11.2: Classes and Instances

Learning Objectives

By the end of this section you should be able to

• Create a class with instance attributes, class attributes, and the __init__() method.
• Use a class definition to create class instances to represent objects.

Classes and instances

In the previous section, a real-world entity, like a person's social media profile, was modeled as a single object. How could a programmer develop a software system that manages millions of profiles? A blueprint that defines the fields and procedures of a profile would be crucial.

In a Python program, a class defines a type of object with attributes (fields) and methods (procedures). A class is a blueprint for creating objects. Individual objects created of the class type are called instances.

Checkpoint: Representing a coffee order with a class

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Concepts in Practice: Classes and instances

Consider the example below:

    class Cat:
      def __init__(self):
        self.name = 'Kitty'
        self.breed = 'domestic short hair'
        self.age = 1
      def print_info(self):
        print(self.name, 'is a ', self.age, 'yr old', self.breed)
    
    pet_1 = Cat()
    pet_2 = Cat()

1.

What is the name of the class?

1. Cat

class Cat

self

2.

Which of the following is an instance of the Cat class?

1. __init__
2. name
3. pet_1

3.

Which of the following is an attribute?

1. breed
2. pet_2
3. print_info

4.

Suppose the programmer wanted to change the class to represent a pet cat. Which is the appropriate name that follows PEP 8 recommendations?

1. petcat
2. pet_cat
3. PetCat

Creating instances with __init__()

___init___() is a special method that is called every time a new instance of a class is created. self refers to the instance of a class and is used in class methods to access the specific instance that called the method. __init__() uses self to define and initialize the instance's attributes.

Checkpoint: Creating multiple coffee orders and changing attributes

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Concepts in Practice: instances and __init__()

Consider the example below:

1

class Rectangle:

2

def __init__(self):

3

self.length = 1

4

self.width = 1

5

def area(self):

6

return self.length * self.width

7

 

8

room_1 = Rectangle()

9

room_1.length = 10

10

room_1.width = 15

11

print("Room 1's area:", room_1.area())

12

room_3 = Rectangle()

13

room_3.length = 12

14

room_3.width = 14

15

print("Room 3's area:", room_3.area())

5.

How many times is __init__() called?

1. 1
2. 2
3. 3

6.

When line 11 executes, execution flow moves to line 5. What does self represent on line 5?

1. the area of room_1
2. the instance room_1
3. the Rectangle class

7.

Which line initializes the instance attribute length?

1. 3
2. 6
3. 9

8.

Suppose line 2 is changed to def __init__():. What would room_1's attributes be initialized to?

1. length = 0, width = 0
2. length = 1, width = 1
3. Error

Instance attributes vs. class attributes

The attributes shown so far have been instance attributes. An instance attribute is a variable that is unique to each instance of a class and is accessed using the format instance_name.attribute_name. Another type of attribute, a class attribute, belongs to the class and is shared by all class instances. Class attributes are accessed using the format class_name.attribute_name.

Checkpoint: Using class attributes for shared coffee order information

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Concepts in Practice: Instances and class attributes

Consider the example above.

9.

Which is an instance attribute?

1. loc

order_id

order_3

10.

Suppose the line order_1.cup_size = 8 is added before order_3.print_order(). What is the new output?

1. Cafe Coffee Order 3 : 8 oz
2. Cafe Coffee Order 3 : 16 oz
3. Error

11.

Suppose the line CoffeeOrder.loc = 'Caffeine Cafe' is added before order_3.print_order(). What is the new output?

1. Caffeine Cafe Order 3 : 16 oz
2. Cafe Coffee Order 3 : 16 oz
3. Error

12.

Suppose the line self.cls_id = 5 is added to the end of __init__()'s definition. What is the new output?

1. Cafe Coffee Order 5 : 16 oz
2. Cafe Coffee Order 3 : 16 oz
3. Error

Try It: Creating a class for an airline's flight tickets

Write a class, FlightTicket, as described below. Default values follow the attributes. Then create a flight ticket and assign each instance attribute with values read from input.

Instance attributes:

• flight_num: 1
• airport: JFK
• gate: T1-1
• time: 8:00
• seat: 1A
• passenger: unknown

Class attributes:

• airline: Oceanic Airlines
• airline_code: OA

Method:

• __init__(): initializes the instance attributes
• print_info(): prints ticket information (provided in template)

Given input:

    2121
    KEF
    D22B
    11:45
    12B
    Jules Laurent

The output is:

    Passenger Jules Laurent departs on flight # 2121 at 11:45 from KEF D22B in seat 12B

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Try It: Creating a class for fantasy books

Write a class, Book, as described below. Then create two instances and assign each instance attribute with values read from input.

Instance attributes:

• title: ''
• author: ''
• year: 0
• pages: 0

Class attribute:

• imprint: Fantasy Tomes

Method:

• __init__(): initializes the instance attributes
• print_info(): prints book information (provided in template)

Given input:

    Lord of the Bracelets
    Blake R. R. Brown
    1999
    423
    A Match of Thrones
    Terry R. R. Thomas
    2020
    761

The output is:

    Lord of the Bracelets by Blake R. R. Brown published by Fantasy Tomes
    in 1999 with 423 pages
    A Match of Thrones by Terry R. R. Thomas published by Fantasy Tomes
    in 2020 with 761 pages

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