H a n d s o n, p r o j e c t b a s e d

Creating a Row of Aliens
We’re ready to generate a full row of aliens. Because our code for making 
a single alien works, we’ll rewrite 
_create_fleet()
to make a whole row of 
aliens:
def _create_fleet(self):
"""Create the fleet of aliens."""
# Create an alien and find the number of aliens in a row.
# Spacing between each alien is equal to one alien width.
u
alien = Alien(self)
v
alien_width = alien.rect.width
w
available_space_x = self.settings.screen_width - (2 * alien_width)
number_aliens_x = available_space_x // (2 * alien_width)
# Create the first row of aliens.
x
for alien_number in range(number_aliens_x):
# Create an alien and place it in the row.
alien = Alien(self)
y
alien.x = alien_width + 2 * alien_width * alien_number
alien.rect.x = alien.x
self.aliens.add(alien)
alien_invasion.py

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Aliens!
261
We’ve already thought through most of this code. We need to know the 
alien’s width and height to place aliens, so we create an alien at u before 
we perform calculations. This alien won’t be part of the fleet, so don’t add 
it to the group 
aliens
. At v we get the alien’s width from its 
rect
attribute 
and store this value in 
alien_width
so we don’t have to keep working through 
the 
rect
attribute. At w we calculate the horizontal space available for aliens 
and the number of aliens that can fit into that space.
Next, we set up a loop that counts from 0 to the number of aliens we 
need to make x. In the main body of the loop, we create a new alien and 
then set its x-coordinate value to place it in the row y. Each alien is pushed 
to the right one alien width from the left margin. Next, we multiply the 
alien width by 2 to account for the space each alien takes up, including the 
empty space to its right, and we multiply this amount by the alien’s position 
in the row. We use the alien’s 
x
attribute to set the position of its rect. Then 
we add each new alien to the group 
aliens
.
When you run Alien Invasion now, you should see the first row of aliens 
appear, as in Figure 13-3.
Figure 13-3: The first row of aliens
The first row is offset to the left, which is actually good for gameplay. 
The reason is that we want the fleet to move right until it hits the edge 
of the screen, then drop down a bit, then move left, and so forth. Like the 
classic game Space Invaders, this movement is more interesting than having 
the fleet drop straight down. We’ll continue this motion until all aliens are 
shot down or until an alien hits the ship or the bottom of the screen.

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