App Basics¶
In this chapter we will cover how to use Textual's App class to create an application. Just enough to get you up to speed. We will go in to more detail in the following chapters.
The App class¶
The first step in building a Textual app is to import the App class and create a subclass. Let's look at the simplest app class:
The run method¶
To run an app we create an instance and call run().
from textual.app import App
class MyApp(App):
pass
if __name__ == "__main__":
app = MyApp()
app.run()
Apps don't get much simpler than this—don't expect it to do much.
Tip
The __name__ == "__main__": condition is true only if you run the file with python command. This allows us to import app without running the app immediately. It also allows the devtools run command to run the app in development mode. See the Python docs for more information.
If we run this app with python simple02.py you will see a blank terminal, something like the following:
When you call App.run() Textual puts the terminal in to a special state called application mode. When in application mode the terminal will no longer echo what you type. Textual will take over responding to user input (keyboard and mouse) and will update the visible portion of the terminal (i.e. the screen).
If you hit Ctrl+C Textual will exit application mode and return you to the command prompt. Any content you had in the terminal prior to application mode will be restored.
Tip
A side effect of application mode is that you may no longer be able to select and copy text in the usual way. Terminals typically offer a way to bypass this limit with a key modifier. On iTerm you can select text if you hold the Option key. See the documentation for your terminal software for how to select text in application mode.
Run inline¶
Added in version 0.55.0
You can also run apps in inline mode, which will cause the app to appear beneath the prompt (and won't go in to application mode). Inline apps are useful for tools that integrate closely with the typical workflow of a terminal.
To run an app in inline mode set the inline parameter to True when you call App.run(). See Style Inline Apps for how to apply additional styles to inline apps.
Events¶
Textual has an event system you can use to respond to key presses, mouse actions, and internal state changes. Event handlers are methods prefixed with on_ followed by the name of the event.
One such event is the mount event which is sent to an application after it enters application mode. You can respond to this event by defining a method called on_mount.
Info
You may have noticed we use the term "send" and "sent" in relation to event handler methods in preference to "calling". This is because Textual uses a message passing system where events are passed (or sent) between components. See events for details.
Another such event is the key event which is sent when the user presses a key. The following example contains handlers for both those events:
from textual.app import App
from textual import events
class EventApp(App):
COLORS = [
"white",
"maroon",
"red",
"purple",
"fuchsia",
"olive",
"yellow",
"navy",
"teal",
"aqua",
]
def on_mount(self) -> None:
self.screen.styles.background = "darkblue"
def on_key(self, event: events.Key) -> None:
if event.key.isdecimal():
self.screen.styles.background = self.COLORS[int(event.key)]
if __name__ == "__main__":
app = EventApp()
app.run()
The on_mount handler sets the self.screen.styles.background attribute to "darkblue" which (as you can probably guess) turns the background blue. Since the mount event is sent immediately after entering application mode, you will see a blue screen when you run this code.
The key event handler (on_key) has an event parameter which will receive a Key instance. Every event has an associated event object which will be passed to the handler method if it is present in the method's parameter list.
Note
It is unusual (but not unprecedented) for a method's parameters to affect how it is called. Textual accomplishes this by inspecting the method prior to calling it.
Some events contain additional information you can inspect in the handler. The Key event has a key attribute which is the name of the key that was pressed. The on_key method above uses this attribute to change the background color if any of the keys from 0 to 9 are pressed.
Async events¶
Textual is powered by Python's asyncio framework which uses the async and await keywords.
Textual knows to await your event handlers if they are coroutines (i.e. prefixed with the async keyword). Regular functions are generally fine unless you plan on integrating other async libraries (such as httpx for reading data from the internet).
Tip
For a friendly introduction to async programming in Python, see FastAPI's concurrent burgers article.
Widgets¶
Widgets are self-contained components responsible for generating the output for a portion of the screen. Widgets respond to events in much the same way as the App. Most apps that do anything interesting will contain at least one (and probably many) widgets which together form a User Interface.
