Introduction to PyGame
Now onto the flashy graphics!
We're going to show you pygame now. Going line by line through a simple example which puts a monkey head onto a window.
If you want to see the full code listing skip to the bottom of the lecture. For now we'll start exploring the first few lines.
Import Modules
Below is the code that imports all the needed modules into your program. Modules are a grouping of code, or a library.
Open a command prompt and go into the pygame examples directory. Then run python.
>>> import pygame, sys,os >>> from pygame.locals import *
Try typing that into your interpreter like this: 
You can look up the documentation for these modules: pygame, os, sys.
You may also want to try using dir() and help() functions on those modules 
>>> dir(pygame)
Initializing pygame
Now we initialize all imported pygame modules. This is done with the pygame.init() function.
>>> pygame.init() (6,0)
We've now initialized the video, the sound, and a number of other pygame modules. If you type it into the interpreter you'll see that 6 modules have initialized correctly, and none have failed.
Setting up the screen
First we are making a window 468 pixels wide, and 60 pixels high. A wide, short window.
window = pygame.display.set_mode((468, 60))
Check out the documentation for pygame.display.set_mode,
Now we set a caption on the window. The caption is the text in the middle of the window bar. Set the text to what ever you like! Change 'Monkey Fever' to 'I am the best python coder in the room!'.
pygame.display.set_caption('Monkey Fever')
Check out the documentation for pygame.display.set_caption,
Finally we get the display surface representing a screen.
screen = pygame.display.get_surface()
A surface represents either the screen or an area in memory where images are held.
Check out the documentation for pygame.Surface,
Constructing the monkey filename
>>> monkey_head_file_name = os.path.join("data","chimp.bmp")
>>> print monkey_head_file_name
data\chimp.bmp
This creates a relative pathname to the file. In this example all the resources are in a "data" subdirectory of the pygame examples directory. By using the os.path.join function, a pathname will be created that works for whatever platform the game is running on.
On windows the pygame examples directory is in: C:\\Program Files\Pygame-Docs\examples\.
The relative path "data\chimp.bmp" is relative to the examples directory. The full path name would be
"C:\\Program Files\Pygame-Docs\examples\data\chimp.bmp"
On debian linux the pygame examples directory is in /usr/share/doc/python2.2-pygame/examples/ The relative path "data/chimp.bmp" is relative to that examples directory. The full path name would be
"/usr/share/doc/python2.2-pygame/examples/data/chimp.bmp"
Hopefully you can see that os.path.join() is one of the python functions which helps you write code which will run on multiple platforms. The same code should run on windows, linux boxes, macs, and other types of machines.
Loading the monkey head image
Now we get to load the monkey head image.
>>> monkey_surface = pygame.image.load(monkey_head_file_name)
simple eh?
Have a look at the documentation for the pygame.image.load function.
Drawing the monkey onto the screen.
screen.blit(monkey_surface, (0,0))
Here we come accross the blit function. blit is just another word for draw.
What we are doing on this line is drawing the image of the monkey, which is contained in the monkey_surface variable, onto the screen.
The second argument to the function is telling blit to draw the monkey at coordinates (0,0). The top left of the screen.
Pygame uses a coordinate system for the screen where x= 0, y=0 is the top left of the screen. (0,20) is twenty pixels below the top of the screen.
Fliping the display
pygame.display.flip()
Here is where we flip the display surface. This updates the whole screen.
There are more complicated things which you can do with updating the display, which we will explain in the upcomming lectures. If you want to learn more about updating the display you can find out here - http://pygame.org/docs/ref/pygame_display.html#flip
Why do you have to flip the display? To see your graphics drawn. Flipping the display is your way of telling pygame that you have finished making changes for that frame, now please show the changes.
For now just be content that you should flip the display after having drawn to it.
Adding a way to quit.
def input(events):
for event in events:
if event.type == QUIT:
sys.exit(0)
else:
print event
We are defining a function with this code. This function does two things:
- looks for a quit event.
- prints other events.
An event is:
- Something that takes place; an occurrence.
- A significant occurrence or happening.
- A social gathering or activity.
Our game isn't likely to get an invitation down to the pub, or the park, but it may be told that the mouse has moved, that certain keys have been pressed or the joystick has been moved.
These are the types of events that happen within our program.
The input() function above loops over the input sequence events, and does a test on each event.
Once a quit event happens the program exits. A quit event can happen by clicking on the close window, or pressing ALT+F4.
If it is not a quit event it prints the event to the console(command line window).
The main loop
while True: input(pygame.event.get())
Here we have an infinite loop. While True is true it will keep looping. As true is going to stay true for a long time, it will keep going on(probably until the program exits).
pygame.event.get is used to see what is happening in the program. It returns a list of events. We pass this list to the input function we defined above.
All the code together.
Below we have all the code together. Copy this into your text editor and save it in the Pygame-Docs\examples directory as monkey_fever.py
Then run it, and see all the events fly by on the console!
import pygame, sys,os
from pygame.locals import *
pygame.init()
window = pygame.display.set_mode((468, 60))
pygame.display.set_caption('Monkey Fever')
screen = pygame.display.get_surface()
monkey_head_file_name = os.path.join("data","chimp.bmp")
monkey_surface = pygame.image.load(monkey_head_file_name)
screen.blit(monkey_surface, (0,0))
pygame.display.flip()
def input(events):
for event in events:
if event.type == QUIT:
sys.exit(0)
else:
print event
while True:
input(pygame.event.get())
As you can see the monkey looks a bit strange. It's got red in the background. The red color is what is called a color key. That is a color in the image which represents transparency. So instead of showing red it should show nothing, and let the black background be seen.
Now try pressing a few keys. You will notice the events being printed out to the console. If you press a mouse button you will get events for that. If you move the mouse you will get events.
Read the pygame docs for events, they are quite good - http://pygame.org/docs/ref/event.html
Exercises
Move the head
- Make the monkeys head be drawn 35 pixels to the right. Then make it drawn 40 pixels from the top of the window.
Quit on any key pressed.
- Find out how to make the program quit when you press any key. Once you find out, make your program quit when any key is pressed.
Find the size of the monkey surface
- Print to the console the size of the monkey surface.
Move the head when pressing a key.
- When pressing the 's' key make the monkey move to x = 0 and y =0. Make it move to x = 35 y = 40 when the 'd' key is pressed.
Read pygame examples
- Read through some of the pygame examples. Run them see what they do. You probably won't understand them all, but you will likely get a feel for some other pygame code. For those new to python you should also have a read through some of the python tutorials:
Next
In the next lecture we will be showing you:
- how to load and play sounds,
- some more advanced stuff with updating the display,
- how to load color keys in images properly.
- an introduction to the pygame sprite class.
Until next time. Have fun!
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