Events and Messages

We've used event handler methods in many of the examples in this guide. This chapter explores events and messages (see below) in more detail.

Messages

Events are a particular kind of message sent by Textual in response to input and other state changes. Events are reserved for use by Textual, but you can also create custom messages for the purpose of coordinating between widgets in your app.

More on that later, but for now keep in mind that events are also messages, and anything that is true of messages is true of events.

Message Queue

Every App and Widget object contains a message queue. You can think of a message queue as orders at a restaurant. The chef takes an order and makes the dish. Orders that arrive while the chef is cooking are placed in a line. When the chef has finished a dish they pick up the next order in the line.

Textual processes messages in the same way. Messages are picked off a queue and processed (cooked) by a handler method. This guarantees messages and events are processed even if your code can not handle them right away.

This processing of messages is done within an asyncio Task which is started when you mount the widget. The task monitors a queue for new messages and dispatches them to the appropriate handler when they arrive.

Tip

The FastAPI docs have an excellent introduction to Python async programming.

By way of an example, let's consider what happens if you were to type "Text" into a Input widget. When you hit the T key, Textual creates a key event and sends it to the widget's message queue. Ditto for E, X, and T.

The widget's task will pick the first message from the queue (a key event for the T key) and call the on_key method with the event as the first argument. In other words it will call Input.on_key(event), which updates the display to show the new letter.

When the on_key method returns, Textual will get the next event from the queue and repeat the process for the remaining keys. At some point the queue will be empty and the widget is said to be in an idle state.

Note

This example illustrates a point, but a typical app will be fast enough to have processed a key before the next event arrives. So it is unlikely you will have so many key events in the message queue.

Default behaviors

You may be familiar with Python's super function to call a function defined in a base class. You will not have to use this in event handlers as Textual will automatically call handler methods defined in a widget's base class(es).

For instance, let's say we are building the classic game of Pong and we have written a Paddle widget which extends Static. When a Key event arrives, Textual calls Paddle.on_key (to respond to Up and Down keys), then Static.on_key, and finally Widget.on_key.

Preventing default behaviors

If you don't want this behavior you can call prevent_default() on the event object. This tells Textual not to call any more handlers on base classes.

Warning

You won't need prevent_default very often. Be sure to know what your base classes do before calling it, or you risk disabling some core features builtin to Textual.

Bubbling

Messages have a bubble attribute. If this is set to True then events will be sent to a widget's parent after processing. Input events typically bubble so that a widget will have the opportunity to respond to input events if they aren't handled by their children.

The following diagram shows an (abbreviated) DOM for a UI with a container and two buttons. With the "No" button focused, it will receive the key event first.

After Textual calls Button.on_key the event bubbles to the button's parent and will call Container.on_key (if it exists).

As before, the event bubbles to its parent (the App class).

The App class is always the root of the DOM, so there is nowhere for the event to bubble to.

Stopping bubbling

Event handlers may stop this bubble behavior by calling the stop() method on the event or message. You might want to do this if a widget has responded to the event in an authoritative way. For instance when a text input widget responds to a key event it stops the bubbling so that the key doesn't also invoke a key binding.

Custom messages

You can create custom messages for your application that may be used in the same way as events (recall that events are simply messages reserved for use by Textual).

The most common reason to do this is if you are building a custom widget and you need to inform a parent widget about a state change.

Let's look at an example which defines a custom message. The following example creates color buttons which—when clicked—send a custom message.

custom01.pyOutput

custom01.py

from textual.app import App, ComposeResult
from textual.color import Color
from textual.message import Message
from textual.widgets import Static


class ColorButton(Static):
    """A color button."""

    class Selected(Message):
        """Color selected message."""

        def __init__(self, color: Color) -> None:
            self.color = color
            super().__init__()

    def __init__(self, color: Color) -> None:
        self.color = color
        super().__init__()

    def on_mount(self) -> None:
        self.styles.margin = (1, 2)
        self.styles.content_align = ("center", "middle")
        self.styles.background = Color.parse("#ffffff33")
        self.styles.border = ("tall", self.color)

    def on_click(self) -> None:
        # The post_message method sends an event to be handled in the DOM
        self.post_message(self.Selected(self.color))

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


class ColorApp(App):
    def compose(self) -> ComposeResult:
        yield ColorButton(Color.parse("#008080"))
        yield ColorButton(Color.parse("#808000"))
        yield ColorButton(Color.parse("#E9967A"))
        yield ColorButton(Color.parse("#121212"))

    def on_color_button_selected(self, message: ColorButton.Selected) -> None:
        self.screen.styles.animate("background", message.color, duration=0.5)


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

Note the custom message class which extends Message. The constructor stores a color object which handler methods will be able to inspect.

The message class is defined within the widget class itself. This is not strictly required but recommended, for these reasons:

• It reduces the amount of imports. If you import ColorButton, you have access to the message class via ColorButton.Selected.
• It creates a namespace for the handler. So rather than on_selected, the handler name becomes on_color_button_selected. This makes it less likely that your chosen name will clash with another message.

