Input¶
This chapter will discuss how to make your app respond to input in the form of key presses and mouse actions.
Quote
More Input!
— Johnny Five
Keyboard input¶
The most fundamental way to receive input is via Key events which are sent to your app when the user presses a key. Let's write an app to show key events as you type.
from textual import events
from textual.app import App, ComposeResult
from textual.widgets import RichLog
class InputApp(App):
"""App to display key events."""
def compose(self) -> ComposeResult:
yield RichLog()
def on_key(self, event: events.Key) -> None:
self.query_one(RichLog).write(event)
if __name__ == "__main__":
app = InputApp()
app.run()
When you press a key, the app will receive the event and write it to a RichLog widget. Try pressing a few keys to see what happens.
Tip
For a more feature rich version of this example, run textual keys from the command line.
Key Event¶
The key event contains the following attributes which your app can use to know how to respond.
key¶
The key attribute is a string which identifies the key that was pressed. The value of key will be a single character for letters and numbers, or a longer identifier for other keys.
Some keys may be combined with the Shift key. In the case of letters, this will result in a capital letter as you might expect. For non-printable keys, the key attribute will be prefixed with shift+. For example, Shift+Home will produce an event with key="shift+home".
Many keys can also be combined with Ctrl which will prefix the key with ctrl+. For instance, Ctrl+P will produce an event with key="ctrl+p".
Warning
Not all keys combinations are supported in terminals and some keys may be intercepted by your OS. If in doubt, run textual keys from the command line.
character¶
If the key has an associated printable character, then character will contain a string with a single Unicode character. If there is no printable character for the key (such as for function keys) then character will be None.
For example the P key will produce character="p" but F2 will produce character=None.
name¶
The name attribute is similar to key but, unlike key, is guaranteed to be valid within a Python function name. Textual derives name from the key attribute by lower casing it and replacing + with _. Upper case letters are prefixed with upper_ to distinguish them from lower case names.
For example, Ctrl+P produces name="ctrl_p" and Shift+P produces name="upper_p".
is_printable¶
The is_printable attribute is a boolean which indicates if the key would typically result in something that could be used in an input widget. If is_printable is False then the key is a control code or function key that you wouldn't expect to produce anything in an input.
aliases¶
Some keys or combinations of keys can produce the same event. For instance, the Tab key is indistinguishable from Ctrl+I in the terminal. For such keys, Textual events will contain a list of the possible keys that may have produced this event. In the case of Tab, the aliases attribute will contain ["tab", "ctrl+i"]
Key methods¶
Textual offers a convenient way of handling specific keys. If you create a method beginning with key_ followed by the key name (the event's name attribute), then that method will be called in response to the key press.
Let's add a key method to the example code.
from textual import events
from textual.app import App, ComposeResult
from textual.widgets import RichLog
class InputApp(App):
"""App to display key events."""
def compose(self) -> ComposeResult:
yield RichLog()
def on_key(self, event: events.Key) -> None:
self.query_one(RichLog).write(event)
def key_space(self) -> None:
self.bell()
if __name__ == "__main__":
app = InputApp()
app.run()
Note the addition of a key_space method which is called in response to the space key, and plays the terminal bell noise.
Note
Consider key methods to be a convenience for experimenting with Textual features. In nearly all cases, key bindings and actions are preferable.
Input focus¶
Only a single widget may receive key events at a time. The widget which is actively receiving key events is said to have input focus.
The following example shows how focus works in practice.
from textual import events
from textual.app import App, ComposeResult
from textual.widgets import RichLog
class KeyLogger(RichLog):
def on_key(self, event: events.Key) -> None:
self.write(event)
class InputApp(App):
"""App to display key events."""
CSS_PATH = "key03.tcss"
def compose(self) -> ComposeResult:
yield KeyLogger()
yield KeyLogger()
yield KeyLogger()
yield KeyLogger()
if __name__ == "__main__":
app = InputApp()
app.run()
The app splits the screen in to quarters, with a RichLog widget in each quarter. If you click any of the text logs, you should see that it is highlighted to show that the widget has focus. Key events will be sent to the focused widget only.
Tip
the :focus CSS pseudo-selector can be used to apply a style to the focused widget.
You can move focus by pressing the Tab key to focus the next widget. Pressing Shift+Tab moves the focus in the opposite direction.
Focusable widgets¶
Each widget has a boolean can_focus attribute which determines if it is capable of receiving focus.
Note that can_focus=True does not mean the widget will always be focusable.
For example, a disabled widget cannot receive focus even if can_focus is True.
Controlling focus¶
Textual will handle keyboard focus automatically, but you can tell Textual to focus a widget by calling the widget's focus() method. By default, Textual will focus the first focusable widget when the app starts.
Focus events¶
When a widget receives focus, it is sent a Focus event. When a widget loses focus it is sent a Blur event.
Bindings¶
Keys may be associated with actions for a given widget. This association is known as a key binding.
To create bindings, add a BINDINGS class variable to your app or widget. This should be a list of tuples of three strings.
The first value is the key, the second is the action, the third value is a short human readable description.
