Writing a pyglet application

Getting started with a new library or framework can be daunting, especially when presented with a large amount of reference material to read. This chapter gives a very quick introduction to pyglet without covering any of the details.

Hello, World

We’ll begin with the requisite “Hello, World” introduction. This program will open a window with some text in it and wait to be closed. You can find the entire program in the examples/programming_guide/hello_world.py file.

Begin by importing the pyglet package:

import pyglet

Create a pyglet.window.Window by calling its default constructor. The window will be visible as soon as it’s created, and will have reasonable default values for all its parameters:

window = pyglet.window.Window()

To display the text, we’ll create a Label. Keyword arguments are used to set the font, position and anchorage of the label:

label = pyglet.text.Label('Hello, world',
                          font_name='Times New Roman',
                          font_size=36,
                          x=window.width//2, y=window.height//2,
                          anchor_x='center', anchor_y='center')

An on_draw() event is dispatched to the window to give it a chance to redraw its contents. pyglet provides several ways to attach event handlers to objects; a simple way is to use a decorator:

@window.event
def on_draw():
    window.clear()
    label.draw()

Within the on_draw() handler the window is cleared to the default background color (black), and the label is drawn.

Finally, call:

pyglet.app.run()

To let pyglet respond to application events such as the mouse and keyboard. Your event handlers will now be called as required, and the run() method will return only when all application windows have been closed.

Note that earlier versions of pyglet required the application developer to write their own event-handling runloop. This is still possible, but discouraged; see The application event loop for details.

Image viewer

Most games will need to load and display images on the screen. In this example we’ll load an image from the application’s directory and display it within the window:

import pyglet

window = pyglet.window.Window()
image = pyglet.resource.image('kitten.jpg')

@window.event
def on_draw():
    window.clear()
    image.blit(0, 0)

pyglet.app.run()

We used the image() function to load the image, which automatically locates the file relative to the source file (rather than the working directory). To load an image not bundled with the application (for example, specified on the command line, you would use pyglet.image.load()).

The blit() method draws the image. The arguments (0, 0) tell pyglet to draw the image at pixel coordinates 0, 0 in the window (the lower-left corner).

The complete code for this example is located in examples/programming_guide/image_viewer.py.

Handling mouse and keyboard events

So far the only event used is the on_draw() event. To react to keyboard and mouse events, it’s necessary to write and attach event handlers for these events as well:

import pyglet

window = pyglet.window.Window()

@window.event
def on_key_press(symbol, modifiers):
    print 'A key was pressed'

@window.event
def on_draw():
    window.clear()

pyglet.app.run()

Keyboard events have two parameters: the virtual key symbol that was pressed, and a bitwise combination of any modifiers that are present (for example, the CTRL and SHIFT keys).

The key symbols are defined in pyglet.window.key:

from pyglet.window import key

@window.event
def on_key_press(symbol, modifiers):
    if symbol == key.A:
        print 'The "A" key was pressed.'
    elif symbol == key.LEFT:
        print 'The left arrow key was pressed.'
    elif symbol == key.ENTER:
        print 'The enter key was pressed.'

See the pyglet.window.key documentation for a complete list of key symbols.

Mouse events are handled in a similar way:

from pyglet.window import mouse

@window.event
def on_mouse_press(x, y, button, modifiers):
    if button == mouse.LEFT:
        print 'The left mouse button was pressed.'

The x and y parameters give the position of the mouse when the button was pressed, relative to the lower-left corner of the window.

There are more than 20 event types that you can handle on a window. The easiest way to find the event name and parameters you need is to add the following line to your program:

window.push_handlers(pyglet.window.event.WindowEventLogger())

This will cause all events received on the window to be printed to the console.

An example program using keyboard and mouse events is in examples/programming_guide/events.py

Playing sounds and music

pyglet makes it easy to play and mix multiple sounds together in your game. The following example plays an MP3 file [1]:

import pyglet

music = pyglet.resource.media('music.mp3')
music.play()

pyglet.app.run()

As with the image loading example presented earlier, media() locates the sound file in the application’s directory (not the working directory). If you know the actual filesystem path (either relative or absolute), use load().

Short sounds, such as a gunfire shot used in a game, should be decoded in memory before they are used, so that they play more immediately and incur less of a CPU performance penalty. Specify streaming=False in this case:

sound = pyglet.resource.media('shot.wav', streaming=False)
sound.play()

The examples/media_player.py example demonstrates playback of streaming audio and video using pyglet. The examples/noisy/noisy.py example demonstrates playing many short audio samples simultaneously, as in a game.

[1]MP3 and other compressed audio formats require AVbin to be installed (this is the default for the Windows and Mac OS X installers). Uncompressed WAV files can be played without AVbin.

Where to next?

The examples presented in this chapter should have given you enough information to get started writing simple arcade and point-and-click-based games.

The remainder of this programming guide goes into quite technical detail regarding some of pyglet’s features. While getting started, it’s recommended that you skim the beginning of each chapter but not attempt to read through the entire guide from start to finish.

To write 3D applications or achieve optimal performance in your 2D applications you’ll need to work with OpenGL directly. The canonical references for OpenGL are The OpenGL Programming Guide and The OpenGL Shading Language.

There are numerous examples of pyglet applications in the examples/ directory of the documentation and source distributions. Keep checking http://www.pyglet.org/ for more examples and tutorials as they are written.

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