In Class: Week 11 Tuesday and Thursday:
Defining Classes

Create a week 11 subdirectory in your cs21/inclass directory by running update21:

$ update21
$ cd
$ cd cs21/class/w11-classes

Topics

• Object-Oriented Programming
• Defining Classes
• Class methods
• The constructor, __init__
• String conversion, __str__
• The implicit self parameter
• Access methods a.k.a. getters
• Mutator methods a.k.a. setters

In-class work

Classes

1. Open up accounts.py. Read through the mostly completed class definition for a bank account. Using this class will be able to define Account objects.

2. First, we will illustrate the use of several functions. You should be able to draw a stack diagram for the constructor showing how self is defined.

3. Open up testAccounts.py and trace the code which creates three separate Account objects using the constructor, prints them using the string conversion method, and prints out information given by the accessor methods getBalance and getName

4. Take some time to implement and test the changePin method. It is very important that you employ unit testing just as you did with the top-down design labs

5. When you are finished, analyze and test the withdraw() function. Once you understand how that works, implement and test the deposit() method. If you have time, implement and test the computeInterest() method.

6. With the time remaining (possible into next week), we will rethink how object-oriented programming could improve our Scattergories program from Week 7. Specifically, we will define a class for creating card objects that store the categories and answers for a user.

Post-class questions

• when constructing our class object, how do decide what to include as a parameter?
  There is no single rule to answer this. But, in general, you need to think about how to initialize all of the data in an object. Some of this data can be a default value, so a parameter is not needed. For example, we could create an Account object without a balance parameter and just initialize self.balance to 0. If, however, some information is needed to initialize the data, we should have a parameter for it. For example, a Snail object needs to be centered somewhere in the window. So, we need to send in a parameter for it


• How do you make classes of graphics?
  The same as any class. I've released a snowman.py file showing how you can define a Snowman object for Lab 5. testSnowClass.py uses this class to create an army of snowmen


• Can classes interact?
  Yes! You will see this in the next lab, where a Ball object interacts with a Paddle object. But you've been doing this for awhile now - GraphWin is an object and we draw Circles et al in it.


• How to decide what kind of information should be stored in a class
  Any data that describes the class/object. What describes an account (name, id, pin, balance,...)? Your answer should go in the Account class.


• can you have multiple classes in one progra
  Yes! See Lab 10 and also our snowman lab - you drew Circles, Rectangles, etc.


• Can we have a class within a class
  Yes! We could create a Bank class that has a list of Accounts in it as well as a branch name and address. Also, the Snail class did this as well.


• What is a class and how does it it make things easier?
  A class is a definition or template for creating a specific type of object. Another way to think of it - it defines a type in Python (like float, int, string, Circle). It makes things easier by encapsulating a concept - it puts data and methods that describe a single concept into one container.


• What are some more practical applications of Classes
  Every major piece of software uses classes. Think of how you used top-down design for labs 8 and 9. For real-world applications, we do the same for very large programs by using classes. Banner (our registration system) would have classes to define Student, Faculty, Course, Department, Building, Room, Staff, etc. Spotify probably has classes to describe a Song and Artist and Album as well as a Playlist (list of Songs with methods like Playlist.shuffle())


• Are there alternatives to self?
  In Python? No, not that I'm familiar with. Most languages use the word this in a similar way.


• What does self mean?
  It is a variable for the object being used. Any data that belongs to the object needs to have the word self before it otherwise it gets lost when the method completes.


• what are the limitations of creating classes to use in functions?
  None. I may be misinterpreting this question so please follow up.


• can we go over recursive again, at least before the next quiz?
  In class? Probably not. David will have some quiz review in lab next week, and the ninjas do quiz review anytime they are asked to. Also, please see me in office hours or by appointment.