In this chapter we’ll finish the Alien Invasion game. We’ll add a Play button to start a game on demand or to restart a game once it ends. We’ll also change the game so it speeds up when the player moves up a level, and we’ll implement a scoring system. By the end of the chapter, you’ll know enough to start writing games that increase in difficulty as a player progresses and that show scores.

TRY IT YOURSELF #1

14-1. Press P to Play: Because Alien Invasion uses keyboard input to control the ship, it’s best to start the game with a keypress. Add code that lets the player press P to start. It may help to move some code from check_play_button() to a start_game() function that can be called from both check_play_button() and check_keydown_events().

14-2. Target Practice: Create a rectangle at the right edge of the screen that moves up and down at a steady rate. Then have a ship appear on the left side of the screen that the player can move up and down while firing bullets at the moving, rectangular target. Add a Play button that starts the game, and when the player misses the target three times, end the game and make the Play button reappear. Let the player restart the game with this Play button.

TRY IT YOURSELF #2

14-3. Challenging Target Practice: Start with your work from Exercise 14-2 (page 298). Make the target move faster as the game progresses, and restart at the original speed when the player clicks Play.

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TRY IT YOURSELF #3

14-4. All-Time High Score: The high score is reset every time a player closes and restarts Alien Invasion. Fix this by writing the high score to a file before calling sys.exit() and reading the high score in when initializing its value in GameStats.

14-5. Refactoring: Look for functions and methods that are doing more than one task, and refactor them to keep your code organized and efficient. For example, move some of the code in check_bullet_alien_collisions(), which starts a new level when the fleet of aliens has been destroyed, to a function called start_new_level(). Also, move the four separate method calls in the __init__() method in Scoreboard to a method called prep_images() to shorten __init__(). The prep_images() method could also help check_play_button() or start_game() if you’ve already refactored check_play_button().

14-6. Expanding Alien Invasion: Think of a way to expand Alien Invasion. For example, you could program the aliens to shoot bullets down at the ship or add shields for your ship to hide behind, which can be destroyed by bullets from either side. Or use something like the pygame.mixer module to add sound effects like explosions and shooting sounds.

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