CancelledError when using yield and TabbedContent.add_pane(TabPane(..., some_child_widget, ...) in a threaded worker

[iscas-zac]

The issue

When I'm working with yielders, the app prints a lot of traceback after I close it.

The sample code is like this, which I run with textual run --dev main.py with main.py containing just a app.run():

class TestApp(App):
    def compose(self) -> ComposeResult:
        yield TabbedContent()
        yield Input()

    async def on_mount(self):
        self.async_caller()

    @work(thread=True, exit_on_error=False)
    async def async_caller(self):
        a = self.call_from_thread(self.yielder)
        next(a)

    def yielder(self):
        tc = self.query_one(TabbedContent)
        # tp = TabPane("1234", id="_1234")
        # tc.add_pane(tp)
        tc.add_pane(TabPane("1234", Static(), id="_1234"))
        yield "the work is done now. close it"

After I Ctrl-C it, it exits and prints

Traceback (most recent call last):
  File "C:\Python310\lib\site-packages\textual\message_pump.py", line 483, in _process_messages   
    if not await self._pre_process():
  File "C:\Python310\lib\site-packages\textual\message_pump.py", line 507, in _pre_process        
    await self._dispatch_message(events.Compose())
  File "C:\Python310\lib\site-packages\textual\message_pump.py", line 614, in _dispatch_message   
    await self.on_event(message)
  File "C:\Python310\lib\site-packages\textual\message_pump.py", line 683, in on_event
    await self._on_message(event)
  File "C:\Python310\lib\site-packages\textual\message_pump.py", line 704, in _on_message
    await invoke(method, message)
  File "C:\Python310\lib\site-packages\textual\_callback.py", line 81, in invoke
    return await _invoke(callback, *params)
  File "C:\Python310\lib\site-packages\textual\_callback.py", line 47, in _invoke
    result = await result
  File "C:\Python310\lib\site-packages\textual\widget.py", line 3384, in _on_compose
    await self.mount(*widgets)
  File "C:\Python310\lib\site-packages\textual\widget.py", line 117, in await_mount
    await wait(aws)
  File "C:\Python310\lib\asyncio\tasks.py", line 384, in wait
    return await _wait(fs, timeout, return_when, loop)
  File "C:\Python310\lib\asyncio\tasks.py", line 491, in _wait
    await waiter
asyncio.exceptions.CancelledError

During handling of the above exception, another exception occurred:

asyncio.exceptions.CancelledError

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "C:\Python310\lib\site-packages\textual\app.py", line 1441, in run_app
    await self.run_async(
  File "C:\Python310\lib\site-packages\textual\app.py", line 1413, in run_async
    await app._shutdown()
  File "C:\Python310\lib\site-packages\textual\app.py", line 2502, in _shutdown
    await self._close_all()
  File "C:\Python310\lib\site-packages\textual\app.py", line 2482, in _close_all
    await self._prune_node(stack_screen)
  File "C:\Python310\lib\site-packages\textual\app.py", line 3030, in _prune_node
    await asyncio.gather(*close_messages)
asyncio.exceptions.CancelledError

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "**********\main.py", line 7, in <module>
    app.run()
  File "C:\Python310\lib\site-packages\textual\app.py", line 1452, in run
    asyncio.run(run_app())
  File "C:\Python310\lib\asyncio\runners.py", line 44, in run
    return loop.run_until_complete(main)
  File "C:\Python310\lib\asyncio\base_events.py", line 646, in run_until_complete
    return future.result()
asyncio.exceptions.CancelledError

Triggering conditions

The issue involves some patterns.

• The add_pane() should sit in a yielder.
• If I remove the worker statement and put the yielder in the async on_mount() function, it won't trigger.
• Also the TabPane must contain a child widget (the child can also be added afterwards instead of as a parameter, both will trigger).

My textual diagnose output

Textual Diagnostics

Versions

Name	Value
Textual	0.45.1
Rich	13.3.5

Python

Name	Value
Version	3.10.7
Implementation	CPython
Compiler	MSC v.1933 64 bit (AMD64)
Executable	C:\Python310\python.exe

Operating System

Name	Value
System	Windows
Release	10
Version	10.0.22635

Terminal

Name	Value
Terminal Application	vscode (1.86.0-insider)
TERM	Not set
COLORTERM	truecolor
FORCE_COLOR	Not set
NO_COLOR	Not set

Rich Console options

Name	Value
size	width=98, height=13
legacy_windows	False
min_width	1
max_width	98
is_terminal	True
encoding	utf-8
max_height	13
justify	None
overflow	None
no_wrap	False
highlight	None
markup	None
height	None

Afterwords

I'm not sure if I use it right. I want the computation-heavy yielder task both to give some feedback when running and to run in a different thread, so I write codes with workers and yield statements. The problem doesn't affect my experience when it runs, so I guess I'll be OK if this isn't fixed.

[rodrigogiraoserrao]

Hey @iscas-zac, I couldn't understand 100% what you're trying to do or demonstrate with your generator yielder.
I see you're calling yielder with self.call_from_thread but the body of your generator only runs on the next line when you call next(a), so it's likely that the instantiation of the TabPane and adding the tab pane is still being made in a non-thread-safe way.

For example, if I create the pane and add it in a function that is “called from thread”, like you can see below, there are no errors:

from textual import work
from textual.app import App, ComposeResult
from textual.widgets import TabbedContent, TabPane, Input, Static


class TestApp(App):
    def compose(self) -> ComposeResult:
        yield TabbedContent()
        yield Input()

    async def on_mount(self):
        self.async_caller()

    @work(thread=True, exit_on_error=False)
    async def async_caller(self):
        a = self.call_from_thread(self.yielder)
        next(a)

    def yielder(self):
        tc = self.query_one(TabbedContent)

        def add_the_pane():
            tc.add_pane(TabPane("1234", Static(), id="_1234"))

        self.call_from_thread(add_the_pane)
        yield "the work is done now. close it"


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

Now, depending on your final objective, it may be possible to remove some of these indirections that my code has (the worker that calls the generator that calls another function with call_from_thread).

[iscas-zac]

Hello, my fault that I have little knowledge on thread-safe thing, and I really appreciate your explanation. Thanks!

[rodrigogiraoserrao]

No worries!
If you want, join our Discord server: https://discord.gg/Enf6Z3qhVr
Folk there will be happy to provide pointers for resources to learn from, give some hints, etc.