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Widgets

In this chapter we will explore widgets in more detail, and how you can create custom widgets of your own.

What is a widget?

A widget is a component of your UI responsible for managing a rectangular region of the screen. Widgets may respond to events in much the same way as an app. In many respects, widgets are like mini-apps.

Information

Every widget runs in its own asyncio task.

Custom widgets

There is a growing collection of builtin widgets in Textual, but you can build entirely custom widgets that work in the same way.

The first step in building a widget is to import and extend a widget class. This can either be Widget which is the base class of all widgets, or one of its subclasses.

Let's create a simple custom widget to display a greeting.

hello01.py
from textual.app import App, ComposeResult, RenderResult
from textual.widget import Widget


class Hello(Widget):
    """Display a greeting."""

    def render(self) -> RenderResult:
        return "Hello, [b]World[/b]!"


class CustomApp(App):
    def compose(self) -> ComposeResult:
        yield Hello()


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

The three highlighted lines define a custom widget class with just a render() method. Textual will display whatever is returned from render in the content area of your widget. We have returned a string in the code above, but there are other possible return types which we will cover later.

Note that the text contains tags in square brackets, i.e. [b]. This is console markup which allows you to embed various styles within your content. If you run this you will find that World is in bold.

CustomApp Hello, World!

This (very simple) custom widget may be styled in the same was as builtin widgets, and targeted with CSS. Let's add some CSS to this app.

hello02.py
from textual.app import App, ComposeResult, RenderResult
from textual.widget import Widget


class Hello(Widget):
    """Display a greeting."""

    def render(self) -> RenderResult:
        return "Hello, [b]World[/b]!"


class CustomApp(App):
    CSS_PATH = "hello02.css"

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


if __name__ == "__main__":
    app = CustomApp()
    app.run()
hello02.css
Screen {
    align: center middle;
}

Hello {
    width: 40;
    height: 9;
    padding: 1 2;
    background: $panel;
    color: $text;
    border: $secondary tall;
    content-align: center middle;
}

The addition of the CSS has completely transformed our custom widget.

CustomApp ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔ Hello, World! ▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁

Static widget

While you can extend the Widget class, a subclass will typically be a better starting point. The Static class is a widget subclass which caches the result of render, and provides an update() method to update the content area.

Let's use Static to create a widget which cycles through "hello" in various languages.

hello03.py
from itertools import cycle

from textual.app import App, ComposeResult
from textual.widgets import Static


hellos = cycle(
    [
        "Hola",
        "Bonjour",
        "Guten tag",
        "Salve",
        "Nǐn hǎo",
        "Olá",
        "Asalaam alaikum",
        "Konnichiwa",
        "Anyoung haseyo",
        "Zdravstvuyte",
        "Hello",
    ]
)


class Hello(Static):
    """Display a greeting."""

    def on_mount(self) -> None:
        self.next_word()

    def on_click(self) -> None:
        self.next_word()

    def next_word(self) -> None:
        """Get a new hello and update the content area."""
        hello = next(hellos)
        self.update(f"{hello}, [b]World[/b]!")


class CustomApp(App):
    CSS_PATH = "hello03.css"

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


if __name__ == "__main__":
    app = CustomApp()
    app.run()
hello03.css
Screen {
    align: center middle;
}

Hello {
    width: 40;
    height: 9;
    padding: 1 2;
    background: $panel;
    border: $secondary tall;
    content-align: center middle;
}

CustomApp ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔ Hola, World! ▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁

Note that there is no render() method on this widget. The Static class is handling the render for us. Instead we call update() when we want to update the content within the widget.

The next_word method updates the greeting. We call this method from the mount handler to get the first word, and from a click handler to cycle through the greetings when we click the widget.

Default CSS

When building an app it is best to keep your CSS in an external file. This allows you to see all your CSS in one place, and to enable live editing. However if you intend to distribute a widget (via PyPI for instance) it can be convenient to bundle the code and CSS together. You can do this by adding a DEFAULT_CSS class variable inside your widget class.

Textual's builtin widgets bundle CSS in this way, which is why you can see nicely styled widgets without having to copy any CSS code.

Here's the Hello example again, this time the widget has embedded default CSS:

hello04.py
from itertools import cycle

from textual.app import App, ComposeResult
from textual.widgets import Static


hellos = cycle(
    [
        "Hola",
        "Bonjour",
        "Guten tag",
        "Salve",
        "Nǐn hǎo",
        "Olá",
        "Asalaam alaikum",
        "Konnichiwa",
        "Anyoung haseyo",
        "Zdravstvuyte",
        "Hello",
    ]
)


class Hello(Static):
    """Display a greeting."""

    DEFAULT_CSS = """
    Hello {
        width: 40;
        height: 9;
        padding: 1 2;
        background: $panel;
        border: $secondary tall;
        content-align: center middle;
    }
    """

    def on_mount(self) -> None:
        self.next_word()

    def on_click(self) -> None:
        self.next_word()

    def next_word(self) -> None:
        """Get a new hello and update the content area."""
        hello = next(hellos)
        self.update(f"{hello}, [b]World[/b]!")


class CustomApp(App):
    CSS_PATH = "hello04.css"

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


if __name__ == "__main__":
    app = CustomApp()
    app.run()
hello04.css
Screen {
    align: center middle;
}

CustomApp ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔ Hola, World! ▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁

Default specificity

CSS defined within DEFAULT_CSS has an automatically lower specificity than CSS read from either the App's CSS class variable or an external stylesheet. In practice this means that your app's CSS will take precedence over any CSS bundled with widgets.

Text in a widget may be marked up with links which perform an action when clicked. Links in console markup use the following format:

"Click [@click='app.bell']Me[/]"

The @click tag introduces a click handler, which runs the app.bell action.

