Assignment 7: Alien Invaders



CS 1110: Introduction to Computing Using Python

Assignment 7:
Alien Invaders

For over a decade, the final assignment in CS 1110 was a recreation of the classic Breakout game. While it has served us well for
many years, we felt that it was time for a change. So your instructor has dug deep into his childhood for another classic arcade
game, and chosen to do Space Invaders this year.
If you have never played Space Invaders before, there are a few versions online as flash games. The Pacxon version is the most
authentic one that we have found. You should play this to get a good idea of the gameplay. Keep in mind that this version is a bit
too authentic and potentially runs afoul of copyright issues.
One of the main challenges with this assignment is its scope is completely up to you. There is a bare minimum of functionality that
you must implement. You must implement a complete, single wave of Space Invaders. But after that point, you are free (and
encouraged) to add more interesting features to your game. The video to above shows our solution, which has several extra features
such as sound and basic animation. You can even look at more advanced shooting games like Galaxian and Galaga for inspiration.
You are permitted to do anything that you want, provided that the basic functionality is there.
Despite any difficulties you might have had with previous assignments, we know that this assignment is easily within your grasp.
You just have to start early, break the problem up into manageable pieces, and program/test incrementally. Below, we discuss
stages of implementation and give suggestions for staying on top of the project. If you follow our advice and test each piece
thoroughly before proceeding to the next, you should be successful.
Our bare minimum solution is about 875 lines of code (including specifications and comments). This is ~225 lines in
(including the ~155 lines already there), ~380 lines in (including the ~75 lines already there), and ~270 lines in (including the ~130 lines already there). So you should expect to write just a little over 500 lines of code for the bare
minimum solution. While this may seem intimidating, a good amount of that consists of headers and specification. Our minimum
solution has ~20 methods across five classes, not including getters and setters.
Important: Some of you experienced problems with the GUI in Assignment 6. There is apparently an issue with Dell XPS,
Windows 10, and high-DPI displays. There is a new driver that fixes this problem. You should download it. See the Piazza post for
more information.
Learning Objectives
This final assignment has several important objectives.
It gives you practice with reading official class documentation and APIs.
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It gives you experience using stateful controllers to handle a complex, interactive application.
It gives you experience with designing helper functions to structure your code properly.
It gives you experience with using constants to make your code more readable.
It gives you experience with programming animation and simple collisions.
It gives you experience with an open-ended project that is not fully specified.
It brings together everything you have learned in this class in one final assignment.
Table of Contents
Author: W. White
Academic Integrity and Collaboration
Organization and Scope
Alien Invaders
The Basic Game
Create a Welcome Screen
Create a Wave of Aliens
Create (and Animate) the Ship
Walk the Aliens Back-and-Forth
Fire Bolts from the Ship
Fire Bolts from the Aliens
Handle Bolt Collisions
Finish the Game
Extending the Game
Completing the Assignment

Organization and Scope
This is a long assignment, similar in length to Assignment 6. Once again the trick is to pace yourself. This can be finished by the
end of classes, if you work a little bit every day (excluding the Thanksgiving break).
While there are no unit tests this time, you should be able to figure out if everything is working simply by playing the game. There
are no tricky “restore everything to how it was” like with Turtles. Just get the game working.
Assignment Source Code
The first thing to do in this assignment is to download the zip file from this link. You should unzip it and put the contents
in a new directory. As with the imager application, this assignment is organized a package with several files. In particular, this
package file contains the following:
This file contains the controller class Invaders. This is the controller that launches the application. It is one of three
modules that you will modify for this assignment. While it is the primary controller class, you will note that it has no script
code. For that, you will use the module below.
This file contains the secondary controller class Wave. This class manages a single wave of aliens. It works as a
subcontroller, just like the example subcontroller from class. It is another of the three modules that you will modify for
this assignment, and the one that will require the most original code.
This file contains the model classes Ship, Alien and Bolt. If you want to add other model classes (e.g. power-ups), then
you should add those here as well. This is the last of the three files you should modify for this assignment.
This is a module filled with constants (global variables that should not ever change). It is used by,, and to ensure that these modules agree on certain important values. It also contains code for adjusting your alien
count and speed. You should only modify this file if you are adding additional constants as part of your extended features.
This module contains the application code for this assignment. It is the module you run from the command line to start the
game. It works the same way that the imager application did in Assignment 6. Do not modify this file!
This package contains the classes you will use to design you game. Technically, this was already installed as part of Cornell
Extensions. However, we have made some important modifications for this assignment. Under no circumstances should you
ever modify this package!
This folder is a list of sound effects that you may wish to use as part of your extensions. You are also free to add more if you
wish; just put them in this folder. All sounds must be WAV files. While we have gotten MP3 to work on Windows, Python
support for OS X is unreliable.
This folder is a collection of True Type Fonts, should you get tired of the default Kivy font. You can put whatever font you
want in this folder, provided it is a .ttf file. Other Font formats (such as .ttc, .otf, or .dfont) are not supported.
This folder is a collection of images for the ship and aliens. The GImage and GSprite classes allow you to include these in
your game. You may also want to include other images, such as a background; just remember to draw the background image
For the basic game, you will only modify the first three files (maybe four) listed above. The class Invaders is a subclass of the
class GameApp. Your model classes should all be subclasses of GObject. As part of this assignment, you are expected to read the
online documentation which describes how to use the basic classes.
Running the Application
This application is very similar to Assignment 6 in that it is organized as a package. To run the application, make sure all of the
files are together in a folder and give the folder a name like invaders. To run the program, change the directory in your command
shell to just outside of the folder invaders and type
python invaders
In this case, Python will run the entire folder. What this really means is that it runs the script in This script imports
each of the other modules in this folder to create a complex application.
