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

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()

Concepts in Practice \(\PageIndex{1}\)

What is the name of the class?

1. Cat
2. class Cat
3. self

Answer

a. Cat follows the class keyword at the top of the class definition and is the name of the class.

Concepts in Practice \(\PageIndex{2}\)

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

1. __init__
2. name
3. pet_1

Answer

c. pet_1 is an instance of the Cat class created by calling Cat(). The other instance is pet_2.

Concepts in Practice \(\PageIndex{3}\)

Which of the following is an attribute?

1. breed
2. pet_2
3. print_info

Answer

a. breed is an instance attribute defined in __init__(). The other instance attributes are name and age.

Concepts in Practice \(\PageIndex{4}\)

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

Answer

c. CapWords style, with all words capitalized and no spaces between words, is recommended for class names. "Pet" and "cat" are separate words, so each is capitalized and one directly follows the other.

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

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())

Concepts in Practice \(\PageIndex{5}\)

How many times is __init__() called?

1. 1
2. 2
3. 3

Answer

b. Two instances, room_1 and room_3, are created by calling Rectangle(), which calls the constructor __init__().

Concepts in Practice \(\PageIndex{6}\)

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

Answer

b. room_1 is the instance that called the method area(), so self represents the specific instance.

Concepts in Practice \(\PageIndex{7}\)

Which line initializes the instance attribute length?

1. 3
2. 6
3. 9

Answer

a. Line 3 is in __init__(), which initializes length and the other instance attributes for each new instance.

Concepts in Practice \(\PageIndex{8}\)

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

Answer

c. __init__() has one required parameter, conventionally called self, which allows the constructor to access the instance.

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

Concepts in Practice: Instances and class attributes

Consider the example above.

Concepts in Practice \(\PageIndex{9}\)

Which is an instance attribute?

1. loc
2. order_id
3. order_3

Answer

b. order_id is an instance attribute defined in __init__(). The other instance attribute is cup_size.

Concepts in Practice \(\PageIndex{10}\)

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

Answer

b. Changing order_1.cup_size does not affect order_3.cup_size. order_1.cup_size changes to 8 and order_3.cup_size is still 16 .

Concepts in Practice \(\PageIndex{11}\)

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

Answer

a. order_1, order_2, and order_3 all share the class attribute CoffeeOrder.loc. Changing loc affects all instances, so Caffeine Cafe is printed as the location.

Concepts in Practice \(\PageIndex{12}\)

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

Answer

b. Trying to access a class attribute using an instance is unsafe. In __init__(), trying to access the class attribute creates a new instance attribute called cls_id. Though possible, programmers should avoid using the same name for class attributes and instance attributes to avoid confusion.

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

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