In Alien Invasion, the player controls a rocket ship that appears at the bottom center of the screen. The player can move the ship right and left using the arrow keys and shoot bullets using the spacebar. When the game begins, a fleet of aliens fills the sky and moves across and down the screen. The player shoots and destroys the aliens. If the player shoots all the aliens, a new fleet appears that moves faster than the previous fleet. If any alien hits the player’s ship or reaches the bottom of the screen, the player loses a ship. If the player loses three ships, the game ends.
The main file, alien_invasion.py, contains the AlienInvasion class. This class creates a number of important attributes used throughout the game: the settings are assigned to settings, the main display surface is assigned to screen, and a ship instance is created in this file as well. The main loop of the game, a while loop, is also stored in this module. The while loop calls _check_events(), ship.update(), and _update_screen().
The _check_events() method detects relevant events, such as keypresses and releases, and processes each of these types of events through the methods _check_keydown_events() and _check_keyup_events(). For now, these methods manage the ship’s movement. The AlienInvasion class also contains _update_screen(), which redraws the screen on each pass through the main loop.
The alien_invasion.py file is the only file you need to run when you want to play Alien Invasion. The other files—settings.py and ship.py—contain code that is imported into this file.
The settings.py file contains the Settings class. This class only has an init() method, which initializes attributes controlling the game’s appearance and the ship’s speed.
The ship.py file contains the Ship class. The Ship class has an init() method, an update() method to manage the ship’s position, and a blitme() method to draw the ship to the screen. The image of the ship is stored in ship.bmp, which is in the images folder.