Let's use markup links in the hello example so that the greeting becomes a link which updates the widget.

hello05.py
from itertools import cycle

from textual.app import App, ComposeResult
from textual.widgets import Static


hellos = cycle(
    [
        "Hola",
        "Bonjour",
        "Guten tag",
        "Salve",
        "Nǐn hǎo",
        "Olá",
        "Asalaam alaikum",
        "Konnichiwa",
        "Anyoung haseyo",
        "Zdravstvuyte",
        "Hello",
    ]
)


class Hello(Static):
    """Display a greeting."""

    def on_mount(self) -> None:
        self.action_next_word()

    def action_next_word(self) -> None:
        """Get a new hello and update the content area."""
        hello = next(hellos)
        self.update(f"[@click='next_word']{hello}[/], [b]World[/b]!")


class CustomApp(App):
    CSS_PATH = "hello05.css"

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


if __name__ == "__main__":
    app = CustomApp()
    app.run()
hello05.css
Screen {
    align: center middle;
}

Hello {
    width: 40;
    height: 9;
    padding: 1 2;
    background: $panel;
    border: $secondary tall;
    content-align: center middle;
}

CustomApp ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔ HolaWorld! ▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁

If you run this example you will see that the greeting has been underlined, which indicates it is clickable. If you click on the greeting it will run the next_word action which updates the next word.

Rich renderables

In previous examples we've set strings as content for Widgets. You can also use special objects called renderables for advanced visuals. You can use any renderable defined in Rich or third party libraries.

Lets make a widget that uses a Rich table for its content. The following app is a solution to the classic fizzbuzz problem often used to screen software engineers in job interviews. The problem is this: Count up from 1 to 100, when the number is divisible by 3, output "fizz"; when the number is divisible by 5, output "buzz"; and when the number is divisible by both 3 and 5 output "fizzbuzz".

This app will "play" fizz buzz by displaying a table of the first 15 numbers and columns for fizz and buzz.

fizzbuzz01.py
from rich.table import Table

from textual.app import App, ComposeResult
from textual.widgets import Static


class FizzBuzz(Static):
    def on_mount(self) -> None:
        table = Table("Number", "Fizz?", "Buzz?")
        for n in range(1, 16):
            fizz = not n % 3
            buzz = not n % 5
            table.add_row(
                str(n),
                "fizz" if fizz else "",
                "buzz" if buzz else "",
            )
        self.update(table)


class FizzBuzzApp(App):
    CSS_PATH = "fizzbuzz01.css"

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


if __name__ == "__main__":
    app = FizzBuzzApp()
    app.run()
fizzbuzz01.css
Screen {
    align: center middle;
}

FizzBuzz {
    width: auto;
    height: auto;
    background: $primary;    
    color: $text;
}

FizzBuzzApp ┏━━━━━━━━┳━━━━━━━┳━━━━━━━┓  Number  Fizz?  Buzz?  ┡━━━━━━━━╇━━━━━━━╇━━━━━━━┩ │ 1      │       │       │ │ 2      │       │       │ │ 3      │ fizz  │       │ │ 4      │       │       │ │ 5      │       │ buzz  │ │ 6      │ fizz  │       │ │ 7      │       │       │ │ 8      │       │       │ │ 9      │ fizz  │       │ │ 10     │       │ buzz  │ │ 11     │       │       │ │ 12     │ fizz  │       │ │ 13     │       │       │ │ 14     │       │       │ │ 15     │ fizz  │ buzz  │ └────────┴───────┴───────┘

Content size

Textual will auto-detect the dimensions of the content area from rich renderables if width or height is set to auto. You can override auto dimensions by implementing get_content_width() or get_content_height().

Let's modify the default width for the fizzbuzz example. By default, the table will be just wide enough to fix the columns. Let's force it to be 50 characters wide.

fizzbuzz02.py
from rich.table import Table

from textual.app import App, ComposeResult
from textual.geometry import Size
from textual.widgets import Static


class FizzBuzz(Static):
    def on_mount(self) -> None:
        table = Table("Number", "Fizz?", "Buzz?", expand=True)
        for n in range(1, 16):
            fizz = not n % 3
            buzz = not n % 5
            table.add_row(
                str(n),
                "fizz" if fizz else "",
                "buzz" if buzz else "",
            )
        self.update(table)

    def get_content_width(self, container: Size, viewport: Size) -> int:
        """Force content width size."""
        return 50


class FizzBuzzApp(App):
    CSS_PATH = "fizzbuzz02.css"

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


if __name__ == "__main__":
    app = FizzBuzzApp()
    app.run()
fizzbuzz02.css
Screen {
    align: center middle;
}

FizzBuzz {
    width: auto;
    height: auto;
    background: $primary;
    color: $text;    
}

FizzBuzzApp ┏━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━┓  Number           Fizz?          Buzz?         ┡━━━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━┩ │ 1               │               │              │ │ 2               │               │              │ │ 3               │ fizz          │              │ │ 4               │               │              │ │ 5               │               │ buzz         │ │ 6               │ fizz          │              │ │ 7               │               │              │ │ 8               │               │              │ │ 9               │ fizz          │              │ │ 10              │               │ buzz         │ │ 11              │               │              │ │ 12              │ fizz          │              │ │ 13              │               │              │ │ 14              │               │              │ │ 15              │ fizz          │ buzz         │ └─────────────────┴───────────────┴──────────────┘

Note that we've added expand=True to tell the Table to expand beyond the optimal width, so that it fills the 50 characters returned by get_content_width.

Compound widgets

TODO: Explanation of compound widgets

Line API

TODO: Explanation of line API