test.py

from rich.markdown import Markdown

from textual import events
from textual.app import App
from textual.widgets import Header, Footer, Placeholder, ScrollView


class MyApp(App):
    """An example of a very simple Textual App"""

    async def on_load(self, event: events.Load) -> None:
        """Bind keys with the app loads (but before entering application mode)"""
        await self.bind("b", "view.toggle('sidebar')", "Toggle sidebar")
        await self.bind("q", "quit", "Quit")
        await self.bind("escape", "quit", "Quit")

    async def on_mount(self, event: events.Mount) -> None:
        """Create and dock the widgets."""

        # A scrollview to contain the markdown file
        body = ScrollView(gutter=1)

        # Header / footer / dock
        await self.view.dock(Header(), edge="top")
        await self.view.dock(Footer(), edge="bottom")
        await self.view.dock(Placeholder(), edge="left", size=30, name="sidebar")

        # Dock the body in the remaining space
        await self.view.dock(body, edge="right")

        async def get_markdown(filename: str) -> None:
            with open(filename, "r", encoding="utf8") as fh:
                readme = Markdown(fh.read(), hyperlinks=True)
            await body.update(readme)

        await self.call_later(get_markdown, "richreadme.md")


MyApp.run(title="Simple App", log="textual.log")