The following example binds the keys R, G, and B to an action which adds a bar widget to the screen.
from textual.app import App, ComposeResult
from textual.color import Color
from textual.widgets import Footer, Static
class Bar(Static):
pass
class BindingApp(App):
CSS_PATH = "binding01.tcss"
BINDINGS = [
("r", "add_bar('red')", "Add Red"),
("g", "add_bar('green')", "Add Green"),
("b", "add_bar('blue')", "Add Blue"),
]
def compose(self) -> ComposeResult:
yield Footer()
def action_add_bar(self, color: str) -> None:
bar = Bar(color)
bar.styles.background = Color.parse(color).with_alpha(0.5)
self.mount(bar)
self.call_after_refresh(self.screen.scroll_end, animate=False)
if __name__ == "__main__":
app = BindingApp()
app.run()
Note how the footer displays bindings and makes them clickable.
Tip
Multiple keys can be bound to a single action by comma-separating them.
For example, ("r,t", "add_bar('red')", "Add Red") means both R and T are bound to add_bar('red').
When you press a key, Textual will first check for a matching binding in the BINDINGS list of the currently focused widget.
If no match is found, it will search upwards through the DOM all the way up to the App looking for a match.
Binding class¶
The tuple of three strings may be enough for simple bindings, but you can also replace the tuple with a Binding instance which exposes a few more options.
Priority bindings¶
Individual bindings may be marked as a priority, which means they will be checked prior to the bindings of the focused widget. This feature is often used to create hot-keys on the app or screen. Such bindings can not be disabled by binding the same key on a widget.
You can create priority key bindings by setting priority=True on the Binding object. Textual uses this feature to add a default binding for Ctrl+Q so there is always a way to exit the app. Here's the BINDINGS from the App base class. Note the quit binding is set as a priority:
Show bindings¶
The footer widget can inspect bindings to display available keys. If you don't want a binding to display in the footer you can set show=False. The default bindings on App do this so that the standard Ctrl+C, Tab and Shift+Tab bindings don't typically appear in the footer.
Mouse Input¶
Textual will send events in response to mouse movement and mouse clicks. These events contain the coordinates of the mouse cursor relative to the terminal or widget.
Information
The trackpad (and possibly other pointer devices) are treated the same as the mouse in terminals.
Terminal coordinates are given by a pair values named x and y. The X coordinate is an offset in characters, extending from the left to the right of the screen. The Y coordinate is an offset in lines, extending from the top of the screen to the bottom.
Coordinates may be relative to the screen, so (0, 0) would be the top left of the screen. Coordinates may also be relative to a widget, where (0, 0) would be the top left of the widget itself.
Mouse movements¶
When you move the mouse cursor over a widget it will receive MouseMove events which contain the coordinate of the mouse and information about what modifier keys (Ctrl, Shift etc) are held down.
The following example shows mouse movements being used to attach a widget to the mouse cursor.
from textual import events
from textual.app import App, ComposeResult
from textual.widgets import RichLog, Static
class Ball(Static):
pass
class MouseApp(App):
CSS_PATH = "mouse01.tcss"
def compose(self) -> ComposeResult:
yield RichLog()
yield Ball("Textual")
def on_mouse_move(self, event: events.MouseMove) -> None:
self.screen.query_one(RichLog).write(event)
self.query_one(Ball).offset = event.screen_offset - (8, 2)
if __name__ == "__main__":
app = MouseApp()
app.run()
If you run mouse01.py you should find that it logs the mouse move event, and keeps a widget pinned directly under the cursor.
The on_mouse_move handler sets the offset style of the ball (a rectangular one) to match the mouse coordinates.
Mouse capture¶
In the mouse01.py example there was a call to capture_mouse() in the mount handler. Textual will send mouse move events to the widget directly under the cursor. You can tell Textual to send all mouse events to a widget regardless of the position of the mouse cursor by calling capture_mouse.
Call release_mouse to restore the default behavior.
Warning
If you capture the mouse, be aware you might get negative mouse coordinates if the cursor is to the left of the widget.
Textual will send a MouseCapture event when the mouse is captured, and a MouseRelease event when it is released.
Enter and Leave events¶
Textual will send a Enter event to a widget when the mouse cursor first moves over it, and a Leave event when the cursor moves off a widget.
Both Enter and Leave bubble, so a widget may receive these events from a child widget.
You can check the initial widget these events were sent to by comparing the node attribute against self in the message handler.
Click events¶
There are three events associated with clicking a button on your mouse. When the button is initially pressed, Textual sends a MouseDown event, followed by MouseUp when the button is released. Textual then sends a final Click event.
If you want your app to respond to a mouse click you should prefer the Click event (and not MouseDown or MouseUp). This is because a future version of Textual may support other pointing devices which don't have up and down states.
Scroll events¶
Most mice have a scroll wheel which you can use to scroll the window underneath the cursor. Scrollable containers in Textual will handle these automatically, but you can handle MouseScrollDown and MouseScrollUp if you want build your own scrolling functionality.
Information
Terminal emulators will typically convert trackpad gestures in to scroll events.