Markdown update doesn't work with Threaded Workers

[jimbob88]

Have you checked closed issues? https://github.com/Textualize/textual/issues?q=is%3Aissue+is%3Aclosed Yes

This is a minimum example:

from textual import work
from textual.app import App, ComposeResult
from textual.containers import VerticalScroll
from textual.widgets import Input, Markdown


class MarkdownExample(App):
    """Test if markdown update doesn't work when threaded"""

    def compose(self) -> ComposeResult:
        yield Input(placeholder="Hello World")
        with VerticalScroll(id="results-container"):
            yield Markdown(id="results")

    async def on_input_submitted(self, message: Input.Changed) -> None:
        """A coroutine to handle a text changed message."""
        if message.value:
            self.do_test_thingy(message.value)
        else:
            # Clear the results
            self.query_one("#results", Markdown).update("")

    @work(exclusive=True)
    def do_test_thingy(self, verse: str) -> None:
        # insert sqlite operations here, doesn't seem to affect whether or not the code fails.
        self.query_one("#results", Markdown).update("markdown")


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

A bit of background to my project: I am building something that grabs values from an SQLite database, formats them, and then displays them in markdown. Originally I was using async but it was extremely slow and unresponsive due to the fact my SQLite wasn't setup to be async. So, I decided to move to threading (I noticed it towards the bottom of the Workers doc). After moving to it, processing is far faster, however I can no longer update the markdown widget. I can make other changes to it, i.e. change its visibility, but I lose access to updating it. I'm not sure if I am just laying out the workers wrong or if this is a bug!

I'm not sure if there is a work around to this? I am happy to use a temporary fix whilst I am working on the project!

Thanks awfully, this library has been an absolute blessing so far. Good error logs, good design, and perfect docs!

~ James

---

It will be helpful if you run the following command and paste the results:

textual diagnose

Textual Diagnostics

Versions

Name	Value
Textual	0.28.1
Rich	13.4.2

Python

Name	Value
Version	3.10.11
Implementation	CPython
Compiler	MSC v.1929 64 bit (AMD64)
Executable	C:\Users\black\Documents\Development\Python\textual_broken_markdown\venv\Scripts\python.exe

Operating System

Name	Value
System	Windows
Release	10
Version	10.0.19045

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

[davep]

You may want to take a look at the final paragraph in the worker docs:

Most Textual functions are not thread-safe which means you will need to use call_from_thread to run them from a thread worker. An exception would be post_message which is thread-safe. If your worker needs to make multiple updates to the UI, it is a good idea to send custom messages and let the message handler update the state of the UI.

In other words, the best thing for you to do here would be to either use call_from_thread or to send a message up to request that the update is performed.