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Reactivity

Textual's reactive attributes are attributes with superpowers. In this chapter we will look at how reactive attributes can simplify your apps.

Quote

With great power comes great responsibility.

— Uncle Ben

Reactive attributes

Textual provides an alternative way of adding attributes to your widget or App, which doesn't require adding them to your class constructor (__init__). To create these attributes import reactive from textual.reactive, and assign them in the class scope.

The following code illustrates how to create reactive attributes:

from textual.reactive import reactive
from textual.widget import Widget

class Reactive(Widget):

    name = reactive("Paul")  # (1)!
    count = reactive(0) # (2)!
    is_cool = reactive(True)  # (3)!
  1. Create a string attribute with a default of "Paul"
  2. Creates an integer attribute with a default of 0.
  3. Creates a boolean attribute with a default of True.

The reactive constructor accepts a default value as the first positional argument.

Information

Textual uses Python's descriptor protocol to create reactive attributes, which is the same protocol used by the builtin property decorator.

You can get and set these attributes in the same way as if you had assigned them in an __init__ method. For instance self.name = "Jessica", self.count += 1, or print(self.is_cool).

Dynamic defaults

You can also set the default to a function (or other callable). Textual will call this function to get the default value. The following code illustrates a reactive value which will be automatically assigned the current time when the widget is created:

from time import time
from textual.reactive import reactive
from textual.widget import Widget

class Timer(Widget):

    start_time = reactive(time)  # (1)!
  1. The time function returns the current time in seconds.

Typing reactive attributes

There is no need to specify a type hint if a reactive attribute has a default value, as type checkers will assume the attribute is the same type as the default.

You may want to add explicit type hints if the attribute type is a superset of the default type. For instance if you want to make an attribute optional. Here's how you would create a reactive string attribute which may be None:

    name: reactive[str | None] = reactive("Paul")

Smart refresh

The first superpower we will look at is "smart refresh". When you modify a reactive attribute, Textual will make note of the fact that it has changed and refresh automatically.

Information

If you modify multiple reactive attributes, Textual will only do a single refresh to minimize updates.

Let's look at an example which illustrates this. In the following app, the value of an input is used to update a "Hello, World!" type greeting.

from textual.app import App, ComposeResult
from textual.reactive import reactive
from textual.widget import Widget
from textual.widgets import Input


class Name(Widget):
    """Generates a greeting."""

    who = reactive("name")

    def render(self) -> str:
        return f"Hello, {self.who}!"


class WatchApp(App):
    CSS_PATH = "refresh01.tcss"

    def compose(self) -> ComposeResult:
        yield Input(placeholder="Enter your name")
        yield Name()

    def on_input_changed(self, event: Input.Changed) -> None:
        self.query_one(Name).who = event.value


if __name__ == "__main__":
    app = WatchApp()
    app.run()
Input {
    dock: top;
    margin-top: 1;
}

Name {
    height: 100%;
    content-align: center middle;
}

WatchApp ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔ Textual ▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁ Hello, Textual!

The Name widget has a reactive who attribute. When the app modifies that attribute, a refresh happens automatically.

Information

Textual will check if a value has really changed, so assigning the same value wont prompt an unnecessary refresh.

Disabling refresh

If you don't want an attribute to prompt a refresh or layout but you still want other reactive superpowers, you can use var to create an attribute. You can import var from textual.reactive.

The following code illustrates how you create non-refreshing reactive attributes.

class MyWidget(Widget):
    count = var(0)  # (1)!
  1. Changing self.count wont cause a refresh or layout.

Layout

The smart refresh feature will update the content area of a widget, but will not change its size. If modifying an attribute should change the size of the widget, you can set layout=True on the reactive attribute. This ensures that your CSS layout will update accordingly.

The following example modifies "refresh01.py" so that the greeting has an automatic width.

from textual.app import App, ComposeResult
from textual.reactive import reactive
from textual.widget import Widget
from textual.widgets import Input


class Name(Widget):
    """Generates a greeting."""

    who = reactive("name", layout=True)  # (1)!

    def render(self) -> str:
        return f"Hello, {self.who}!"


class WatchApp(App):
    CSS_PATH = "refresh02.tcss"

    def compose(self) -> ComposeResult:
        yield Input(placeholder="Enter your name")
        yield Name()

    def on_input_changed(self, event: Input.Changed) -> None:
        self.query_one(Name).who = event.value


if __name__ == "__main__":
    app = WatchApp()
    app.run()
  1. This attribute will update the layout when changed.
Input {
    dock: top;
    margin-top: 1;
}

Name {
    width: auto;
    height: auto;
    border: heavy $secondary;
}

WatchApp ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔ name ▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁ ━━━━━━━━━━━━ Hello, name! ━━━━━━━━━━━━

If you type in to the input now, the greeting will expand to fit the content. If you were to set layout=False on the reactive attribute, you should see that the box remains the same size when you type.