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Assignment Organization
This assignment follows the model-view-controller pattern discussed in class. The modules are clearly organized so that each holds
models, the view, or a controller. The organization of these files is shown below. The arrows in this diagram mean “imports”. So
the Invaders controller imports the view and Wave subcontroller. The Wave controller imports the view and the models. This
leads to an important separation of files. Invaders is never permitted to access anything in and Wave is never
permitted to access anything in This is an important rule that we will enforce while grading.
You will notice that the models module imports the view because it needs the parent class GObject to perform any drawing. In
practice, we often like to separate the model and view to cut down on the number of arrows (less meetings between the various
programmers). However, that would make this assignment a lot harder. Fortunately, the view does not import anything (and should
not be modified). This means there are no cycles in this architecture (e.g. A imports B imports C imports A). Cyclical imports are
very dangerous and you have to be careful with them in large applications. Avoiding cycles is one of the reasons we draw pictures
like the one below.
In addition to the four main modules, there is another module with no class or function definitions. It only has constants, which are
global variables that do not change. This is imported by the models module and the two controllers; it is a way to help them share
Assignment Scope
As we explained in class, your game is a subclass of GameApp. The parent class does a lot of work for you. You just need to
implement three main methods. They are as follows:
Method Description
start() Initializes the game state and attributes. Because of how Kivy works, initialization code should go
here and not in the constructor (which is called before the window is sized properly).
Updates the models for the next animation frame. The speed at which this is called is determined by
the (immutable) attribute fps, which is set by the constructor. The parameter dt is time in seconds
since the last call to update.
draw() Draws all of the models to the screen. This is called when update is complete. Implementing this
method should be as simple as calling the method draw inherited from GObject.
Obviously, you are not going to put all of your code in those three methods. The result would be an unreadable mess. An important
part of this assignment is developing new methods whenever you need them so that each method is small and manageable. Your
grade will depend partly on the design of your program. As one guideline, points will be deducted for methods that are more
than 30 lines long (not including specifications).
You will also need to add methods and attributes to the class Wave in, as well as Ship, Alien, and Bolt in
These classes are completely empty, though we have given you a lot of hints in the class specification. You should read all these
As you write the assignment, you may find that you need additional attributes. Whenever you add a new attribute to a class, you
must fully state the invariant in your specification. All instance attributes should be hidden. While you do not need to enforce the
invariants in the getters and setters, you must must have them if the attributes are accessed by another class. For example, if the
Wave class needs to check the direction of a laser bolt, then you are going to need a getter for the velocity in the Bolt class.
When approaching this assignment, you should always be thinking about “what code goes where?” If you do not know what file to
put things in, please ask on Piazza (but do not post code). In general, you should follow these guidelines:
This controller does very little. All it does is keep track of the game state (e.g. whether or not the game is paused). Most of the time
it just calls the methods of Wave, and Wave does all the work. However, if you need anything between games, like a paused
message or a high score, this goes here.
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This class does all the hard work. In addition to the initializer, it needs its own update and draw methods. This is a subcontroller,
and you should use the subcontroller example from class as a template.
The most complex method will be the update and you will certainly violate the 30-line rule if you do not break it up into headers.
For the basic game, this method will need to do the follow:
Move the ship according to player input
March the aliens across the screen
Fire a laser bolt from either the ship or an alien
Move any laser bolts across the screen
Resolve any collisions with a laser bolt
In our code, each one of these is a separate helper. You should think about doing this in your code as well.
The Models
The models just keep track of data. Most of the time, models just have attributes, with getters and setters. Think Image and
ImageHistory from the previous assignment. However, sometimes models have additional methods that perform complex
computation on the data, like increment or undo in class ImageHistory.
The models in this assignment are the game objects on screen: the ship, any aliens, and any laser bolts. Of these three, it is more
important to have a class for Bolt. Bolt needs an additional attribute for its velocity, and you need some extra methods to perform
calculations with this velocity.
The classes Ship and Alien are subclasses of GImage. You should not need any new attributes for these two classes. However,
you will want to write a collides method in each of these classes to detect collisions with a laser bolt.
If your added features include scoring, you will probably need extra attributes in the Alien class to track point value. If you add
boss aliens or motherships, then you may need additional model classes to display and track them. If in doubt about whether or not
you need a new class, ask us on Piazza.
Pacing Yourself
You should start as soon as possible. If you wait until the day before this assignment is due, you will have a hard time completing
it. If you work on a little bit of it every day or every other day, thenyou will enjoy it and get it done on time. We have also tried not
to include your Thanksgiving Break in this assignment.
The hard part of this assignment may be “finishing up”: designing the final reorganization in order to incorporate lives in your
game. We have budgeted you one day for this, but you have up to three days if you decide just to do the bare minimum and not add
any extensions.
You should implement the application in stages, as described in these instructions. Do not try to get everything working all at once.
Make sure that each stage is working before moving on to the next stage.
Set up a schedule. We have suggested some milestones, but make up your own schedule. Leave time for learning things and asking
questions. Above all, do not try to extend the program until you get the basic functionality working. If you add extensions too early,
debugging may get very difficult.
Getting Help
We have tried to give you as much guidance in this document as we can. However, if you are still lost, please see someone
immediately. Like the last few assignments, this is a fairly involved project, and you should get started early. To get help, you may
talk to the course instructor, a TA, or a consultant. See the staff page for more information.
In addition, you should always check Piazza for student questions as the assignment progresses. We may also periodically post
announcements regarding this assignment on Piazza and the course website.
Alien Invaders
The initial configuration of the Alien Invaders is shown in the picture below. There are aliens arrange in rows and columns on the
left side of the screen. At the bottom of the screen is the player’s ship. There is also a horizontal line at the bottom of the screen.
This is the defense line. If the aliens make it past this line, they have successfully invaded and you have lost the game.