Widgets can be as simple as a piece of text, a button, or a fully-fledged component like a text editor or file browser (which may contain widgets of their own).
Composing¶
To add widgets to your app implement a compose() method which should return an iterable of Widget instances. A list would work, but it is convenient to yield widgets, making the method a generator.
The following example imports a builtin Welcome widget and yields it from App.compose().
from textual.app import App, ComposeResult
from textual.widgets import Welcome
class WelcomeApp(App):
def compose(self) -> ComposeResult:
yield Welcome()
def on_button_pressed(self) -> None:
self.exit()
if __name__ == "__main__":
app = WelcomeApp()
app.run()
When you run this code, Textual will mount the Welcome widget which contains Markdown content and a button:
Notice the on_button_pressed method which handles the Button.Pressed event sent by a button contained in the Welcome widget. The handler calls App.exit() to exit the app.
Mounting¶
While composing is the preferred way of adding widgets when your app starts it is sometimes necessary to add new widget(s) in response to events. You can do this by calling mount() which will add a new widget to the UI.
Here's an app which adds a welcome widget in response to any key press:
from textual.app import App
from textual.widgets import Welcome
class WelcomeApp(App):
def on_key(self) -> None:
self.mount(Welcome())
def on_button_pressed(self) -> None:
self.exit()
if __name__ == "__main__":
app = WelcomeApp()
app.run()
When you first run this you will get a blank screen. Press any key to add the welcome widget. You can even press a key multiple times to add several widgets.
Awaiting mount¶
When you mount a widget, Textual will mount everything the widget composes.
Textual guarantees that the mounting will be complete by the next message handler, but not immediately after the call to mount().
This may be a problem if you want to make any changes to the widget in the same message handler.
Let's first illustrate the problem with an example. The following code will mount the Welcome widget in response to a key press. It will also attempt to modify the Button in the Welcome widget by changing its label from "OK" to "YES!".
from textual.app import App
from textual.widgets import Button, Welcome
class WelcomeApp(App):
def on_key(self) -> None:
self.mount(Welcome())
self.query_one(Button).label = "YES!" # (1)!
if __name__ == "__main__":
app = WelcomeApp()
app.run()
- See queries for more information on the
query_onemethod.
If you run this example, you will find that Textual raises a NoMatches exception when you press a key. This is because the mount process has not yet completed when we attempt to change the button.
To solve this we can optionally await the result of mount(), which requires we make the function async.
This guarantees that by the following line, the Button has been mounted, and we can change its label.
from textual.app import App
from textual.widgets import Button, Welcome
class WelcomeApp(App):
async def on_key(self) -> None:
await self.mount(Welcome())
self.query_one(Button).label = "YES!"
if __name__ == "__main__":
app = WelcomeApp()
app.run()
Here's the output. Note the changed button text:
Exiting¶
An app will run until you call App.exit() which will exit application mode and the run method will return. If this is the last line in your code you will return to the command prompt.
The exit method will also accept an optional positional value to be returned by run(). The following example uses this to return the id (identifier) of a clicked button.
from textual.app import App, ComposeResult
from textual.widgets import Label, Button
class QuestionApp(App[str]):
def compose(self) -> ComposeResult:
yield Label("Do you love Textual?")
yield Button("Yes", id="yes", variant="primary")
yield Button("No", id="no", variant="error")
def on_button_pressed(self, event: Button.Pressed) -> None:
self.exit(event.button.id)
if __name__ == "__main__":
app = QuestionApp()
reply = app.run()
print(reply)
Running this app will give you the following:
Clicking either of those buttons will exit the app, and the run() method will return either "yes" or "no" depending on button clicked.