Validation

The next superpower we will look at is validation, which can check and potentially modify a value you assign to a reactive attribute.

If you add a method that begins with validate_ followed by the name of your attribute, it will be called when you assign a value to that attribute. This method should accept the incoming value as a positional argument, and return the value to set (which may be the same or a different value).

A common use for this is to restrict numbers to a given range. The following example keeps a count. There is a button to increase the count, and a button to decrease it. The validation ensures that the count will never go above 10 or below zero.

from textual.app import App, ComposeResult
from textual.containers import Horizontal
from textual.reactive import reactive
from textual.widgets import Button, RichLog


class ValidateApp(App):
    CSS_PATH = "validate01.tcss"

    count = reactive(0)

    def validate_count(self, count: int) -> int:
        """Validate value."""
        if count < 0:
            count = 0
        elif count > 10:
            count = 10
        return count

    def compose(self) -> ComposeResult:
        yield Horizontal(
            Button("+1", id="plus", variant="success"),
            Button("-1", id="minus", variant="error"),
            id="buttons",
        )
        yield RichLog(highlight=True)

    def on_button_pressed(self, event: Button.Pressed) -> None:
        if event.button.id == "plus":
            self.count += 1
        else:
            self.count -= 1
        self.query_one(RichLog).write(f"count = {self.count}")


if __name__ == "__main__":
    app = ValidateApp()
    app.run()
#buttons {
    dock: top;
    height: auto;
}

ValidateApp ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔ +1-1 ▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁

If you click the buttons in the above example it will show the current count. When self.count is modified in the button handler, Textual runs validate_count which performs the validation to limit the value of count.

Watch methods

Watch methods are another superpower. Textual will call watch methods when reactive attributes are modified. Watch method names begin with watch_ followed by the name of the attribute, and should accept one or two arguments. If the method accepts a single argument, it will be called with the new assigned value. If the method accepts two positional arguments, it will be called with both the old value and the new value.

The following app will display any color you type in to the input. Try it with a valid color in Textual CSS. For example "darkorchid" or "#52de44".

from textual.app import App, ComposeResult
from textual.color import Color, ColorParseError
from textual.containers import Grid
from textual.reactive import reactive
from textual.widgets import Input, Static


class WatchApp(App):
    CSS_PATH = "watch01.tcss"

    color = reactive(Color.parse("transparent"))  # (1)!

    def compose(self) -> ComposeResult:
        yield Input(placeholder="Enter a color")
        yield Grid(Static(id="old"), Static(id="new"), id="colors")

    def watch_color(self, old_color: Color, new_color: Color) -> None:  # (2)!
        self.query_one("#old").styles.background = old_color
        self.query_one("#new").styles.background = new_color

    def on_input_submitted(self, event: Input.Submitted) -> None:
        try:
            input_color = Color.parse(event.value)
        except ColorParseError:
            pass
        else:
            self.query_one(Input).value = ""
            self.color = input_color  # (3)!


if __name__ == "__main__":
    app = WatchApp()
    app.run()
  1. Creates a reactive color attribute.
  2. Called when self.color is changed.
  3. New color is assigned here.
Input {
    dock: top;
    margin-top: 1;
}

#colors {
    grid-size: 2 1;
    grid-gutter: 2 4;
    grid-columns: 1fr;
    margin: 0 1;
}

#old {
    height: 100%;
    border: wide $secondary;
}

#new {
    height: 100%;
    border: wide $secondary;
}

WatchApp ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔ darkorchid ▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁ ▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁ ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔

The color is parsed in on_input_submitted and assigned to self.color. Because color is reactive, Textual also calls watch_color with the old and new values.

When are watch methods called?

Textual only calls watch methods if the value of a reactive attribute changes. If the newly assigned value is the same as the previous value, the watch method is not called. You can override this behaviour by passing always_update=True to reactive.

