Textual Knowledge

Provides patterns, API constraints, and working examples for building terminal applications with the Textual framework.

Scope

Consult python-engineering:standards-for-python-development for the shared rules (architecture, typing, testing, CLI rules).

TRIGGER: Activate when the user asks about Textual — building TUI apps, widgets, CSS styling, reactive attributes, testing, or concurrency.

COVERS:

• App class, run/exit/suspend, title/subtitle, CSS loading
• Screen stack (push, pop, switch), modal screens, modes, screen data return
• Custom widgets, builtins, Line API, compound widgets, data flow
• Layout types (vertical, horizontal, grid), docking, layers, offsets
• CSS selectors, pseudo-classes, combinators, specificity, variables, nesting
• Events, message queue, bubbling, custom messages, @on decorator
• Actions, action strings, namespaces, bindings, dynamic actions
• Reactive attributes, smart refresh, validation, watch methods, compute, data binding
• Testing with run_test, Pilot API (press, click, hover, pause)
• Snapshot testing with pytest-textual-snapshot
• Workers (@work, run_worker, thread workers, worker events)

DOES NOT COVER:

• Rich library internals (see Rich docs)
• Textual Web deployment
• Custom CSS parsers or non-Textual terminal frameworks

Workflow

flowchart TD
    Task([Task received]) --> Q1{Task type?}
    Q1 -->|App structure or lifecycle| App[Load app-and-screens.md]
    Q1 -->|Widget creation or builtins| Wid[Load widgets.md]
    Q1 -->|Layout or CSS styling| CSS[Load layout-and-css.md]
    Q1 -->|Events, messages, or actions| Evt[Load events-and-actions.md]
    Q1 -->|Reactive attributes or data binding| React[Load reactivity.md]
    Q1 -->|Testing or background tasks| Test[Load testing-and-workers.md]
    Q1 -->|Multiple concerns| Multi[Load relevant files]

Reference Files

App and Screens

App class basics, run/exit/suspend, CSS loading, screen stack (push/pop/switch), modal screens, returning data from screens, and modes.

Load when the user asks about App setup, screen navigation, modal dialogs, or multi-screen apps.

../python3-cli/references/textual/app-and-screens.md

Widgets

Custom widget creation, Static, Default CSS, focusability, bindings, Rich renderables, Line API, compound widgets, coordinating via messages, and the builtin widget catalog.

Load when the user asks about creating widgets, choosing a builtin widget, or structuring compound UIs.

../python3-cli/references/textual/widgets.md

Layout and CSS

Layout types (vertical, horizontal, grid), utility containers, docking, layers, offsets, CSS selectors, pseudo-classes, combinators, specificity, variables, nested CSS, and the style property reference.

Load when the user asks about arranging widgets, CSS syntax, selectors, or style properties.

../python3-cli/references/textual/layout-and-css.md

Events and Actions

Message queue, event handler naming convention, @on decorator, handler arguments, async handlers, event bubbling, custom messages, preventing messages, actions, action strings, namespaces, key bindings, dynamic actions, and the builtin actions list.

Load when the user asks about handling user input, defining key bindings, creating custom events, or action strings.

../python3-cli/references/textual/events-and-actions.md

Reactivity

Reactive attributes, var, smart refresh, layout-triggering reactives, validation methods, watch methods, dynamic watchers, recompose, compute methods, set_reactive, mutable reactives (mutate_reactive), and data binding.

Load when the user asks about reactive state, auto-refresh, watchers, computed values, or binding parent state to child widgets.

../python3-cli/references/textual/reactivity.md

Testing and Workers

run_test, Pilot API (press, click, hover, pause), terminal size simulation, snapshot testing with pytest-textual-snapshot, run_worker, the @work decorator, worker state lifecycle, thread workers, and worker events.

Load when the user asks about writing tests, simulating input, snapshot comparisons, background HTTP requests, or preventing UI blocking.

../python3-cli/references/textual/testing-and-workers.md

Quick Reference

# Minimal app
from textual.app import App, ComposeResult
from textual.widgets import Label


class MyApp(App):
    CSS = "Screen { align: center middle; }"

    def compose(self) -> ComposeResult:
        yield Label("Hello!")


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

# Reactive + watch
from textual.reactive import reactive
from textual.widget import Widget


class Counter(Widget):
    count = reactive(0)

    def watch_count(self, value: int) -> None:
        self.refresh()

    def render(self) -> str:
        return str(self.count)

# Worker for background HTTP
from textual import work
import httpx


class MyApp(App):
    @work(exclusive=True)
    async def fetch(self, url: str) -> None:
        async with httpx.AsyncClient() as client:
            response = await client.get(url)
        self.query_one("#result").update(response.text)

# Test with Pilot
async def test_button() -> None:
    app = MyApp()
    async with app.run_test() as pilot:
        await pilot.click("#submit")
        assert app.query_one("#result").renderable == "done"