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Building full screen applications

prompt_toolkit can be used to create complex full screen terminal applications. Typically, an application consists of a layout (to describe the graphical part) and a set of key bindings.

The sections below describe the components required for full screen applications (or custom, non full screen applications), and how to assemble them together.

Before going through this page, it could be helpful to go through :ref:`asking for input <asking_for_input>` (prompts) first. Many things that apply to an input prompt, like styling, key bindings and so on, also apply to full screen applications.

Note

Also remember that the examples directory of the prompt_toolkit repository contains plenty of examples. Each example is supposed to explain one idea. So, this as well should help you get started.

Don't hesitate to open a GitHub issue if you feel that a certain example is missing.

A simple application

Every prompt_toolkit application is an instance of an :class:`~prompt_toolkit.application.Application` object. The simplest full screen example would look like this:

from prompt_toolkit import Application

app = Application(full_screen=True)
app.run()

This will display a dummy application that says "No layout specified. Press ENTER to quit.".

Note

If we wouldn't set the full_screen option, the application would not run in the alternate screen buffer, and only consume the least amount of space required for the layout.

An application consists of several components. The most important are:

  • I/O objects: the input and output device.
  • The layout: this defines the graphical structure of the application. For instance, a text box on the left side, and a button on the right side. You can also think of the layout as a collection of 'widgets'.
  • A style: this defines what colors and underline/bold/italic styles are used everywhere.
  • A set of key bindings.

We will discuss all of these in more detail below.

I/O objects

Every :class:`~prompt_toolkit.application.Application` instance requires an I/O objects for input and output:

Both are optional and normally not needed to pass explicitly. Usually, the default works fine.

There is a third I/O object which is also required by the application, but not passed inside. This is the event loop, an :class:`~prompt_toolkit.eventloop.EventLoop` instance. This is basically a while-true loop that waits for user input, and when it receives something (like a key press), it will send that to the the appropriate handler, like for instance, a key binding.

When :func:`~prompt_toolkit.application.Application.run()` is called, the event loop will run until the application is done. An application will quit when :func:`~prompt_toolkit.application.Application.exit()` is called.

The layout

A layered layout architecture

There are several ways to create a prompt_toolkit layout, depending on how customizable you want things to be. In fact, there are several layers of abstraction.

Containers and controls

The biggest difference between containers and controls is that containers arrange the layout by splitting the screen in many regions, while controls are responsible for generating the actual content.

Note

Under the hood, the difference is:

Abstract base class Examples
:class:`~prompt_toolkit.layout.Container` :class:`~prompt_toolkit.layout.HSplit` :class:`~prompt_toolkit.layout.VSplit` :class:`~prompt_toolkit.layout.FloatContainer` :class:`~prompt_toolkit.layout.Window`
:class:`~prompt_toolkit.layout.UIControl` :class:`~prompt_toolkit.layout.BufferControl` :class:`~prompt_toolkit.layout.FormattedTextControl`

The :class:`~prompt_toolkit.layout.Window` class itself is particular: it is a :class:`~prompt_toolkit.layout.Container` that can contain a :class:`~prompt_toolkit.layout.UIControl`. Thus, it's the adaptor between the two. The :class:`~prompt_toolkit.layout.Window` class also takes care of scrolling the content and wrapping the lines if needed.

Finally, there is the :class:`~prompt_toolkit.layout.Layout` class which wraps the whole layout. This is responsible for keeping track of which window has the focus.

Here is an example of a layout that displays the content of the default buffer on the left, and displays "Hello world" on the right. In between it shows a vertical line:

from prompt_toolkit import Application
from prompt_toolkit.buffer import Buffer
from prompt_toolkit.layout.containers import VSplit, Window
from prompt_toolkit.layout.controls import BufferControl, FormattedTextControl
from prompt_toolkit.layout.layout import Layout

buffer1 = Buffer()  # Editable buffer.

root_container = VSplit([
    # One window that holds the BufferControl with the default buffer on
    # the left.
    Window(content=BufferControl(buffer=buffer1)),

    # A vertical line in the middle. We explicitly specify the width, to
    # make sure that the layout engine will not try to divide the whole
    # width by three for all these windows. The window will simply fill its
    # content by repeating this character.
    Window(width=1, char='|'),

    # Display the text 'Hello world' on the right.
    Window(content=FormattedTextControl(text='Hello world')),
])

layout = Layout(root_container)

app = Application(layout=layout, full_screen=True)
app.run() # You won't be able to Exit this app

Notice that if you execute this right now, there is no way to quit this application yet. This is something we explain in the next section below.

More complex layouts can be achieved by nesting multiple :class:`~prompt_toolkit.layout.VSplit`, :class:`~prompt_toolkit.layout.HSplit` and :class:`~prompt_toolkit.layout.FloatContainer` objects.

If you want to make some part of the layout only visible when a certain condition is satisfied, use a :class:`~prompt_toolkit.layout.ConditionalContainer`.