Dynamically watching reactive attributes

You can programmatically add watchers to reactive attributes with the method watch. This is useful when you want to react to changes to reactive attributes for which you can't edit the watch methods.

The example below shows a widget Counter that defines a reactive attribute counter. The app that uses Counter uses the method watch to keep its progress bar synced with the reactive attribute:

from textual.app import App, ComposeResult
from textual.reactive import reactive
from textual.widget import Widget
from textual.widgets import Button, Label, ProgressBar


class Counter(Widget):
    DEFAULT_CSS = "Counter { height: auto; }"
    counter = reactive(0)  # (1)!

    def compose(self) -> ComposeResult:
        yield Label()
        yield Button("+10")

    def on_button_pressed(self) -> None:
        self.counter += 10

    def watch_counter(self, counter_value: int):
        self.query_one(Label).update(str(counter_value))


class WatchApp(App[None]):
    def compose(self) -> ComposeResult:
        yield Counter()
        yield ProgressBar(total=100, show_eta=False)

    def on_mount(self):
        def update_progress(counter_value: int):  # (2)!
            self.query_one(ProgressBar).update(progress=counter_value)

        self.watch(self.query_one(Counter), "counter", update_progress)  # (3)!


if __name__ == "__main__":
    WatchApp().run()
  1. counter is a reactive attribute defined inside Counter.
  2. update_progress is a custom callback that will update the progress bar when counter changes.
  3. We use the method watch to set update_progress as an additional watcher for the reactive attribute counter.

WatchApp 30 ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔ +10 ▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁ ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━30%

Recompose

An alternative to a refresh is recompose. If you set recompose=True on a reactive, then Textual will remove all the child widgets and call compose() again, when the reactive attribute changes. The process of removing and mounting new widgets occurs in a single update, so it will appear as though the content has simply updated.

The following example uses recompose:

from textual.app import App, ComposeResult
from textual.reactive import reactive
from textual.widget import Widget
from textual.widgets import Input, Label


class Name(Widget):
    """Generates a greeting."""

    who = reactive("name", recompose=True)  # (1)!

    def compose(self) -> ComposeResult:  # (2)!
        yield Label(f"Hello, {self.who}!")


class WatchApp(App):
    CSS_PATH = "refresh02.tcss"

    def compose(self) -> ComposeResult:
        yield Input(placeholder="Enter your name")
        yield Name()

    def on_input_changed(self, event: Input.Changed) -> None:
        self.query_one(Name).who = event.value


if __name__ == "__main__":
    app = WatchApp()
    app.run()
  1. Setting recompose=True will cause all child widgets to be removed and compose called again to add new widgets.
  2. This compose() method will be called when who is changed.
Input {
    dock: top;
    margin-top: 1;
}

Name {
    width: auto;
    height: auto;
    border: heavy $secondary;
}

WatchApp ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔ Paul ▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁ ━━━━━━━━━━━━ Hello, Paul! ━━━━━━━━━━━━

While the end-result is identical to refresh02.py, this code works quite differently. The main difference is that recomposing creates an entirely new set of child widgets rather than updating existing widgets. So when the who attribute changes, the Name widget will replace its Label with a new instance (containing updated content).

Warning

You should avoid storing a reference to child widgets when using recompose. Better to query for a child widget when you need them.

It is important to note that any child widgets will have their state reset after a recompose. For simple content, that doesn't matter much. But widgets with an internal state (such as DataTable, Input, or TextArea) would not be particularly useful if recomposed.

Recomposing is slightly less efficient than a simple refresh, and best avoided if you need to update rapidly or you have many child widgets. That said, it can often simplify your code. Let's look at a practical example. First a version without recompose:

from datetime import datetime

from textual.app import App, ComposeResult
from textual.reactive import reactive
from textual.widgets import Digits


class Clock(App):

    CSS = """
    Screen {align: center middle}
    Digits {width: auto}
    """

    time: reactive[datetime] = reactive(datetime.now, init=False)

    def compose(self) -> ComposeResult:
        yield Digits(f"{self.time:%X}")

    def watch_time(self) -> None:  # (1)!
        self.query_one(Digits).update(f"{self.time:%X}")

    def update_time(self) -> None:
        self.time = datetime.now()

    def on_mount(self) -> None:
        self.set_interval(1, self.update_time)  # (2)!


if __name__ == "__main__":
    app = Clock()
    app.run()
  1. Called when the time attribute changes.
  2. Update the time once a second.