Return type¶
You may have noticed that we subclassed App[str] rather than the usual App.
from textual.app import App, ComposeResult
from textual.widgets import Label, Button
class QuestionApp(App[str]):
def compose(self) -> ComposeResult:
yield Label("Do you love Textual?")
yield Button("Yes", id="yes", variant="primary")
yield Button("No", id="no", variant="error")
def on_button_pressed(self, event: Button.Pressed) -> None:
self.exit(event.button.id)
if __name__ == "__main__":
app = QuestionApp()
reply = app.run()
print(reply)
The addition of [str] tells mypy that run() is expected to return a string. It may also return None if App.exit() is called without a return value, so the return type of run will be str | None. Replace the str in [str] with the type of the value you intend to call the exit method with.
Note
Type annotations are entirely optional (but recommended) with Textual.
Return code¶
When you exit a Textual app with App.exit(), you can optionally specify a return code with the return_code parameter.
What are return codes?
Returns codes are a standard feature provided by your operating system.
When any application exits it can return an integer to indicate if it was successful or not.
A return code of 0 indicates success, any other value indicates that an error occurred.
The exact meaning of a non-zero return code is application-dependant.
When a Textual app exits normally, the return code will be 0. If there is an unhandled exception, Textual will set a return code of 1.
You may want to set a different value for the return code if there is error condition that you want to differentiate from an unhandled exception.
Here's an example of setting a return code for an error condition:
The app's return code can be queried with app.return_code, which will be None if it hasn't been set, or an integer.
Textual won't explicitly exit the process.
To exit the app with a return code, you should call sys.exit.
Here's how you might do that:
Suspending¶
A Textual app may be suspended so you can leave application mode for a period of time. This is often used to temporarily replace your app with another terminal application.
You could use this to allow the user to edit content with their preferred text editor, for example.
Info
App suspension is unavailable with textual-web.
Suspend context manager¶
You can use the App.suspend context manager to suspend your app. The following Textual app will launch vim (a text editor) when the user clicks a button:
from os import system
from textual import on
from textual.app import App, ComposeResult
from textual.widgets import Button
class SuspendingApp(App[None]):
def compose(self) -> ComposeResult:
yield Button("Open the editor", id="edit")
@on(Button.Pressed, "#edit")
def run_external_editor(self) -> None:
with self.suspend(): # (1)!
system("vim")
if __name__ == "__main__":
SuspendingApp().run()
- All code in the body of the
withstatement will be run while the app is suspended.
Suspending from foreground¶
On Unix and Unix-like systems (GNU/Linux, macOS, etc) Textual has support for the user pressing a key combination to suspend the application as the foreground process.
Ordinarily this key combination is Ctrl+Z;
in a Textual application this is disabled by default, but an action is provided (action_suspend_process) that you can bind in the usual way.
For example:
from textual.app import App, ComposeResult
from textual.binding import Binding
from textual.widgets import Label
class SuspendKeysApp(App[None]):
BINDINGS = [Binding("ctrl+z", "suspend_process")]
def compose(self) -> ComposeResult:
yield Label("Press Ctrl+Z to suspend!")
if __name__ == "__main__":
SuspendKeysApp().run()
Note
If suspend_process is called on Windows, or when your application is being hosted under Textual Web, the call will be ignored.
CSS¶
Textual apps can reference CSS files which define how your app and widgets will look, while keeping your Python code free of display related code (which tends to be messy).
Info
Textual apps typically use the extension .tcss for external CSS files to differentiate them from browser (.css) files.
The chapter on Textual CSS describes how to use CSS in detail. For now let's look at how your app references external CSS files.
The following example enables loading of CSS by adding a CSS_PATH class variable:
from textual.app import App, ComposeResult
from textual.widgets import Button, Label
class QuestionApp(App[str]):
CSS_PATH = "question02.tcss"
def compose(self) -> ComposeResult:
yield Label("Do you love Textual?", id="question")
yield Button("Yes", id="yes", variant="primary")
yield Button("No", id="no", variant="error")
def on_button_pressed(self, event: Button.Pressed) -> None:
self.exit(event.button.id)
if __name__ == "__main__":
app = QuestionApp()
reply = app.run()
print(reply)
Note
We also added an id to the Label, because we want to style it in the CSS.
