Teach Your Kids To Code: A Parent-friendly Guide to Python Programming

User Interaction: Get into the Game 219

set up all the starting values we

need in our sprite object. The

two underscores on either side

of the __init__() function name

have special meaning in Python.

In this case, __init__() is the spe-

cial function name that is used to

initialize a class. We tell Python

how each Smiley object should be

initialized in this function, and

every time we create a Smiley,

this special __init__() function

does its job behind the scenes,

setting up variables and more

for each Smiley object.

There are a number of items we need to set up in our __init__()

function. First, we’ll determine what parameters we need to pass

to our __init__() function. For our random smiley faces, we might

pass in a position and the starting x- and y-velocities. Because

our Smiley is a class and all our smiley face sprites will be objects

of the Smiley type, the first parameter in all the functions in the

class will be the smiley sprite object itself. We label this parameter

self, because it connects __init__() and the other functions to the

object’s own data. Look at the code for our __init__() function:

def init(self, pos, xvel, yvel):
u pygame.sprite.Sprite.init(self)
v self.image = pic
self.rect = self.image.get_rect()
w self.pos = pos
x self.rect.x = pos[0] - self.scale/2
self.rect.y = pos[1] - self.scale/2
y self.xvel = xvel
self.yvel = yvel

The four parameters for our __init__() function are the object

itself, self; the position where we want the smiley to appear, pos;

and xvel and yvel, its horizontal and vertical speed values. Next,

at u, we call the initialization function for the main Sprite class

so that our object can take advantage of the properties of sprite