Clock  ┓ ┏━┓    ┓ ╺━┓   ╺━┓╺━┓  ┃ ┃ ┃ :  ┃  ━┫ :  ━┫ ━┫ ╺┻╸┗━┛   ╺┻╸╺━┛   ╺━┛╺━┛

This displays a clock which updates once a second. The code is straightforward, but note how we format the time in two places: compose() and watch_time(). We can simplify this by recomposing rather than refreshing:

from datetime import datetime

from textual.app import App, ComposeResult
from textual.reactive import reactive
from textual.widgets import Digits


class Clock(App):

    CSS = """
    Screen {align: center middle}
    Digits {width: auto}
    """

    time: reactive[datetime] = reactive(datetime.now, recompose=True)

    def compose(self) -> ComposeResult:
        yield Digits(f"{self.time:%X}")

    def update_time(self) -> None:
        self.time = datetime.now()

    def on_mount(self) -> None:
        self.set_interval(1, self.update_time)


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

Clock  ┓ ┏━┓    ┓ ╺━┓   ╺━┓╺━┓  ┃ ┃ ┃ :  ┃  ━┫ :  ━┫ ━┫ ╺┻╸┗━┛   ╺┻╸╺━┛   ╺━┛╺━┛

In this version, the app is recomposed when the time attribute changes, which replaces the Digits widget with a new instance and updated time. There's no need for the watch_time method, because the new Digits instance will already show the current time.

Compute methods

Compute methods are the final superpower offered by the reactive descriptor. Textual runs compute methods to calculate the value of a reactive attribute. Compute methods begin with compute_ followed by the name of the reactive value.

You could be forgiven in thinking this sounds a lot like Python's property decorator. The difference is that Textual will cache the value of compute methods, and update them when any other reactive attribute changes.

The following example uses a computed attribute. It displays three inputs for each color component (red, green, and blue). If you enter numbers in to these inputs, the background color of another widget changes.

from textual.app import App, ComposeResult
from textual.color import Color
from textual.containers import Horizontal
from textual.reactive import reactive
from textual.widgets import Input, Static


class ComputedApp(App):
    CSS_PATH = "computed01.tcss"

    red = reactive(0)
    green = reactive(0)
    blue = reactive(0)
    color = reactive(Color.parse("transparent"))

    def compose(self) -> ComposeResult:
        yield Horizontal(
            Input("0", placeholder="Enter red 0-255", id="red"),
            Input("0", placeholder="Enter green 0-255", id="green"),
            Input("0", placeholder="Enter blue 0-255", id="blue"),
            id="color-inputs",
        )
        yield Static(id="color")

    def compute_color(self) -> Color:  # (1)!
        return Color(self.red, self.green, self.blue).clamped

    def watch_color(self, color: Color) -> None:  # (2)
        self.query_one("#color").styles.background = color

    def on_input_changed(self, event: Input.Changed) -> None:
        try:
            component = int(event.value)
        except ValueError:
            self.bell()
        else:
            if event.input.id == "red":
                self.red = component
            elif event.input.id == "green":
                self.green = component
            else:
                self.blue = component


if __name__ == "__main__":
    app = ComputedApp()
    app.run()
  1. Combines color components in to a Color object.
  2. The watch method is called when the result of compute_color changes.
#color-inputs {
    dock: top;
    height: auto;
}

Input {
    width: 1fr;
}

#color {
    height: 100%;
    border: tall $secondary;
}

ComputedApp ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔ 000 ▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁ ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔ ▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁

Note the compute_color method which combines the color components into a Color object. It will be recalculated when any of the red , green, or blue attributes are modified.

When the result of compute_color changes, Textual will also call watch_color since color still has the watch method superpower.

Note

Textual will first attempt to call the compute method for a reactive attribute, followed by the validate method, and finally the watch method.

Note

It is best to avoid doing anything slow or CPU-intensive in a compute method. Textual calls compute methods on an object when any reactive attribute changes.

Setting reactives without superpowers

You may find yourself in a situation where you want to set a reactive value, but you don't want to invoke watchers or the other super powers. This is fairly common in constructors which run prior to mounting; any watcher which queries the DOM may break if the widget has not yet been mounted.

To work around this issue, you can call set_reactive as an alternative to setting the attribute. The set_reactive method accepts the reactive attribute (as a class variable) and the new value.