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Starting Position
Once the game begins, the aliens march back and forth across the screen. In the beginning they march to the right, moving
ALIEN_H_WALK (a variable in pixels at a time. When they reach the right hand side of the screen they move down
ALIEN_V_WALK pixels, and then start marching to the left. They continue this pattern back and forth, dropping down one step
whenever they reach the edge of the screen, until they are all destroyed or make it past the defense line. An illustration of the aliens
breaking the defense line is shown below.
Breaching the Defense
To protect against the invaders, the player can move the ship left and right, and fire laser bolts. A laser bolt starts from the tip of the
ship and moves up in a straight line. If the bolt collides with an alien, that alien is destroyed, as shown below.
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Alien Destroyed
The aliens are not defenseless. Every time the aliens take step, there is a random chance that one of the aliens will fire a laser bolt
back. That laser bolt will always come from the bottom alien in a column chosen at random. If a laser bolt from an alien collides
with the ship, the ship is destroyed. This is shown below.
Ship Destroyed
A player has up to three lives, where one life is lost each time a ship is destroyed. If the player has any lives remaining when the
ship is destroyed, the game will briefly pause before starting again. The wave continues until one of three things happens:
1. The last alien is destroyed.
2. The ship is destroyed and there are no lives remaining.
3. Any alien touches the defensive line.
In case 1, the player wins the game. In the other two, the player loses. The video below shows the basic game, with no extensions.
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Game State
One of the challenges with making an application like this is keeping track of the game state. In the description above, we can
identity several distinct phases of the game:
Before the game starts, and the alien wave has not started
When the aliens are set up, but have not started to move
While the game is ongoing, and the aliens are on the march
While the game is paused (e.g. to show a message)
While the game is creating a new ship to replace the old one
After the game is over
Keeping these phases straight is an important part of implementing the game. You need this information to implement update in
Invaders correctly. For example, whenever the game is ongoing, the method update should instruct the Wave object to move the
ship. However, if the game has just started, there is no Wave object yet, and the method update should create one.
For your convenience, we have provided you with constants for six states:
STATE_INACTIVE, before a a wave has started
STATE_NEWWAVE, when it is time to create a new wave of aliens
STATE_ACTIVE, when the game is ongoing and the aliens are marching
STATE_PAUSED, when the game is paused to display a message
STATE_CONTINUE, when the player is waiting for a new ship
STATE_COMPLETE, when the game is over
All of these constants are available in The current application state should be stored in the attribute _state inside
Invaders. You are free to add more states when you work on your game extensions. However, your basic game should stick to
these six states.
The rules for changing between these six states are outlined in the specification of method update in Invaders. You should read
that in its entirety. However, we will cover these rules in the instructions below as well.
The Basic Game
We have divided these instructions into two parts. The first part covers the basic things that you must implement just to get the
game running. Once you do that, the assignment gets more interesting. You should try to finish everything in this part of the
assignment by Friday, December 1 (the end of the last week of class). If you do that, you will be in good shape to add extensions.
You should focus most of your effort on the basic game. Without any extensions at all, you can get 92/100 on this assignment. The
extensions are the last eight points. However, particularly good extensions will count as extra credit, giving you even more points
on the assignment. However, except in extreme cases, you cannot earn more than a 100.
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Before You Do Anything
The very first thing that you should do is read the file If you ever need a value like the size of the ship, the size of the
game window, or so on, this is where you go. When writing code, you should always use the constants, not raw numbers (or
“magic numbers,” as we call them). Magic numbers make your code hard to debug, and if you make a change (e.g. to make the
ship bigger), you have no idea about all of the locations in your code that need to be changed.
With that said, you are welcome to change any of these numbers if you wish. You are also encouraged to add more constants if you
think of other numeric values that you need. Anytime that you find yourself putting a number in your code, ask yourself whether or
not it would make sense as a constant.
Create a Welcome Screen
We start with a simple warm-up to get you used to defining state and drawing graphics elements. When the player starts the
application, they should be greeted by a welcome screen. When you work on your extensions, you can embellish your welcome
screen to be as fancy as you wish. But for now, keep it simple. Your initial welcome screen is a simple text message.
Because the welcome message is before any game has started, it belongs in the Invaders class, not the Wave class. You are
already seeing how we separate what goes where.
The text message will look something like the one above. It does not need to say “Press ‘S’ to play”. It could say something else, as
long as it is clear that the user press a key on the keyboard to continue. However, we recommend against allowing the user to press
any key, since in later steps that will make it easy for the user to accidentally miss an important message.
To create a text message, you need to create a GLabel and store in it an attribute. If you read the specification for class Invaders,
you will see an attribute named _text. This is for any messages to display to the player. If you wish you may rename this attribute,
as long as you make it clear in the class specification.
Since the welcome message should appear as soon as you start the game, it should be created in the method start, the first
important method of the class Invaders. When creating your message, you will want to set things like the font size and position
of the text. As you can see from the documentation for GLabel and GObject, graphics objects have a lot of attributes to specify
things such as position, size, color, font style, and so on.
You should experiment with these attributes to get the welcome screen that you want. The key thing to remember is that — in Kivy
— screen coordinates start from the bottom-left corner of the window (and not the top right as with most graphics applications).
Note that x and y are the center of the label. If you want to place the left edge, use the attribute left instead of x.
Simply adding this code to start is not enough. If you were to run the application right now, all you would see is a blank white
window. You have to tell Python what to draw. To do this, simply add the line
to the method draw in Invaders. The (non-hidden) attribute view is a reference to the window. Hence this method call instructs
Python to draw this text label in the window. This attribute has an “invisible” getter like the attributes un RGB. Now run the
application and check if you see your welcome message.
Initializing Game State
The other thing that you have to do in the beginning is initialize the game state. The attribute _state (included in the class
specification) should start out as STATE_INACTIVE. That way we know that the game is not ongoing, and the program should (not
yet) be attempting to animate anything on the screen. In addition, the other attributes listed (particularly _game) should be None;
we have not done anything yet!
