textual/docs/introduction.md

# Introduction

Welcome to the Textual Introduction!

This is a very gentle introduction to creating Textual applications. By the end of this document you should have an understanding of the basic concepts involved in using the Textual framework.

## Pre-requisites

- Python 3.7 or later. If you have a choice, pick the most recent version.
- Installed `textual` from Pypi.
- Basic Python skills.

## A Simple App

Let's looks at the simplest possible Textual app. It doesn't do much, but will demonstrate the basic steps you will need to create any application.

If you would like to follow along and run the examples, navigate to the `docs/examples/introduction` directory from the command prompt. We will be looking at `intro01.py`, which you can see here:

```python title="intro01.py"
--8<-- "docs/examples/introduction/intro01.py"
```

Enter the following command to run the application:

```bash
python intro01.py
```

The command prompt should disappear and you will see a blank screen. It will look something like the following:

```{.textual path="docs/examples/introduction/intro01.py"}

```

Hit ++ctrl+c++ to exit and return to the command prompt.

The first step in all Textual applications is to import the `App` class from `textual.app` and extend it:

```python hl_lines="1 2 3 4 5" title="intro01.py"
--8<-- "docs/examples/introduction/intro01.py"
```

There will be a single App class in any Textual application. The App class is responsible for loading data, setting up the screen, managing events etc. In a real app most of the core logic of your application will be contained within methods on the this class.

The last two lines create an instance of the application and calls `run()`:

```python hl_lines="8 9" title="intro01.py"
--8<-- "docs/examples/introduction/intro01.py"
```

The `run` method will put your terminal in to "application mode" which disables the prompt and allows Textual to take over input and output. When you press ++ctrl+c++ the application will exit application mode and re-enable the command prompt.

## Handling Events

In the previously example our app did next to nothing. Most applications will contain event handler methods, which are called in response to user actions such as key presses and mouse movements in addition to other changes your app needs to know about such as terminal resize, scrolling, timers, etc.

!!! note

    Although `intro01.py` did not explicitly define any event handlers, Textual still had to respond to events to catch ++ctrl+c++, otherwise you wouldn't be able to exit the app.

In our next example, we are going to handle two events; `Mount` and `Key`. The `Mount` event is sent when the app is first run, and a `Key` event is sent when the user presses a key on the keyboard. Try running `intro02.py` in the `docs/examples/introduction`:

```python title="intro02.py"
--8<-- "docs/examples/introduction/intro02.py"
```

When you run this app you should see a blue screen in your terminal, like the following:

```{.textual path="docs/examples/introduction/intro02.py"}

```

If you hit any of the number keys ++0++-++9++, the background will change color and you should hear a beep. As before, pressing ++ctrl+c++ will exit the app and return you to your prompt.

!!! note

    The "beep" is your terminal's *bell*. Some terminals may be configured to play different noises or a visual indication of a bell rather than a noise.

There are two event handlers in this app. Event handlers start with the text `on_` followed by the name of the event in lower case. Hence `on_mount` is called for the `Mount` event, and `on_key` is called for the `Key` event.

The first event handler to run is `on_mount`:

```python hl_lines="19 20" title="intro02.py"
--8<-- "docs/examples/introduction/intro02.py"
```

This `on_mount` method sets the `background` attribute of `self.styles` to `"darkblue"` which updates the background color. There are a lot of other properties on the Styles object, which define how your app looks. We will explore what you can do with this object later.

!!! note

    You may have noticed there was no function call to repaint the screen in this examples. Textual will detect when a refresh is required, and do it automatically.

The second event handler will receive `Key` events whenever you press a key on the keyboard:

```python hl_lines="22 23 24 25" title="intro02.py"
--8<-- "docs/examples/introduction/intro02.py"
```

This method has an `event` positional argument which contains information regarding the key that was pressed. The body of the method sets the background to a corresponding color int the `COLORS` list when you press one of the digit keys. It also calls `bell()` which is a method on App that plays your terminal's bell.

!!! note

    Every event has a corresponding `Event` object, but Textual knows to only call the event handler with the event object if you have it in the argument list. It does this by inspecting the handler method prior to calling it.

## Widgets

Most Textual applications will also make use of one or more `Widget` classes. A Widget is a self contained component responsible for defining how a given part of the screen should look. Widgets respond to events in much the same way as the App does.

Let's look at an app with a simple Widget to show the current time and date. Here is the code for `"clock01.py"` which is in the same directory as the previous examples:

```python title="clock01.py"
--8<-- "docs/examples/introduction/clock01.py"
```

Here's what you will see if you run this code:

```{.textual path="docs/examples/introduction/clock01.py"}

```

This script imports App as before, but also the `Widget` class from `textual.widget`, which is the base class for all Widgets. To create a Clock widget we extend from the Widget base class:

```python title="clock01.py" hl_lines="7 8 9 10 11 12 13"
--8<-- "docs/examples/introduction/clock01.py"
```

Widgets support many of the same events as the Application itself, and can be thought of as mini-applications in their own right. The Clock widget also responds to a Mount event which is the first event received when a widget is _mounted_ (added to the App). The code in `Clock.on_mount` sets `styles.content_align` to tuple of `("center", "middle")` which tells Textual to display the Widget's content aligned to the horizontal center, and in the middle vertically. If you resize the terminal, you should find the time remains in the center.

The second line in `on_mount` calls `self.set_interval` which tells Textual to invoke the `self.refresh` function once a second to refresh the Clock widget.

When Textual refreshes a widget it calls it's `render` method:

```python title="clock01.py" hl_lines="12 13"
--8<-- "docs/examples/introduction/clock01.py"
```

The Clocks `render` method uses the datetime module to format the current date and time. It returns a string, but can also return a _Rich renderable_. Don't worry if you aren't familiar with [Rich](https://github.com/Textualize/rich), we will cover that later.

Before a Widget can be displayed, it must first be mounted on the app. This is typically done within the applications Mount handler, so that an application's widgets are added when the application first starts:

```python title="clock01.py" hl_lines="17 18"
--8<-- "docs/examples/introduction/clock01.py"
```

In the case of the clock application, we call `mount` with an instance of the `Clock` widget.

That's all there is to this Clock example. It will display the current time until you hit ++ctrl+c++