Let's look at an example. The following app is intended to cycle through various greeting when you press Space, however it contains a bug.

set_reactive01.py
from textual.app import App, ComposeResult
from textual.containers import Horizontal
from textual.reactive import reactive, var
from textual.widgets import Label

GREETINGS = [
    "Bonjour",
    "Hola",
    "こんにちは",
    "你好",
    "안녕하세요",
    "Hello",
]


class Greeter(Horizontal):
    """Display a greeting and a name."""

    DEFAULT_CSS = """
    Greeter {
        width: auto;
        height: 1;
        & Label {
            margin: 0 1;
        }
    }
    """
    greeting: reactive[str] = reactive("")
    who: reactive[str] = reactive("")

    def __init__(self, greeting: str = "Hello", who: str = "World!") -> None:
        super().__init__()
        self.greeting = greeting  # (1)!
        self.who = who

    def compose(self) -> ComposeResult:
        yield Label(self.greeting, id="greeting")
        yield Label(self.who, id="name")

    def watch_greeting(self, greeting: str) -> None:
        self.query_one("#greeting", Label).update(greeting)  # (2)!

    def watch_who(self, who: str) -> None:
        self.query_one("#who", Label).update(who)


class NameApp(App):

    CSS = """
    Screen {
        align: center middle;
    }   
    """
    greeting_no: var[int] = var(0)
    BINDINGS = [("space", "greeting")]

    def compose(self) -> ComposeResult:
        yield Greeter(who="Textual")

    def action_greeting(self) -> None:
        self.greeting_no = (self.greeting_no + 1) % len(GREETINGS)
        self.query_one(Greeter).greeting = GREETINGS[self.greeting_no]


if __name__ == "__main__":
    app = NameApp()
    app.run()
  1. Setting this reactive attribute invokes a watcher.
  2. The watcher attempts to update a label before it is mounted.

If you run this app, you will find Textual raises a NoMatches error in watch_greeting. This is because the constructor has assigned the reactive before the widget has fully mounted.

The following app contains a fix for this issue:

from textual.app import App, ComposeResult
from textual.containers import Horizontal
from textual.reactive import reactive, var
from textual.widgets import Label

GREETINGS = [
    "Bonjour",
    "Hola",
    "こんにちは",
    "你好",
    "안녕하세요",
    "Hello",
]


class Greeter(Horizontal):
    """Display a greeting and a name."""

    DEFAULT_CSS = """
    Greeter {
        width: auto;
        height: 1;
        & Label {
            margin: 0 1;
        }
    }
    """
    greeting: reactive[str] = reactive("")
    who: reactive[str] = reactive("")

    def __init__(self, greeting: str = "Hello", who: str = "World!") -> None:
        super().__init__()
        self.set_reactive(Greeter.greeting, greeting)  # (1)!
        self.set_reactive(Greeter.who, who)

    def compose(self) -> ComposeResult:
        yield Label(self.greeting, id="greeting")
        yield Label(self.who, id="name")

    def watch_greeting(self, greeting: str) -> None:
        self.query_one("#greeting", Label).update(greeting)

    def watch_who(self, who: str) -> None:
        self.query_one("#who", Label).update(who)


class NameApp(App):

    CSS = """
    Screen {
        align: center middle;
    }   
    """
    greeting_no: var[int] = var(0)
    BINDINGS = [("space", "greeting")]

    def compose(self) -> ComposeResult:
        yield Greeter(who="Textual")

    def action_greeting(self) -> None:
        self.greeting_no = (self.greeting_no + 1) % len(GREETINGS)
        self.query_one(Greeter).greeting = GREETINGS[self.greeting_no]


if __name__ == "__main__":
    app = NameApp()
    app.run()
  1. The attribute is set via set_reactive, which avoids calling the watcher.

NameApp HelloTextual

The line self.set_reactive(Greeter.greeting, greeting) sets the greeting attribute but doesn't immediately invoke the watcher.

Data binding

Reactive attributes from one widget may be bound (connected) to another widget, so that changes to a single reactive will automatically update another widget (potentially more than one).

To bind reactive attributes, call data_bind on a widget. This method accepts reactives (as class attributes) in positional arguments or keyword arguments.

Let's look at an app that could benefit from data binding. In the following code we have a WorldClock widget which displays the time in any given timezone.