The _state attribute is an important part of many of the invariants in this game. In particular, we want your new attribute for the
welcome message to have the following invariant:
If the state is STATE_INACTIVE, then there is a welcome message
If the state is not STATE_INACTIVE, the welcome message is None.
Does your definition of start satisfy this invariant?
Dismissing the Welcome Screen
The welcome screen should not show up forever. The player should be able to dismiss the welcome screen (and start a new game)
when he or she presses a key. To respond to keyboard events, you will need the attribute input, which is an instance of GInput.
This class has several methods for identifying what keys are currently pressed.
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When using the attribute input, remember the issues that we discussed in class. The method update is called every 16
millisecond. If you hold a key down, then you see a lot of key presses. You just want the first press! That means you need some
way to determine whether or not the key was pressed this animation frame and not in the previous one. See the demo
from class on how to do this. This may require you to add a new attribute to Invaders.
If you detect a key press, then you should change the state STATE_INACTIVE to STATE_NEWWAVE. This will start a new game.
You are not ready to actually write the code to start the game, but switching states is an important first activity.
Invariants must be satisfied at the end of every method, so you need to assign None to the welcome message as well. This will
require a simple change to method draw to keep it from crashing. Once you have done that, run the application. Does the message
disappear when you press a key?
Important Considerations
This first part of the assignment looks relatively straightforward, but it gets you used to having to deal with controller state. In this
part, you likely had to add attributes beyond the ones that we have provided. Whenever you a new attribute, you must add it and its
corresponding invariant to the class specification. Add it just after the comment stating “ADD MORE ATTRIBUTES”, to make it
easier for the graders (and you) to find them. We will deduct style points for instance attributes that are not specified.
Try to finish this part by Friday, November 17 (e.g. the day after starting the assignment). You will spend most of your time
reading the online documentation, but this will give you a solid understanding of how this application works.
Create a Wave of Aliens
The state STATE_NEWWAVE is only supposed to last one animation frame. When you are in this state, you should construct a new
Wave object and assign it to the attribute _wave. During the next animation frame, you should switch to the state STATE_ACTIVE.
Do not worry about this second state; you will deal with it in the next task.
Right now, the constructor subcontroller Wave does not do anything. That is because you have not written an initializer yet.
Eventually, your initializer is going to create the aliens and the ship. Right now, we are just going to focus on the aliens.
Creating a Single Alien
While you have not yet completed the definition of class Alien yet, we have gotten you started in the module In
particular, the class Alien is a subclass of GImage. That means it inherits all of its attributes and methods, including the initializer.
Hence (unless you override methods to do otherwise), you create and draw aliens the same way you create and draw a GImage
To define an image, use the attributes x, y, width, height, and source to specify how it looks on screen. The first four attributes
are just like GLabel, while source specifies an image file in the Images folder. As with the label, you can either assign the
attributes after the object is created or assign them in the constructor using keywords. Keyword arguments work like default
arguments in that you write param = value. See the online documentation for an example of how to approach this.
Creating a Wave of Aliens
Read the specification for Wave. You will see that it contains a two-dimensional list of Alien objects. Your method __init__
should fill this 2d list with aliens. Because there are other things you will need to do in __init__, you should probably make a
helper method that does this initialization. Otherwise, __init__ may go over 30 lines when you add extra feature later.
Look at the constants in You need to draw ALIEN_ROWS rows of aliens with ALIENS_IN_ROW many aliens in each
row. The module also includes constants for how big to make the aliens and how much space to put between them.
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When you are ready, you should set up the aliens as shown above. Each row of aliens will be a list of Alien objects, and each row
is an element of the attribute _aliens. You can store the rows bottom-up or top-down; it does not matter. The alien positions
should line up neatly into rows and columns. The left edge of the wave should be ALIEN_H_SEP from the left edge of the window,
and the top edge of the wave should be ALIEN_CEILING from the top of the window.
You should be able to code this as a straightforward nested loop, like those that you worked with in Assignment 6. There are only
three things to keep track of in the loop:
The x position of the next alien
The y position of the next alien
The image of the next alien
All other values are defined by constants. To compute the first two, remember that the aliens should be ALIEN_H_SEP and
ALIEN_V_SEP apart from each other. For the images, however, you should do something different. All aliens in a row should use
the same image and the image should be constant for two rows. Images should go from bottom to top. So if there are five rows, that
is two rows of alien1.png, another two rows of alien2.png, and just one row of alien3.png.
We have only provided you with three alien images. However, in the extensions you are free to add more. Regardless, you should
be prepared to draw any number of rows of aliens. So when you run out of aliens, you need to loop back to the beginning. For
example, suppose you had four alien images but needed nine rows of aliens. You would have two rows of alien1.png,
alien2.png, alien3.png, and alien4.png, in that order. Then you would have one last row of alien1.png at the top. See
the example below for three images and nine rows. You will find the constant ALIEN_IMAGES helpful here.
Drawing Aliens
Once again, creating Alien objects is not enough to draw them on the screen. But drawing them is a bit more complicated than
drawing the welcome message. The aliens are (hidden) attributes in Wave. Writing
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for row in self._wave._aliens:
for alien in row:
works, but it is not allowed. We will take off style points if a class of one module ever accesses the hidden attributes of an
object of a class in a different module.
This is the purpose of adding a draw method to class Wave. The draw method in Invaders calls the draw method in Wave,
which calls the draw method for each alien (defined in GObject).
However, only Invaders has access to the attribute view, which is necessary for drawing. The class Wave cannot directly access
any attributes in Invaders. If a method in Wave needs an attribute from Invaders, then Invaders must provide that attribute as
an argument in the method call. This means that the draw method in Wave needs to have view as a parameter, just like the draw
method in GObject.
