │                                                 │
│ │ widgets = [Label(), Label(), Label(), Label(), Label(), Label(), Label(), Label()] │                                                 │
│ ╰────────────────────────────────────────────────────────────────────────────────────╯                                                 │
│                                                                                                                                        │
│ /home/comzyh/.virtualenvs/textual/lib/python3.11/site-packages/textual/widget.py:1016 in mount_all                                     │
│                                                                                                                                        │
│   1013 │   │   """                                                                                                                     │
│   1014 │   │   if self.app._exit:                                                                                                      │
│   1015 │   │   │   return AwaitMount(self, [])                                                                                         │
│ ❱ 1016 │   │   await_mount = self.mount(*widgets, before=before, after=after)                                                          │
│   1017 │   │   return await_mount                                                                                                      │
│   1018 │                                                                                                                               │
│   1019 │   if TYPE_CHECKING:                                                                                                           │
│                                                                                                                                        │
│ ╭────────────────────────────────────── locals ──────────────────────────────────────╮                                                 │
│ │   after = None                                                                     │                                                 │
│ │  before = None                                                                     │                                                 │
│ │    self = Vertical()                                                               │                                                 │
│ │ widgets = [Label(), Label(), Label(), Label(), Label(), Label(), Label(), Label()] │                                                 │
│ ╰────────────────────────────────────────────────────────────────────────────────────╯                                                 │
│                                                                                                                                        │
│ /home/comzyh/.virtualenvs/textual/lib/python3.11/site-packages/textual/widget.py:947 in mount                                          │
│                                                                                                                                        │
│    944 │   │   if self._closing:                                                                                                       │
│    945 │   │   │   return AwaitMount(self, [])                                                                                         │
│    946 │   │   if not self.is_attached:                                                                                                │
│ ❱  947 │   │   │   raise MountError(f"Can't mount widget(s) before {self!r} is mounted")                                               │
│    948 │   │   # Check for duplicate IDs in the incoming widgets                                                                       │
│    949 │   │   ids_to_mount = [                                                                                                        │
│    950 │   │   │   widget_id for widget in widgets if (widget_id := widget.id) is not None                                             │
│                                                                                                                                        │
│ ╭────────────────────────────────────── locals ──────────────────────────────────────╮                                                 │
│ │   after = None                                                                     │                                                 │
│ │  before = None                                                                     │                                                 │
│ │    self = Vertical()                                                               │                                                 │
│ │ widgets = (Label(), Label(), Label(), Label(), Label(), Label(), Label(), Label()) │                                                 │
│ ╰────────────────────────────────────────────────────────────────────────────────────╯                                                 │
╰────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────╯
MountError: Can't mount widget(s) before Vertical() is mounted

In my environment, it only reproduces if I use reactive recompose.

If I remove the reactive add and uncomment 2 lines in add_widgets,

    async def add_widgets(self) -> None:
        for i in range(32):
            v = self.query_one(Vertical)
            v.mount(Label(f"Test line {i}"))
            await asyncio.sleep(0.01)

it won't happen.

Textual diagnose

$ textual diagnose

Textual Diagnostics
Versions



Name
Value




Textual
0.71.0


Rich
13.6.0



Python



Name
Value




Version
3.11.0rc1


Implementation
CPython


Compiler
GCC 11.2.0


Executable
/home/comzyh/.virtualenvs/textual/bin/python3.11



Operating System



Name
Value




System
Linux


Release
5.15.133.1-microsoft-standard-WSL2


Version
#1 SMP Thu Oct 5 21:02:42 UTC 2023



Terminal



Name
Value




Terminal Application
Windows Terminal


TERM
xterm-256color


COLORTERM
Not set


FORCE_COLOR
Not set


NO_COLOR
Not set



Rich Console options



Name
Value




size
width=138, height=78


legacy_windows
False


min_width
1


max_width
138


is_terminal
True


encoding
utf-8


max_height
78


justify
None


overflow
None


no_wrap
False


highlight
None


markup
None


height
None

[willmcgugan]

Recomposing will remove all children and mount new widgets. Your trigger action is getting a vertical, that has not yet completed mounting.

You might want to elaborate on what problem you are attempting to solve with concurrency here.