Sending messages

To send a message call the post_message() method. This will place a message on the widget's message queue and run any message handlers.

It is common for widgets to send messages to themselves, and allow them to bubble. This is so a base class has an opportunity to handle the message. We do this in the example above, which means a subclass could add a on_color_button_selected if it wanted to handle the message itself.

Preventing messages

You can temporarily disable posting of messages of a particular type by calling prevent, which returns a context manager (used with Python's with keyword). This is typically used when updating data in a child widget and you don't want to receive notifications that something has changed.

The following example will play the terminal bell as you type. It does this by handling Input.Changed and calling bell(). There is a Clear button which sets the input's value to an empty string. This would normally also result in a Input.Changed event being sent (and the bell playing). Since we don't want the button to make a sound, the assignment to value is wrapped within a prevent context manager.

Tip

In reality, playing the terminal bell as you type would be very irritating -- we don't recommend it!

prevent.pyOutput

prevent.py

from textual.app import App, ComposeResult
from textual.widgets import Button, Input


class PreventApp(App):
    """Demonstrates `prevent` context manager."""

    def compose(self) -> ComposeResult:
        yield Input()
        yield Button("Clear", id="clear")

    def on_button_pressed(self) -> None:
        """Clear the text input."""
        input = self.query_one(Input)
        with input.prevent(Input.Changed):  # (1)!
            input.value = ""

    def on_input_changed(self) -> None:
        """Called as the user types."""
        self.bell()  # (2)!


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

1. Clear the input without sending an Input.Changed event.
2. Plays the terminal sound when typing.

Message handlers

Most of the logic in a Textual app will be written in message handlers. Let's explore handlers in more detail.

Handler naming

Textual uses the following scheme to map messages classes on to a Python method.

• Start with "on_".
• Add the message's namespace (if any) converted from CamelCase to snake_case plus an underscore "_".
• Add the name of the class converted from CamelCase to snake_case.

Messages have a namespace if they are defined as a child class of a Widget. The namespace is the name of the parent class. For instance, the builtin Input class defines its Changed message as follows:

class Input(Widget):
    ...
    class Changed(Message):
        """Posted when the value changes."""
        ...

Because Changed is a child class of Input, its namespace will be "input" (and the handler name will be on_input_changed). This allows you to have similarly named events, without clashing event handler names.

Tip

If you are ever in doubt about what the handler name should be for a given event, print the handler_name class variable for your event class.

Here's how you would check the handler name for the Input.Changed event:

>>> from textual.widgets import Input
>>> Input.Changed.handler_name
'on_input_changed'

On decorator

In addition to the naming convention, message handlers may be created with the on decorator, which turns a method into a handler for the given message or event.

For instance, the two methods declared below are equivalent:

@on(Button.Pressed)
def handle_button_pressed(self):
    ...

def on_button_pressed(self):
    ...

While this allows you to name your method handlers anything you want, the main advantage of the decorator approach over the naming convention is that you can specify which widget(s) you want to handle messages for.

Let's first explore where this can be useful. In the following example we have three buttons, each of which does something different; one plays the bell, one toggles dark mode, and the other quits the app.

on_decorator01.pyon_decorator.tcssOutput

on_decorator01.py

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


class OnDecoratorApp(App):
    CSS_PATH = "on_decorator.tcss"

    def compose(self) -> ComposeResult:
        """Three buttons."""
        yield Button("Bell", id="bell")
        yield Button("Toggle dark", classes="toggle dark")
        yield Button("Quit", id="quit")

    def on_button_pressed(self, event: Button.Pressed) -> None:  # (1)!
        """Handle all button pressed events."""
        if event.button.id == "bell":
            self.bell()
        elif event.button.has_class("toggle", "dark"):
            self.theme = (
                "textual-dark" if self.theme == "textual-light" else "textual-light"
            )
        elif event.button.id == "quit":
            self.exit()


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

1. The message handler is called when any button is pressed

on_decorator.tcss

Screen {
    align: center middle;
    layout: horizontal;
}

Button {
    margin: 2 4;
}

Note how the message handler has a chained if statement to match the action to the button. While this works just fine, it can be a little hard to follow when the number of buttons grows.

The on decorator takes a CSS selector in addition to the event type which will be used to select which controls the handler should work with. We can use this to write a handler per control rather than manage them all in a single handler.

The following example uses the decorator approach to write individual message handlers for each of the three buttons:

on_decorator02.pyon_decorator.tcssOutput

on_decorator02.py

from textual import on
from textual.app import App, ComposeResult
from textual.widgets import Button


class OnDecoratorApp(App):
    CSS_PATH = "on_decorator.tcss"

    def compose(self) -> ComposeResult:
        """Three buttons."""
        yield Button("Bell", id="bell")
        yield Button("Toggle dark", classes="toggle dark")
        yield Button("Quit", id="quit")

    @on(Button.Pressed, "#bell")  # (1)!
    def play_bell(self):
        """Called when the bell button is pressed."""
        self.bell()