Section 6.1 CS 106 Victor Norman IQ Unknown

The Big Q What do we get by being able to define a class?! Or Do we really need this?!

Ancient History (last Tuesday) A card is a tuple with 2 parts, a suit (one of “s”, “d”, “c”, “h”) and a number (2 – 14). We create a card by making a tuple. We access the suit via card[0] and numbervia card[1]. What is good and what is bad about this implementation?

What types of variables can we make? Is this good enough?

Second big Q What defines a type? Data + operations – what you can store. – what you can do to or with it.

Lecture... a class definition is like a recipe (or template). – you don't eat the recipe, right? an object is an instantiation of that class – that's what you eat. Or, a class is a new type.

Class (or Type) Marriage of data and methods. Boundary between caller and implementer – perspective on the world. self is a reference to the object, from within the class code. Each object has its own namespace, accessible via self. (In fact, isn't an object just a namespace? Could be...)

Creating class instances Q: One creates a new instance of a class by calling the class ______________. A: constructor

What does constructor code do? Q: When code instantiates a class, the __init__ method is called. This method's "primary purpose is to establish initial values for the ____________ of the newly created object." A: attributes

Code for Car constructor Q: I have a class Car that I instantiate this way: car = Car () Write the method definition for Car’s constructor. A: def __init__ ( self ): # code here to initialize attributes to # default values … self. _color = “ black ”

“Setter” Signature Q: After creating a Car instance (i.e., object), my main code wants to set the Car's color to a new color. Write the signature of the Car member function, setColor(), to set the car's color. A: def setColor(self, color): ‘’’set the color of this car to the given color’’’ self._color = color

“Getter” Signature Q: Then, my main code wants to get the color from my car instance. Write the signature for the member function getColor(). A: def getColor(self): ‘’’return the color of this Car to the caller.’’’ return self._color # my color

Syntax errors? How many syntax errors are there in this code?: class Car def _init_(make, model, year): self.myMake = make self.myModel = model myYear = year

Answer me this. Given this class definition: class Car: def __init__(self, make, model, year): self.myMake = make self.myModel = model self.myYear = year which is legal code to make a new Car instance? A.aCar = Car.__init__(self, “Honda”, “Odyssey”, 2001) B.aCar = Car.__init__("Honda", "Odyssey", 2001) C.aCar = Car("Honda", "Odyssey", 2001) D.aCar = Car(self, "Honda", "Odyssey", 2001) E.Car(aCar, "Honda", "Odyssey", 2001)

Why use classes? What are the advantages/disadvantages of defining a class/type? Advantages: Can control the state of each object – the setter methods allow only some attributes to be changed. – Sets up a sort-of “firewall” around each object. Can control what a user can do with an object. Is more “polite”: code “asks” an object to do something for it. Other advantages: subclassing, etc. Disadvantages: More syntax. More code.

Accessors and no mutators? Q: What would be the advantage to only giving the user accessor functions and no mutator functions? A: You can restrict what attributes can be changed – i.e., you can make them immutable

Example class Student: def __init__(self, name, id, grades): self._name = name self._id = id self._grades = grades def setName(self, newName): “””Change the student’s name””” self._name = newName # getName(), getId(), and getGrades() here. # No setId() here: a student’s id can never # be changed after the object has been created.

self Note that there is code “on the inside” – code inside the class definition. Code in __init__, getName(), etc. This code refers to the object as self. Then, there is code on the outside: stud1 = Student( “Dan”, , [100, 90, 80] ) This code refers to the object as stud1.

Example class Student: def __init__(self, name, id, grades): self._name = name self._id = id self._grades = grades def getName(self): return self._name def setName(self, newName): self._name = newName def getId(self): return self._id # Create a student student1 = Student( “Angelina”, 10, [] )

Designing a Class When you design a class, you decide: what are the important properties (or attributes, characteristics, or state) of an instance. what information a caller should be able to get from an object what state a caller should be able to change in an object what a caller should be able to ask an object to do to itself.

What goes in a class definition? Q: So it is my understanding that a class definition is a series of function definitions nested within the class definition… Is there anything else that goes into it? A: No. The function definitions in the class definition define the operations that the objects provide. The attributes for each object are set in the constructor method.

Naming Conventions A class name always starts with a capital letter. Each word in the class starts with a capital letter. – class MyFathersCar: – class Cs106Lab: A class attribute always starts with an underscore: _ – self._x_loc, self._y_loc, self._color

Naming Conventions local variables must start with a lowercase letter. global variables must also start with a lowercase letter, unless they are CONSTANTS, which must be all uppercase. function names start with a lowercase letter. Either use camelCase for function names and variables or use _'s between words in a name. – myXLocation, or, my_x_location – getLocation(), or, get_location()

Order of methods in a class Q: Does the order the methods are defined in a class matter? A: No. By convention the constructor method is always first, however.

Attribute names Q: Why would the coder choose _x and _y instead of x and y? A: It is a convention to name your attributes with names starting with _. The reader of the code can then know what something is when it sees the _ in front of the name.