Testing Your Code
When you are testing the later parts of this assignment, you should play with just 3-4 aliens per row and 1-2 rows of aliens. This
will save time and let you quickly see whether you can successfully win or lose. If you play with the default number of aliens (5
rows and 12 aliens per row), then each game will take a while to test.
You might assume that testing in this manner requires you to change the values of the global constants that give the number of rows
and number of aliens in a row. This is also undesirable, a you might forget to change them back. Instead, we have added some
clever code in that allows you to change these constants when you start the application.
When you run your application (again, assuming that it is in a folder called invaders) try the command
python invaders 2 3
When you do this, Python changes the value of ALIEN_ROWS to 2 and the value of ALIENS_IN_ROW to 2.
You should make sure that your creation of the rows of aliens works with any number of rows and any number of aliens in each
row (e.g. 1, 2, …, 5, and perhaps more). This is one of the things we will be testing when we run your program. Technically, the
player should lose if the number of aliens causes them to drop below the defense line, but you can ignore that issue in this step.
Try to finish this part by Sunday, November 19. All you need to do is to produce the alien diagram shown above (after the
welcome screen). Once you have done this, you should be an expert with graphics objects. This will give you considerable
confidence that you can get the rest done.
Create (and Animate) the Ship
Next you need to create the player ship. Again, this is to be stored in an attribute of class _wave. That means that you must create it
in the __init__ method of Wave and modify your drawing code so that it appears. The ship should be an object of class Ship,
which is once again a subclass of GImage.
Creating the Ship
The ship dimensions an position are fully specified by constants in The ship should be centered horizontally and the
bottom should be SHIP_BOTTOM from the bottom of the window. You are free to change these constants as you wish, but use them
when making the ship.
Creating the Defense Line
The defense line is the line above the ship that it is defending from the aliens. It is DEFENSE_LINE pixels above the bottom of the
window. You should create and draw this line. A line is represented by a GPath object.
The primary attributes for GPath are points and linewidth. The linewidth is the width of the line. Make it larger than 1 for a
thicker line. The attribute points is an even-length list of numbers defining the line segment. So if you want the line segment
from (0,10) to (20,30), the points attribute would be [0,10,20,30].
Animating the Ship
To animate the ship, you will need to take into account the player’s key presses. The ship only moves when the player presses (or
holds down) a key to make it move. By default, we assume that the player will use the left and right arrow keys to move the ship.
However, if you prefer WASD controls or some other control scheme, that is okay.
To see how to control the ship, you should look at the sample from class. This example shows how to check if the
arrow keys are presse, and how to use that to animate a shape. Note that this is actually easier than dismissing the welcome
message. We do not care if a key press is the first one. The ship will continue to move so long as we hold down a key.
The ship movement takes place every animation frame. That is why you want to put it the update method of Wave. Remember
that this method must be called within the update method of Invaders, or nothing will happen. Again, see the subcontroller
example from class to understand what we are asking for.
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To check the keyboard, the method update in Wave will need to access the input attribute Invaders, which is an instance of
GInput. Again, since Wave is not allowed to access any of the attributes of Invaders, that means you need to pass input as an
argument in this method call.
In the basic game, the ship should only move left or right, not up and down. Add to the x attribute to get it to move right, and
subtract from it to move it left. The amount you add or subtract is up to you, though we have provided the constant
SHIP_MOVEMENT as a suggestion. If the value is too small, then the ship will be slow and sluggish. If the value is too large, then
the ship will zip across the screen and be hard to control.
Important Considerations
You should ensure that the ship stays completely on the board even if the player continues to hold down a key. If you do not do
this, the ship is going to be completely lost once it goes off screen. Our code for this extra feature is 3 lines long; it uses the
functions min and max.
Your implementation should only allow the ship to be moved when the state is active. This means that update in Invaders is
starting to get more complicatd. At this point you might want to start thinking about helper methods to organize your code better.
Look at the demo from lecture for ideas on how to organize your code. In particular, you want a method to determine the
current state, and then helpers for each of the states.
Draw the ship and get it move by Monday, November 20. This is a really straightforward challenge if you understand the arrows
demo. Given all the work you have done so far, you should be able to complete this in two hours or so.
Walk the Aliens Back-and-Forth
Now that the set-up is complete, it is time to start invading. To invade, the aliens have to march back and forth across the screen.
Fortunately, you have already gotten the ship moving, so you know how to move things. The aliens are similar, with just a few
important differences.
Moving the Aliens to the Right
When aliens move, they always move the same amount: ALIEN_H_WALK. To move the aliens to the right, simply add this value to
the x position of each alien. However, aliens have one important feature that makes them different from the ship. They do not move
every animation frame
You will notice that has an attribute called ALIEN_SPEED. This is how fast the aliens walk. If the value is 1.0, they
make a step every full second. If it is 0.5, they move every half of a second. The smaller this number is, the faster they move
(increasing the difficulty).
This means that you have to keep track of how long it has been between steps before you move the aliens again. How do you do
that? In the specification of Wave, you will see an attribute called _time. This counts the number of seconds since the last alien
step. At the start, and each time the aliens move, it is reset to 0. Otherwise, you add the number of seconds that have passed to
_time, and you do not move the aliens. When this value is bigger than ALIEN_SPEED, you move the aliens again.
So your next question should be how to count the numbers of seconds that have passed. You will notice that update in Invaders
has an attribute called dt. This counts the number of seconds that have passed since the last animation frame (at 60 frames as
second, it is somewhere around 0.017 each time). This is the value that you should be summing and adding to the attribute _time
in Wave. You can do this via a setter or by passing dt as an argument to the update method in Wave.
As you work on this code, you will discover that the update method in Wave is starting to get very long. It is time to start thinking
about breaking this method into helpers.
Moving the Aliens Back
Right now, your aliens will march all the way off the screen. You do not want that. When the wave of aliens reaches the right edge
of the screen, you want to move the aliens down by ALIEN_V_WALK. Then you need to start walking them back to the left.
