Raspberry Pi A User Manual page 204

188
a
beneath that should be familiar from the
to close down and exit.
The line beginning
evaluated when a previous
instruction is used to see if pygame is reporting a
the user is pressing a key on the keyboard. As with the
when an
tation the
something to do when the user presses a key:
These instructions modify the value of the
direction the player's snake is travelling during the game. Using
allows more than one evaluation to be made. In this case, it provides two ways of controlling
the snake: the player can use the cursor keys, or the W, A, S and D keys to make the snake go
up, right, down or left. Until a key is pressed, the snake travels to the right according to the
value set for
If you look back at the variables you initialised at the start of the program, you'll see that
there's another variable called
if the instruction the user has given is valid. The snake should not be allowed to turn imme-
diately back on itself—if it does, the snake dies and the game is over. To prevent this from
happening, the direction requested by the player—stored in
pared to the current direction in which the snake is travelling—stored in
they are opposite directions, the instruction is ignored and the snake continues in the same
direction as before. Type the following lines to set up the comparisons:
P A R T I I I
P R O G R A M M I N G W I T H T H E R A S P B E R R Y P I
message (which happens when the user presses the Escape key). The two lines
QUIT
is used to extend
elif
is true should be indented by an additional four spaces plus whatever inden-
elif
instruction itself has. Type the following lines to give the
elif
if event.key == K_RIGHT or event.key == ord('d'):
changeDirection = 'right'
if event.key == K_LEFT or event.key == ord('a'):
changeDirection = 'left'
if event.key == K_UP or event.key == ord('w'):
changeDirection = 'up'
if event.key == K_DOWN or event.key == ord('s'):
changeDirection = 'down'
if event.key == K_ESCAPE:
pygame.event.post(pygame.event.Event(QUIT))
changeDirection
if changeDirection == 'right' and not direction == 'left':
direction = changeDirection
gameOver
loops. Short for else if, an
if
instruction was found to be false. In this case, the
if
KEYDOWN
changeDirection
at the start of the program.
. This is used alongside
direction
function: they tell pygame and Python
elif
event, which is returned when
instruction, code to be executed
if
elif
variable, used to control the
with an
or
changeDirection
changeDirection
direction
instruction is
elif
instruction
statement
if
to see
—is com-
. If