Note

This example uses the pytz library for working with timezones. You can install pytz with pip install pytz.

from datetime import datetime

from pytz import timezone

from textual.app import App, ComposeResult
from textual.reactive import reactive
from textual.widget import Widget
from textual.widgets import Digits, Label


class WorldClock(Widget):

    time: reactive[datetime] = reactive(datetime.now)

    def __init__(self, timezone: str) -> None:
        self.timezone = timezone
        super().__init__()

    def compose(self) -> ComposeResult:
        yield Label(self.timezone)
        yield Digits()

    def watch_time(self, time: datetime) -> None:
        localized_time = time.astimezone(timezone(self.timezone))
        self.query_one(Digits).update(localized_time.strftime("%H:%M:%S"))


class WorldClockApp(App):
    CSS_PATH = "world_clock01.tcss"

    time: reactive[datetime] = reactive(datetime.now)

    def compose(self) -> ComposeResult:
        yield WorldClock("Europe/London")
        yield WorldClock("Europe/Paris")
        yield WorldClock("Asia/Tokyo")

    def update_time(self) -> None:
        self.time = datetime.now()

    def watch_time(self, time: datetime) -> None:
        for world_clock in self.query(WorldClock):  # (1)!
            world_clock.time = time

    def on_mount(self) -> None:
        self.update_time()
        self.set_interval(1, self.update_time)


if __name__ == "__main__":
    app = WorldClockApp()
    app.run()
  1. Update the time reactive attribute of every WorldClock.
Screen {
    align: center middle;
}

WorldClock {
    width: auto;
    height: auto;
    padding: 1 2;
    background: $panel;
    border: wide $background;

    & Digits {
        width: auto;
        color: $secondary;
    }
}

WorldClockApp ▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁ Europe/London  ┓ ┏━┓    ┓ ╺━┓   ╺━┓╺━┓  ┃ ┃ ┃ :  ┃  ━┫ :  ━┫ ━┫ ╺┻╸┗━┛   ╺┻╸╺━┛   ╺━┛╺━┛ ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔ ▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁ Europe/Paris  ┓  ┓     ┓ ╺━┓   ╺━┓╺━┓  ┃  ┃  :  ┃  ━┫ :  ━┫ ━┫ ╺┻╸╺┻╸   ╺┻╸╺━┛   ╺━┛╺━┛ ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔ ▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁ Asia/Tokyo  ┓ ┏━┓    ┓ ╺━┓   ╺━┓╺━┓  ┃ ┣━┫ :  ┃  ━┫ :  ━┫ ━┫ ╺┻╸┗━┛   ╺┻╸╺━┛   ╺━┛╺━┛ ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔

We've added three world clocks for London, Paris, and Tokyo. The clocks are kept up-to-date by watching the app's time reactive, and updating the clocks in a loop.

While this approach works fine, it does require we take care to update every WorldClock we mount. Let's see how data binding can simplify this.

The following app calls data_bind on the world clock widgets to connect the app's time with the widget's time attribute:

from datetime import datetime

from pytz import timezone

from textual.app import App, ComposeResult
from textual.reactive import reactive
from textual.widget import Widget
from textual.widgets import Digits, Label


class WorldClock(Widget):

    time: reactive[datetime] = reactive(datetime.now)

    def __init__(self, timezone: str) -> None:
        self.timezone = timezone
        super().__init__()

    def compose(self) -> ComposeResult:
        yield Label(self.timezone)
        yield Digits()

    def watch_time(self, time: datetime) -> None:
        localized_time = time.astimezone(timezone(self.timezone))
        self.query_one(Digits).update(localized_time.strftime("%H:%M:%S"))


class WorldClockApp(App):
    CSS_PATH = "world_clock01.tcss"

    time: reactive[datetime] = reactive(datetime.now)

    def compose(self) -> ComposeResult:
        yield WorldClock("Europe/London").data_bind(WorldClockApp.time)  # (1)!
        yield WorldClock("Europe/Paris").data_bind(WorldClockApp.time)
        yield WorldClock("Asia/Tokyo").data_bind(WorldClockApp.time)

    def update_time(self) -> None:
        self.time = datetime.now()

    def on_mount(self) -> None:
        self.update_time()
        self.set_interval(1, self.update_time)


if __name__ == "__main__":
    WorldClockApp().run()
  1. Bind the time attribute, so that changes to time will also change the time attribute on the WorldClock widgets. The data_bind method also returns the widget, so we can yield its return value.
Screen {
    align: center middle;
}