This means that at each step, you need to find the rightmost alien in the wave. Check if its right edge is closer than ALIEN_H_SEP
to the right edge of the window. If so, move all the aliens all down by ALIEN_V_SEP. You should also move them back to the left
so that there is ALIEN_H_SEP distance between the closest alien and the right of the window (the aliens should never go
Repeat this process back and forth. When the aliens get to far to the left, move them down and start moving right again. This
suggests that Wave needs an extra attribute that keeps track of whether the aliens are moving right or left.
Testing the Speed
The speed is what determines the difficulty of the game, so it is a good idea to test the game at different speeds. Just as you can
change the number of rows and aliens per row, you can also change the speed from the command line.
When you run your application (again, assuming that it is in a folder called invaders) try the command
python invaders 2 3 0.5
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When you do this, Python changes the value of ALIEN_ROWS to 2, the value of ALIENS_IN_ROW to 3, and the value of
ALIEN_SPEED 0.5. By varying this third number, you can make the aliens march faster or slower. If your aliens do not move at
different speeds when you try this, you have a problem that you need to fix before continuing.
Try to finish this part by Tuesday, November 21, before you leave for Thanksgiving. The speed delay is the hardest part of this
challenge, and it is a great thing for you to get help on during the open office hours (replacing the labs) this week. If you get this
far, you will be in great shape to finish up the last week of class.
Fire Bolts from the Ship
Now that you are back from Thanksgiving break, it is time to fight back against the aliens. You are going to give the ship the ability
to fire laser bolts.
Creating a Laser Bolt
The game should create a laser bolt when the player presses a fire key. The standard keys for firing the laser are space or up-arrow.
However, it can be anything that you want so long as it is clear to the player.
When you detect a fire command, you should create a Bolt object. This Bolt object should have the same x position as the ship,
and it should be placed right in front of the ship’s nose. Use the constants in to calculate this from the ship and bolt
Unlike the ship and alien classes, Bolt is a subclass of GRectangle. It is very similar to GImage, except that you specify a
fillcolor and a linecolor instead of a image file. We do not expect your laser bolt to be anything fancy.
Bolt objects are stored in the _bolts attribute of Wave. This is a one-dimensional list that may be empty (because there are no
bolts on the screen). You draw the bolts in much the same way that you draw the aliens.
Customizing the Bolt Class
Up until now, we have not needed to add additional attributes to any of our model classes. While the aliens march across the
screen, they all march in lock-step. However, some laser bolts will be moving upwards, and some will be moving downwards.
Therefore, we need to add an attribute in Bolt to keep track of the velocity.
This means that you will need to override the __init__ method in GRectangle and provide your own. We do not care what the
parameters for this method are. That is up to you. The velocity should either be BOLT_SPEED for bolts that are going up, or –
BOLT_SPEED for bolts going down.
Because we will later have the aliens fire their own bolts, you want to be able to distinguish bolts shot from the player from those
shot from an alien. We suggest that you add a method like isPlayerBolt to the class. Remember that all bolts that travel up are
fired by the player.
Moving the Laser Bolt
You move a laser bolt in much the same way that you moved the ship and aliens. You just add the velocity to the y position. Like
the ship, and unlike the aliens, you move the bolt every animation frame. You do not have to stagger the movement.
Removing the Laser Bolt
Right now, your laser bolt will pass through aliens. That is okay. We will worry about the aliens later. However, we do want you to
delete the bolt when it goes off screen. To do this, simply check if the bottom of the bolt is above the height of the game window. If
so, delete the bolt from the list _bolts in Wave. The bolt will no longer be drawn and the object will be deleted. See the sample
code for an example of how to do this.
Important Considerations
There is an important restriction on laser bolts that you must implement at this time. The player may only have one laser bolt on the
screen at a time. The player cannot fire a new bolt until this bolt goes off screen and is deleted.
Keep in mind that _bolts will eventually have multiple laser bolts in the list even though only one of them will belong to the
player. This is the reason for the isPlayerBolt method we suggested. Look at every bolt in the list. If one of them is a player
bolt, the player cannot fire. Otherwise, the player is free to fire.
Try to finish this part by Monday, November 27, after coming back from break.
Fire Bolts from the Aliens
The ship should not be only thing to fire laser bolts. The aliens get to fire bolts as well. The process should be very similar to the
laser bolts for the ship. You create a Bolt object (though this time its velocity is -BOLT_SPEED), add it to the list _bolts, draw it,
and move it across the screen. An alien bolt should be removed when it goes off the screen, which is when the top of the bolt is less
than 0.
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However, there is a tricky part about the alien bolts. There a lot of aliens. So you need to control which aliens fire and when. First
of all, aliens should only fire when they walk. That means that the faster they walk, the faster they fire. Alien speed is the main
difficulty in Space Invaders, so this makes sense.
Picking When to Fire
While we could have the aliens fire every time they step, that would make the game hard. But we also do not want them to fire too
slow, as that makes the game to easy. What we want is for the aliens to fire randomly. Sometimes they fire slow and sometimes
they fire fast.
In you will see the BOLT_RATE. This is the number of steps between alien shots. So if it is 5, the aliens can take up to
five steps before one of them shoots. If it is 10, they can take up to ten steps between shots.
At the start of the wave, you should use the random module to pick a number between 1 and BOLT_RATE. You have already had
some experience with this module in a lab. That number should be stored in an attribute in Wave; it is the number of steps until the
aliens fire. When the aliens are ready to fire, you create a Bolt. You will also generate another number between 1 and BOLT_RATE
for the next laser bolt.
Picking Who to Fire
The laser bolts have to come from an alien. To pick an alien, you should first pick a nonempty column of aliens at random. Note the
invariant of _aliens says that the table of aliens can have None as an entry (which happens when an alien is destroyed). You do
not want to pick a column where all of the rows are None. But otherwise, you should use the random module to pick the column.
