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" in to 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 Left and Right 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, MessageTarget
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.containers import Horizontal
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 messages' namespace (if any) converted from CamelCase to snake_case plus an underscore "_"
• Add the name of the class converted from CamelCase to snake_case.

Handler arguments

Message handler methods can be written with or without a positional argument. If you add a positional argument, Textual will call the handler with the event object. The following handler (taken from custom01.py above) contains a message parameter. The body of the code makes use of the message to set a preset color.

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

If the body of your handler doesn't require any information in the message you can omit it from the method signature. If we just want to play a bell noise when the button is clicked, we could write our handler like this:

    def on_color_button_selected(self) -> None:
        self.app.bell()

This pattern is a convenience that saves writing out a parameter that may not be used.

Async handlers

Message handlers may be coroutines. If you prefix your handlers with the async keyword, Textual will await them. This lets your handler use the await keyword for asynchronous APIs.

If your event handlers are coroutines it will allow multiple events to be processed concurrently, but bear in mind an individual widget (or app) will not be able to pick up a new message from its message queue until the handler has returned. This is rarely a problem in practice; as long as handlers return within a few milliseconds the UI will remain responsive. But slow handlers might make your app hard to use.

Info

To re-use the chef analogy, if an order comes in for beef wellington (which takes a while to cook), orders may start to pile up and customers may have to wait for their meal. The solution would be to have another chef work on the wellington while the first chef picks up new orders.

Network access is a common cause of slow handlers. If you try to retrieve a file from the internet, the message handler may take anything up to a few seconds to return, which would prevent the widget or app from updating during that time. The solution is to launch a new asyncio task to do the network task in the background.

Let's look at an example which looks up word definitions from an api as you type.

Note

You will need to install httpx with pip install httpx to run this example.

dictionary.pydictionary.cssOutput

dictionary.py

import asyncio

try:
    import httpx
except ImportError:
    raise ImportError("Please install httpx with 'pip install httpx' ")

from rich.json import JSON

from textual.app import App, ComposeResult
from textual.containers import VerticalScroll
from textual.widgets import Input, Static


class DictionaryApp(App):
    """Searches a dictionary API as-you-type."""

    CSS_PATH = "dictionary.css"

    def compose(self) -> ComposeResult:
        yield Input(placeholder="Search for a word")
        yield VerticalScroll(Static(id="results"), id="results-container")

    async def on_input_changed(self, message: Input.Changed) -> None:
        """A coroutine to handle a text changed message."""
        if message.value:
            # Look up the word in the background
            asyncio.create_task(self.lookup_word(message.value))
        else:
            # Clear the results
            self.query_one("#results", Static).update()

    async def lookup_word(self, word: str) -> None:
        """Looks up a word."""
        url = f"https://api.dictionaryapi.dev/api/v2/entries/en/{word}"
        async with httpx.AsyncClient() as client:
            results = (await client.get(url)).text

        if word == self.query_one(Input).value:
            self.query_one("#results", Static).update(JSON(results))


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

dictionary.css

Screen {
    background: $panel;
}

Input {
    dock: top;    
    width: 100%;
    height: 1;
    padding: 0 1;
    margin: 1 1 0 1;    
}

#results {
    width: auto;   
    min-height: 100%;
}

#results-container {
    background: $background 50%;
    overflow: auto;    
    margin: 1 2;    
    height: 100%;
}

Note the highlighted line in the above code which calls asyncio.create_task to run a coroutine in the background. Without this you would find typing in to the text box to be unresponsive.