Raspberry Pi A, User Manual, Page: 205 / 314

C H A P T E R 1 2
A N I N T R O D U C T I O N T O P Y T H O N
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if changeDirection == ‘left’ and not direction == ‘right’:
direction = changeDirection
if changeDirection == ‘up’ and not direction == ‘down’:
direction = changeDirection
if changeDirection == ‘down’ and not direction == ‘up’:
direction = changeDirection
With the user’s input checked to make sure it makes sense, the snake—which appears on the
screen as a series of blocks—can be moved. During each turn, the snake moves a distance equal
to the size of one of its blocky segments. With each segment measuring 20 pixels, you can tell
pygame to move the snake a single segment in any direction. Type in the following code:
if direction == ‘right’:
snakePosition[0] += 20
if direction == ‘left’:
snakePosition[0] -= 20
if direction == ‘up’:
snakePosition[1] -= 20
if direction == ‘down’:
snakePosition[1] += 20
The
+=
and
-=
operators are used to change the value of a variable by a certain amount:
+=
sets
the variable to its previous value plus the new value, while
-=
sets the variable to its previous
value minus the new value. By way of example,
snakePosition[0] += 20
is a shorthand way
of writing
snakePosition[0] = snakePosition[0] + 20
. The number in square brackets
following the
snakePosition
variable name is the position in the list being affected: the first
value in the
snakePosition
list stores the snake’s position along the X axis, while the second
value stores the position along the Y axis. Python begins counting at zero, so the X axis is con-
trolled with
snakePosition[0]
and the Y axis with
snakePosition[1]
. If the list were lon-
ger, additional entries could be affected by increasing the number:
[2]
,
[3]
and so on.
Although the
snakePosition
list is always two values long, another list created at the start
of the program is not:
snakeSegments
. This list stores the location of the snake’s body,
behind the head. As the snake eats raspberries and grows longer, this list increases in size
and provides the difficulty in the game: as the player progresses, it becomes harder to avoid
hitting the body of the snake with the head. If the head hits the body, the snake dies and the
game is over. Type the following line to make the snake’s body grow:
snakeSegments.insert(0,list(snakePosition))