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== 11.2#h(0.6em)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 #strong[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 #strong[instances].


    #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[
#emph[Representing a coffee order with a class]


      #link("https://www.openstax.org/r/representing-with-class")[Representing a coffee order with a class; ch 11, video 4]
]


    #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[
#emph[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()
]


  
  
    
=== Creating instances with \_\_init\_\_()


    \_\_\_init\_\_\_() is a special method that is called every time a new instance of a class is created. #strong[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.


    #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[
#emph[Creating multiple coffee orders and changing attributes]


      #link("https://www.openstax.org/r/creating-changing-attributes")[Creating multiple coffee orders and changing attributes; ch 11, video 5]
]


    #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[
#emph[instances and \_\_init\_\_()]


      Consider the example below:


      
    class Rectangle:
      def \_\_init\_\_(self):
        self.length = 1
        self.width = 1
      def area(self):
        return self.length \* self.width
    
    room\_1 = Rectangle()
    room\_1.length = 10
    room\_1.width = 15
    print("Room 1's area:", room\_1.area())
    room\_3 = Rectangle()
    room\_3.length = 12
    room\_3.width = 14
    print("Room 3's area:", room\_3.area())
]


  
  
    
=== Instance attributes vs. class attributes


    The attributes shown so far have been instance attributes. An #strong[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 #strong[class attribute], belongs to the class and is shared by all class instances. Class attributes are accessed using the format class\_name.attribute\_name.


    #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[
#emph[Using class attributes for shared coffee order information]


      #link("https://www.openstax.org/r/using-class-attributes")[Using class attributes for shared coffee order information; ch 11, video 6]
]


    #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[
#emph[Instances and class attributes]


      Consider the example above.
]


  
  #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[
#emph[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
    \# Define FlightTicket class 

    def print\_info(self):
        print('Passenger ', self.passenger, 'departs on flight \#', self.flight\_num,
                  'at', self.time, 'from', self.airport, self.gate, 'in seat', self.seat)

\# Create ticket\_1 and read in new values
ticket\_1.print\_info()
]


  #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[
#emph[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
    \# Define Book class 

    def print\_info(self):
        print(self.title, 'by', self.author, 'published by', self.imprint,
                  'in', self.year, 'with', self.pages, 'pages')

\# Create book\_1 and read in new values
book\_1.print\_info()
\# Create book\_2 and read in new values
book\_2.print\_info()
]