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Classes and instances

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
  2. class Cat
  3. 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
  2. order_id
  3. 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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