Objects

What is an object?

• Any data with state (attributes or value) and defined behavior (methods)

Common Objects

• str
• int
• list
• range

State

• Objects are containers for state and behavior
• They may have a value
• They may have attributes

Functions

• Functions are containers for behavior
• They do not typically also store state

Attribute and Method Access

• Attributes and methods are accessed using “.”

Attribute Example

r = range(1, 10, 2)

print(r.start)
print(r.stop)
print(r.step)

Methods

• A function that is a member of an object
• When called as an attribute of an object, the function can operate on the object state

Method Example

l = [3, 1, 4, 2, 5]

l.sort()

Class

• A template for creating user-defined objects
• Class definitions normally contain method definitions which operate on instances of the class
• Class names are written in upper camel case by convention

Example

class Player:
    hp = 100

alice = Player()
print(alice.hp)

Exercise

Create a Student class with class_level and gpa attributes

Methods

• Functions defined in the class body become attached to objects
• When called, these have access to the object instance

Example

class Vehicle:
    speed = 0

    def accelerate(self):
        self.speed += 1

car = Vehicle()
car.accelerate()
print(car.speed)

Exercise

Add a set_gpa method to your Student class that will set their GPA to a supplied number

Constructor

• A special method may be called when creating a new object from a class
• This is called the constructor

Example

class Length:
    def __init__(self, meters):
        self.meters = meters
        self.feet = meters * 3.2808

l = Length(2)
print(l.feet)

self

• The first argument passed to a method will be a reference to the object instance
• self is chosen as the name for this parameter by convention