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Solutions - Chapter 14

There are a few things that can be helpful to know as you work on the exercises for Chapters 12-14:

  • The solutions for Chapters 12-14 are kept in the solution_files directory, because every exercise is a mini project. These pages contain links to individual solutions in the repository.
  • If you make a mistake when working through the project and can't get it back to a working state, it can be really frustrating to start over from scratch. There are some resources that can help with this:
    • In the online resources, there's a complete version of the Alien Invasion project as it looks at the end of each main section in Chapters 12-14.
    • For example if you're working on getting the ship to move and everything stops working, you can look at the versions from Chapter 12, then click on the adding_ship_image folder, and you'll have a working copy of the project as it looks at the beginning of the section about making the ship move.
    • If you want to compare your files to what they should look like at the end of the Piloting the Ship section, click on the piloting_the_ship folder.
    • If you want to know how to make snapshots of a project like this, make time to work through Appendix D, Using Git for Version Control. It will be well worth your time, and it's something you'll use your whole life as a programmer.
  • If you haven't already seen the cheat sheets, there's a sheet that focuses on Pygame which might be helpful when working on these exercises. (These will be fully updated to match the third edition shortly, but the Pygame sheets will not change much.)
  • It can be helpful to look through some of the Pygame documentation as you work on these exercises. There are also direct links to specific pages in the documentation that are helpful for certain exercises.

14-1: Press P to Play

Because Alien Invasion uses keyboard input to control the ship, it would be useful to start the game with a keypress. Add code that lets the player press P to start. It might help to move some code from _check_play_button() to a _start_game() method that can be called from _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 on the left side of the screen, create a ship that the player can move up and down while firing bullets at the 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.


14-3: Challenging Target Practice

Start with your work from Exercise 14-2 (page 283). Make the target move faster as the game progresses, and restart the target at the original speed when the player clicks Play.


14-4: Difficulty Levels

Make a set of buttons for Alien Invasion that allows the player to select an appropriate starting difficulty level for the game. Each button should assign the appropriate values for the attributes in Settings needed to create different difficulty levels.

There are two solutions for this exercise. The first solution has simple difficulty buttons that change the game's settings, but don't change color when you click them. You may also be interested in a slightly more complex solution which highlights the button that indicates the current difficulty settings.

14-5: 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 in the high score when initializing its value in GameStats.


14-6: Refactoring

Look for methods that are doing more than one task, and refactor them to organize your code and make it 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 simplify _check_play_button() or start_game() if you’ve already refactored _check_play_button().

Note: Before attempting to refactor the project, see Appendix D to learn how to restore the project to a working state if you introduce bugs while refactoring.


14-7: Expanding the Game

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

For a guide on how to add sounds, see here. You can also automate game play, and watch your computer play Alien Invasion.