Next, you should identify the bottommost alien in the column. Again, because aliens may be destroyed, you should not pick a
position where the alien is None. Search the column and find the alien on the bottom.
Once you have done that, you can create the Bolt object. The bolt should have the same x position as the alien. The top of the bolt
should be just below the feet of the alien.
Try to finish this part by Tuesday, November 28. This is one of the trickier parts of the assignment, but you have now built up
enough skills that you should be able to do this in a day.
Handle Bolt Collisions
You should have a lot of laser bolts flying back and forth, but they are all pretty useless right now. It is time to give them some bite.
You need to detect collisions and remove any aliens (and the player) that are killed by a bolt.
How do you detect collisions? Suppose the bolts were a single point (x,y) rather than a rectangle. Then, for any GObject gobj, the
method call
returns True if the point is inside of the object and False if it is not. Since both Ship and Alien are sub(sub)classes of
GObject, they inherit this method.
However, the bolt is not a single point. It occupies physical area, so it may collide with something on the screen even though its
center does not. The easiest thing to do — which is typical of the simplifying assumptions made in real computer games — is to
check the four corners of the bolt and see if any of them are inside of the object. Because the bolt is smaller than either the ship or
an alien, we can guarantee that one of these four corners is inside of the object during a collision.
You should add the method collides to both the class Ship and Alien. This method returns True if a bolt (fired by the other
side) has collided with the instance self of that class. For example, here is our specification of collides in Alien:
def collides(self,bolt):
Returns: True if the bolt was fired by the player and collides with this alien

Parameter bolt: The laser bolt to check
Precondition: bolt is of class Bolt
These methods will be similar, except for one important difference. The method in Ship will only return True if the bolt was fired
by an alien (e.g. it is going down). In Alien it will only return True if the bolt was fired by the ship (e.g. it is going up).
If you detect a collision between an bolt and an alien, you should set that position of the table _aliens to None and remove the
bolt from the list. Similarly,if you detect a collision between a bolt and the ship, you should set _ship to None and remove the bolt
from the list.
When you make this change, you are probably going to have to go back and make some changes to other parts of your code. You
are going to start running into NoneType errors in the code for marching the aliens and drawing the aliens. That is because some of
the aliens may now be None. You have to prepare your code to deal with this. Every time you access _ship or an element of
_aliens, you must check for None before continuing.
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Once you have completed this, you should be able to start playing a game.
Try to finish this part by Wednesday, November 29.. This is the vast majority of the assignment and will allow to you
completely finish everything by the end of classes.
Finish the Game
You now have a (mostly) working game. However, there are two minor details left for you to take care of before you can say that
the game is truly finished.
Player Lives
When the _ship attribute becomes None, the player cannot really do much any more. What should happen in this case is that the
player should lose a life. The player should have three lives before losing the game. You will notice an attribute call _lives in
Wave for managing these lives.
If the player still has lives left after losing a ship, the update method in Invaders should change the state to STATE_PAUSED and
display a message (as you did on the welcome screen) that the player press ‘S’ to continue. As soon as the player presses this key,
switch the state back to STATE_ACTIVE and start the game again. The wave should continue where it left off.
Winning or Losing
Eventually the wave of aliens will end. Each time the player loses a life, you need to check if there are any lives left. If not, the
game is over. Additionally, the game is over if (1) all the aliens are killed or (2) any alien dips below the defense line. Both of the
latter need to be checked in the update method of Wave. You might want to add an extra attribute to keep track of when either of
these happen.
When the wave ends, and the player has either won or lost, you should put up one last message. Use a GLabel to put up a
congratulating (or admonishing) message. Finally, you should change the state one last time to indicate that the game is over. This
is the purpose of the state STATE_COMPLETE.
Try to finish this part by Thursday, November 30.
Extending the Game
If you have followed our suggested timeline, you now have three days to extend the game and try to make it more fun. In doing
this, you might find yourself reorganizing a lot of the code above. You may add new methods or change any of the methods you
have written. You may add new classes. For example, you might want to add a class for a boss alien that is different from a regular
alien. You can even completely rewrite existing classes like Alien to support cool animations.
You are allowed to change anything you want so long as you update the specifications to reflect the changes. There are only
four things that you are not allowed to change.
The win (destroy all aliens) and lose (lose all lives, break the defense line) conditions must remain the same.
The aliens must be generated two rows at a time, bottom to top (though each row can have some special aliens).
The aliens must start marching at the speed ALIEN_SPEED
The ship should never move off screen (aliens can, if creating attack waves).
Everything else is fair game. However, we highly suggest that you save a copy of the basic game in a separate folder before you
start to make major changes. That way you have something to revert to if things go seriously awry when implementing your
extensions. Also, we suggest that you make sure to comment your code well in order to keep track of where you are in the coding
Extensions are worth roughly eight points of your final grade. With no extensions at all, you can still make a 92/100 on this
assignment (which is an A- for an assignment grade). If you give us at least two solid extensions (see the suggestions below), we
will give you those eight points. If you are unsure of whether an extension is “good enough,” ask on Piazza.
If you give us more than two extensions, we will treat this as a form of extra credit. While we will not allow you to go over the
maximum score, this will allow you to earn back any points that you might miss for implementing a feature incorrectly. However,
your cannot get back any points lost for writing bad specifications or for violating the 30-line rule, no matter how good your
extensions are.
Possible Extensions
Here are some possible ways to extend the game, though you should not be constrained by any of them. Make the game you want
to make. We will reward originality more than we will reward quantity. While this is a fairly simple game, the design space is wide
open with possibilities.
Multiple Waves
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The easiest extension is to implement multiple waves. If the player completes a wave without losing all the ship lives, it is time for
a new wave of aliens. This is really easy, since all you have to do is make a new Wave object.