Focusing windows

Focussing something can be done by calling the :meth:`~prompt_toolkit.layout.Layout.focus` method. This method is very flexible and accepts a :class:`~prompt_toolkit.layout.Window`, a :class:`~prompt_toolkit.buffer.Buffer`, a :class:`~prompt_toolkit.layout.controls.UIControl` and more.

In the following example, we use :func:`~prompt_toolkit.application.get_app` for getting the active application.

from prompt_toolkit.application import get_app

# This window was created earlier.
w = Window()

# ...

# Now focus it.
get_app().layout.focus(w)

Changing the focus is something which is typically done in a key binding, so read on to see how to define key bindings.

Key bindings

In order to react to user actions, we need to create a :class:`~prompt_toolkit.key_binding.KeyBindings` object and pass that to our :class:`~prompt_toolkit.application.Application`.

There are two kinds of key bindings:

Global key bindings

Key bindings can be passed to the application as follows:

from prompt_toolkit import Application
from prompt_toolkit.key_binding import KeyBindings

kb = KeyBindings()
app = Application(key_bindings=kb)
app.run()

To register a new keyboard shortcut, we can use the :meth:`~prompt_toolkit.key_binding.KeyBindings.add` method as a decorator of the key handler:

from prompt_toolkit import Application
from prompt_toolkit.key_binding import KeyBindings

kb = KeyBindings()

@kb.add('c-q')
def exit_(event):
    """
    Pressing Ctrl-Q will exit the user interface.

    Setting a return value means: quit the event loop that drives the user
    interface and return this value from the `CommandLineInterface.run()` call.
    """
    event.app.exit()

app = Application(key_bindings=kb, full_screen=True)
app.run()

The callback function is named exit_ for clarity, but it could have been named _ (underscore) as well, because the we won't refer to this name.

:ref:`Read more about key bindings ...<key_bindings>`

Modal containers

All container objects, like :class:`~prompt_toolkit.layout.VSplit` and :class:`~prompt_toolkit.layout.HSplit` take a modal argument.

If this flag has been set, then key bindings from the parent account are not taken into account if one of the children windows has the focus.

This is useful in a complex layout, where many controls have their own key bindings, but you only want to enable the key bindings for a certain region of the layout.

The global key bindings are always active.

More about the Window class

As said earlier, a :class:`~prompt_toolkit.layout.Window` is a :class:`~prompt_toolkit.layout.Container` that wraps a :class:`~prompt_toolkit.layout.UIControl`, like a :class:`~prompt_toolkit.layout.BufferControl` or :class:`~prompt_toolkit.layout.FormattedTextControl`.

Note

Basically, windows are the leafs in the tree structure that represent the UI.

A :class:`~prompt_toolkit.layout.Window` provides a "view" on the :class:`~prompt_toolkit.layout.UIControl`, which provides lines of content. The window is in the first place responsible for the line wrapping and scrolling of the content, but there are much more options.

  • Adding left or right margins. These are used for displaying scroll bars or line numbers.
  • There are the cursorline and cursorcolumn options. These allow highlighting the line or column of the cursor position.
  • Alignment of the content. The content can be left aligned, right aligned or centered.
  • Finally, the background can be filled with a default character.

More about buffers and BufferControl

Input processors

A :class:`~prompt_toolkit.layout.processors.Processor` is used to postprocess the content of a :class:`~prompt_toolkit.layout.BufferControl` before it's displayed. It can for instance highlight matching brackets or change the visualisation of tabs and so on.

A :class:`~prompt_toolkit.layout.processors.Processor` operates on individual lines. Basically, it takes a (formatted) line and produces a new (formatted) line.

Some build-in processors:

Processor Usage:
:class:`~prompt_toolkit.layout.processors.HighlightSearchProcessor` Highlight the current search results.
:class:`~prompt_toolkit.layout.processors.HighlightSelectionProcessor` Highlight the selection.
:class:`~prompt_toolkit.layout.processors.PasswordProcessor` Display input as asterisks. (* characters).
:class:`~prompt_toolkit.layout.processors.BracketsMismatchProcessor` Highlight open/close mismatches for brackets.
:class:`~prompt_toolkit.layout.processors.BeforeInput` Insert some text before.
:class:`~prompt_toolkit.layout.processors.AfterInput` Insert some text after.
:class:`~prompt_toolkit.layout.processors.AppendAutoSuggestion` Append auto suggestion text.
:class:`~prompt_toolkit.layout.processors.ShowLeadingWhiteSpaceProcessor` Visualise leading whitespace.
:class:`~prompt_toolkit.layout.processors.ShowTrailingWhiteSpaceProcessor` Visualise trailing whitespace.
:class:`~prompt_toolkit.layout.processors.TabsProcessor` Visualise tabs as n spaces, or some symbols.

A :class:`~prompt_toolkit.layout.BufferControl` takes only one processor as input, but it is possible to "merge" multiple processors into one with the :func:`~prompt_toolkit.layout.processors.merge_processors` function.