If the path is relative (as it is above) then it is taken as relative to where the app is defined. Hence this example references "question01.tcss" in the same directory as the Python code. Here is that CSS file:
Screen {
layout: grid;
grid-size: 2;
grid-gutter: 2;
padding: 2;
}
#question {
width: 100%;
height: 100%;
column-span: 2;
content-align: center bottom;
text-style: bold;
}
Button {
width: 100%;
}
When "question02.py" runs it will load "question02.tcss" and update the app and widgets accordingly. Even though the code is almost identical to the previous sample, the app now looks quite different:
Classvar CSS¶
While external CSS files are recommended for most applications, and enable some cool features like live editing, you can also specify the CSS directly within the Python code.
To do this set a CSS class variable on the app to a string containing your CSS.
Here's the question app with classvar CSS:
from textual.app import App, ComposeResult
from textual.widgets import Label, Button
class QuestionApp(App[str]):
CSS = """
Screen {
layout: grid;
grid-size: 2;
grid-gutter: 2;
padding: 2;
}
#question {
width: 100%;
height: 100%;
column-span: 2;
content-align: center bottom;
text-style: bold;
}
Button {
width: 100%;
}
"""
def compose(self) -> ComposeResult:
yield Label("Do you love Textual?", id="question")
yield Button("Yes", id="yes", variant="primary")
yield Button("No", id="no", variant="error")
def on_button_pressed(self, event: Button.Pressed) -> None:
self.exit(event.button.id)
if __name__ == "__main__":
app = QuestionApp()
reply = app.run()
print(reply)
Title and subtitle¶
Textual apps have a title attribute which is typically the name of your application, and an optional sub_title attribute which adds additional context (such as the file your are working on).
By default, title will be set to the name of your App class, and sub_title is empty.
You can change these defaults by defining TITLE and SUB_TITLE class variables. Here's an example of that:
from textual.app import App, ComposeResult
from textual.widgets import Button, Header, Label
class MyApp(App[str]):
CSS_PATH = "question02.tcss"
TITLE = "A Question App"
SUB_TITLE = "The most important question"
def compose(self) -> ComposeResult:
yield Header()
yield Label("Do you love Textual?", id="question")
yield Button("Yes", id="yes", variant="primary")
yield Button("No", id="no", variant="error")
def on_button_pressed(self, event: Button.Pressed) -> None:
self.exit(event.button.id)
if __name__ == "__main__":
app = MyApp()
reply = app.run()
print(reply)
Note that the title and subtitle are displayed by the builtin Header widget at the top of the screen:
You can also set the title attributes dynamically within a method of your app. The following example sets the title and subtitle in response to a key press:
from textual.app import App, ComposeResult
from textual.events import Key
from textual.widgets import Button, Header, Label
class MyApp(App[str]):
CSS_PATH = "question02.tcss"
TITLE = "A Question App"
SUB_TITLE = "The most important question"
def compose(self) -> ComposeResult:
yield Header()
yield Label("Do you love Textual?", id="question")
yield Button("Yes", id="yes", variant="primary")
yield Button("No", id="no", variant="error")
def on_button_pressed(self, event: Button.Pressed) -> None:
self.exit(event.button.id)
def on_key(self, event: Key):
self.title = event.key
self.sub_title = f"You just pressed {event.key}!"
if __name__ == "__main__":
app = MyApp()
reply = app.run()
print(reply)
If you run this app and press the T key, you should see the header update accordingly:
Info
Note that there is no need to explicitly refresh the screen when setting the title attributes. This is an example of reactivity, which we will cover later in the guide.
What's next¶
In the following chapter we will learn more about how to apply styles to your widgets and app.