For this to count as a proper extension, we want each wave to increase the alien speed. The alien speed is what makes the game
difficult, and the game should get more difficult with each wave. You will have to add some attributes to Wave to keep track of this.
You will also need modify the __init__ method so that Invaders can increase the speed each time it makes a new wave.
Implement Sound Effects
Another easy extension is to add appropriate sounds for game events. We have provided several audio files in You will
want to look at them, but you are not restricted to only those sounds. Remember that it is a violation of the Academic Integrity
Policy to use unlicensed copyrighted material.
To load an audio file, you simply create a Sound object as follows:
pewSound = Sound(‘pew1.wav’)
Once it is loaded, you can play it whenever you want (such as when the ship fires the laser) by calling The
sound might get monotonous after awhile, so make the sounds vary, and figure out a way to let the user turn sound off (and on).
Read the online specification to see how to use Sound objects. In particular, if you want to play the same sound multiple times
simultaneously (such as if two aliens fire simultaneously), you will need two different Sound objects for the same sound file.
Dynamically Speed Up the Aliens
The aliens in Space Invaders do not just speed up when you start a new wave. They also speed up as you kill aliens. This is actually
a result of a bug in the original Space Invaders, but it was left in because it made the game fun.
To make the aliens move faster, you need to make the speed value smaller. A speed value of 1.0 means they march every second,
while 0.5 means they march every half second. So the easiest way to increase the speed is to multiply by some number less than
one each time you kill an alien. Do not make the multiplication factor too small, or else the aliens will jump to lightning speed. You
will discover that a factor of 0.97 is reasonably challenging.
Keep Track of Score
A large part of the challenge of Space Invaders is getting a high score. Aliens in the front are worth a few points and aliens in the
back are worth less. You should display the score at all times using a GLabel object. Where you display it is up to you (except do
not keep the player from seeing the aliens or ship). Please do not make a new GLabel object each time the score changes. This
will slow down the program tremendously. Simply change the text attribute in your GLabel object.
In classic Space Invaders, this score carries over between waves. If you chose to implement multiple waves, then each wave after
the first should start with the score from the previous wave.
Animate the Aliens
In the video at the start of the animation, you will notice that the aliens are animated as they move and that they have simple
explosion animations. These animations are actually made available to you as filmstrips. A filmstrip is single image file that holds
multiple animation frames. These frames are arranged in a rectangular grid where each frame has equal size. For example, alienstrip1.png is 3×2 grid of animation frames. The first row is the alien walking. The other two rows animate the alien explosion.
To use a filmstrip, you need to make a GSprite object as follows:
alien = GSprite(width=…,height=…,source=’alien-strip1.png’,format=(3,2))
The GSprite class has an attribute called frame which tracks which frame in the filmstrip is currently displayed. At the start, the
frame is always 0, showing the alien in the top-left corner. To walk the alien, you just add the following code to your march update
alien.frame = (alien.frame+1) % 2
You are free to use these filmstrips (also called sprite sheets) or make your own. The Stitches website is a great resource for
making filmstrips.
Defense Barriers
The classic Space Invaders has defense barriers that the player can hide behind. These barriers are damaged by laser bolts fired by
either the player or the aliens. They provide a bit of respite when the aliens are moving very fast.
The original game destroys the barriers a pixel at a time. This is extremely hard to do with the classes that we have provided you,
and impossible without accessing hidden attributes (you have to perform texture blitting on the _texture attribute in GImage or
GSprite). So we do not recommend this approach at all.
However, a simpler approach is to just give the defense barriers a health meter. As the bolts hit the barrier it is weakened and
eventually destroyed. You can even use GSprite to change the barrier appearance as it weakens.
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Use your imagination
What else have you always wanted a game like this to do? Do you want to have swooping aliens like in Galaxian and Galaga?
Should aliens drop power-ups that give the ship rapid laser fire? Do not go too wild with the power-ups, however. We much prefer
a few innovations that greatly improvevthe play as opposed to a screen filled with gizmos.
Again, you can make any modifications to the gameplay you want, but the core gameplay of ships and marching aliens should be
there. Please do not submit a copy of Asteroids and expect to receive credit.
Completing the Assignment
Before submitting anything, test your program to see that it works. Play for a while and make sure that as many parts of it as you
can check are working. Remember to check both of the lose conditions, not just the loss of three lives.
When you are done, reread the specifications of all your methods and functions (including those we stubbed in for you), and be
sure that your specifications are clear and that your functions follow their specifications. If you implemented extensions, make sure
your documentation makes it very clear what your extensions are.
As part of this assignment, we expect you to follow our style guidelines:
1. There are no tabs in the file, only spaces
2. Classes are separated from each other by two blank lines
3. Methods are separated from each other by a single blank line
4. Class contents are ordered as follows: getters/setters, initializer, non-hidden methods, hidden methods
5. Lines are short enough that horizontal scrolling is not necessary (about 80 chars is long enough)
6. The specifications for all of the methods and properties are complete
7. Specifications are immediately after the method header and indented
8. No method is more than 30 lines long, not including the specification
Turning it In
You are potentially modifying a lot of files in this assignment. At a bare minimum, your are modifying,, and You might be modifying You might have extra art and sound files.
In addition, you should create a text file called extensions.txt. In this file, you should write a brief description of your
extensions. Tell us what you were trying to do and how you did it. If you used any art or sound assets that required an attribution
license, you should put that attribution here.
To simplify the submission process, we are not asking you upload each individual file. Instead, put all your files in a zip file called and submit this instead. We need to be able to play your game, and if anything is missing, we cannot play it.
One last time, we need you to do a survey. The survey should be done individually (even if you worked in a group). As always, the
survey will ask about things such as how long you spent on the assignment and your impression of the difficulty. Please try to
complete the survey within a day of turning in this assignment. Remember that participation in surveys comprises 1% of your final


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