WorldClock {
    width: auto;
    height: auto;
    padding: 1 2;
    background: $panel;
    border: wide $background;

    & Digits {
        width: auto;
        color: $secondary;
    }
}

WorldClockApp ▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁ Europe/London  ┓ ┏━┓    ┓ ╺━┓   ╺━┓╻ ╻  ┃ ┃ ┃ :  ┃  ━┫ :  ━┫┗━┫ ╺┻╸┗━┛   ╺┻╸╺━┛   ╺━┛  ╹ ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔ ▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁ Europe/Paris  ┓  ┓     ┓ ╺━┓   ╺━┓╻ ╻  ┃  ┃  :  ┃  ━┫ :  ━┫┗━┫ ╺┻╸╺┻╸   ╺┻╸╺━┛   ╺━┛  ╹ ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔ ▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁ Asia/Tokyo  ┓ ┏━┓    ┓ ╺━┓   ╺━┓╻ ╻  ┃ ┣━┫ :  ┃  ━┫ :  ━┫┗━┫ ╺┻╸┗━┛   ╺┻╸╺━┛   ╺━┛  ╹ ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔

Note how the addition of the data_bind methods negates the need for the watcher in world_clock01.py.

Note

Data binding works in a single direction. Setting time on the app updates the clocks. But setting time on the clocks will not update time on the app.

In the previous example app, the call to data_bind(WorldClockApp.time) worked because both reactive attributes were named time. If you want to bind a reactive attribute which has a different name, you can use keyword arguments.

In the following app we have changed the attribute name on WorldClock from time to clock_time. We can make the app continue to work by changing the data_bind call to data_bind(clock_time=WorldClockApp.time):

from datetime import datetime

from pytz import timezone

from textual.app import App, ComposeResult
from textual.reactive import reactive
from textual.widget import Widget
from textual.widgets import Digits, Label


class WorldClock(Widget):

    clock_time: reactive[datetime] = reactive(datetime.now)

    def __init__(self, timezone: str) -> None:
        self.timezone = timezone
        super().__init__()

    def compose(self) -> ComposeResult:
        yield Label(self.timezone)
        yield Digits()

    def watch_clock_time(self, time: datetime) -> None:
        localized_time = time.astimezone(timezone(self.timezone))
        self.query_one(Digits).update(localized_time.strftime("%H:%M:%S"))


class WorldClockApp(App):
    CSS_PATH = "world_clock01.tcss"

    time: reactive[datetime] = reactive(datetime.now)

    def compose(self) -> ComposeResult:
        yield WorldClock("Europe/London").data_bind(
            clock_time=WorldClockApp.time  # (1)!
        )
        yield WorldClock("Europe/Paris").data_bind(clock_time=WorldClockApp.time)
        yield WorldClock("Asia/Tokyo").data_bind(clock_time=WorldClockApp.time)

    def update_time(self) -> None:
        self.time = datetime.now()

    def on_mount(self) -> None:
        self.update_time()
        self.set_interval(1, self.update_time)


if __name__ == "__main__":
    WorldClockApp().run()
  1. Uses keyword arguments to bind the time attribute of WorldClockApp to clock_time on WorldClock.
Screen {
    align: center middle;
}

WorldClock {
    width: auto;
    height: auto;
    padding: 1 2;
    background: $panel;
    border: wide $background;

    & Digits {
        width: auto;
        color: $secondary;
    }
}

WorldClockApp ▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁ Europe/London  ┓ ┏━┓    ┓ ╺━┓   ╺━┓╻ ╻  ┃ ┃ ┃ :  ┃  ━┫ :  ━┫┗━┫ ╺┻╸┗━┛   ╺┻╸╺━┛   ╺━┛  ╹ ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔ ▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁ Europe/Paris  ┓  ┓     ┓ ╺━┓   ╺━┓╻ ╻  ┃  ┃  :  ┃  ━┫ :  ━┫┗━┫ ╺┻╸╺┻╸   ╺┻╸╺━┛   ╺━┛  ╹ ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔ ▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁ Asia/Tokyo  ┓ ┏━┓    ┓ ╺━┓   ╺━┓╻ ╻  ┃ ┣━┫ :  ┃  ━┫ :  ━┫┗━┫ ╺┻╸┗━┛   ╺┻╸╺━┛   ╺━┛  ╹ ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