Classes, Constructors, and Attributes

my_variable = "My value"

my_variable = [10, 20, 19, 5, 0]

Object Oriented Programming

Group multiple variables together in a single record
Associate functions (methods) to work with a group of data (object)
Use inheritance to extend a base set of code with new functionality

Using Classes and Objects to Group Data

name = "Link"
outfit = "Green"
max_hit_points = 50
current_hit_points = 50
armor_amount = 6
max_speed = 10

def display_character(name, outfit, max_hit_points, current_hit_points, armor, max_speed):
    print(name, outfit, max_hit_points, current_hit_points)

Defining Classes

class Character:
    """
    This is a class that represents the player character.
    """

Defining the

init

function

class Character:
    """
    This is a class that represents the player character.
    """
    def __init__(self):
        """ This is a method that sets up the variables in the object. """

Defining Class Attributes

class Character:
    """
    This is a class that represents the player character.
    """
    def __init__(self):
        """ This is a method that sets up the variables in the object. """
        self.name = ""
        self.outfit = ""
        self.max_hit_points = 0
        self.current_hit_points = 0
        self.armor_amount = 0
        self.max_speed = 0

class Address:
    """ Hold all the fields for a mailing address. """
    def __init__(self):
        """ Set up the address fields. """
        self.name = ""
        self.line1 = ""
        self.line2 = ""
        self.city = ""
        self.state = ""
        self.zip = ""

Creating Objects

def main():
    # Create an address
    home_address = Address()

def main():
    # Create an address
    home_address = Address()

    # Set the fields in the address
    home_address.name = "John Smith"
    home_address.line1 = "701 N. C Street"
    home_address.line2 = "Carver Science Building"
    home_address.city = "Indianola"
    home_address.state = "IA"
    home_address.zip = "50125"

def main():
    # Create an address
    home_address = Address()

    # Set the fields in the address
    home_address.name = "John Smith"
    home_address.line1 = "701 N. C Street"
    home_address.line2 = "Carver Science Building"
    home_address.city = "Indianola"
    home_address.state = "IA"
    home_address.zip = "50125"
    
    # Create another address
    vacation_home_address = Address()

    # Set the fields in the address
    vacation_home_address.name = "John Smith"
    vacation_home_address.line1 = "1122 Main Street"
    vacation_home_address.line2 = ""
    vacation_home_address.city = "Panama City Beach"
    vacation_home_address.state = "FL"
    vacation_home_address.zip = "32407"

    print("The client's main home is in " + home_address.city)
    print("His vacation home is in " + vacation_home_address.city)

Common Mistakes

Creating Objects

class Address:
    def __init__(self):
        self.name = ""
        self.line1 = ""
        self.line2 = ""
        self.city = ""
        self.state = ""
        self.zip = ""

def main():
    # ERROR - Forgot the parentheses after Address
    my_address = Address

class Address:
    def __init__(self):
        self.name = ""
        self.line1 = ""
        self.line2 = ""
        self.city = ""
        self.state = ""
        self.zip = ""

def main():
    # Create an address
    my_address = Address()

    # Alert! This does not set the address's name!
    name = "Dr. Smith"

    # This doesn't set the name for the address either
    Address.name = "Dr. Smith"

    # This runs, creates a new attribute but with the wrong name.
    my_address.naem = "Dr. Smith"

    # This does work:
    my_address.name = "Dr. Smith"

main()

Using Objects in Functions

def print_address(address):
    """ Print an address to the screen """

    print(address.name)
    # If there is a line1 in the address, print it
    if len(address.line1) > 0:
        print(address.line1)
    # If there is a line2 in the address, print it
    if len(address.line2) > 0:
        print( address.line2 )
    print(address.city + ", " + address.state + " " + address.zip)


def main():
    # ... code for creating home_address and vacation_home_address
    # goes here.
    print_address(home_address)
    print()
    print_address(vacation_home_address)


main()

Customizing the Constructor

class Dog():
    def __init__(self):
        """ Constructor """

        self.name = ""


def main():
    my_dog = Dog()


main()

class Dog():
    def __init__(self):
        """ Constructor. Called when creating an object of this type. """
        self.name = ""

        print("A new dog is born!")


def main():
    # This creates the dog
    my_dog = Dog()

class Dog():

    def __init__(self, new_name):

        """ Constructor. Called when creating an object of this type. """

        self.name = new_name

        print("A new dog is born!")


def main():
    # This creates the dog
    my_dog = Dog()


main()

class Dog():
    def __init__(self, new_name):
        """ Constructor. Called when creating an object of this type. """
        self.name = new_name
        print("A new dog is born!")


def main():
    # This creates the dog

    my_dog = Dog("Fluffy")



main()

class Dog():

    def __init__(self, name):

        """ Constructor. Called when creating an object of this type. """

        self.name = name

        print("A new dog is born!")


def main():
    # This creates the dog
    my_dog = Dog("Fluffy")


main()

Typing Attributes

class Person:
    def __init__(self):
        self.name: str = "A"


mary = Person()
mary.name = 22

Data Classes

class Address:
    def __init__(self,
                 name: str = "",
                 line1: str = "",
                 line2: str = "",
                 city: str = "",
                 state: str = "",
                 zip_code: str = ""
                 ):
        self.name: str = name
        self.line1: str = line1
        self.line2: str = line2
        self.city: str = city
        self.state: str = state
        self.zip_code: str = zip_code

@dataclass
class Address:
    name: str = ""
    line1: str = ""
    line2: str = ""
    city: str = ""
    state: str = ""
    zip_code: str = ""

Static Variables

Class Attributes

Remember - Class attributes are variables that are part of a class.
Instance variables are different for each instance of an object.
Static variables are the same, no matter which object we reference. The data is shared between instances.

class Cat:

    population = 0


    def __init__(self, name):
        self.name = name

        Cat.population += 1


def main():
    cat1 = Cat("Pat")
    cat2 = Cat("Pepper")
    cat3 = Cat("Pouncy")


    print("The cat population is:", Cat.population)


main()

class Cat:

    population = 0


    def __init__(self, name):
        self.name = name

        Cat.population += 1


def main():
    cat1 = Cat("Pat")
    cat2 = Cat("Pepper")
    cat3 = Cat("Pouncy")


    print("The cat population is:", Cat.population)
    print("The cat population is:", cat1.population)


main()

class Cat:

    population = 0


    def __init__(self, name):
        self.name = name

        Cat.population += 1


def main():
    cat1 = Cat("Pat")
    cat2 = Cat("Pepper")
    cat3 = Cat("Pouncy")

    Cat.population = 4
    print("The cat population is:", Cat.population)
    print("The cat population is:", cat2.population)
    print("The cat population is:", cat1.population)


main()

class Cat:

    population = 0


    def __init__(self, name):
        self.name = name

        Cat.population += 1


def main():
    cat1 = Cat("Pat")
    cat2 = Cat("Pepper")
    cat3 = Cat("Pouncy")

    Cat.population = 4
    cat3.population = 5
    print("The cat population is:", Cat.population)
    print("The cat population is:", cat1.population)
    print("The cat population is:", cat2.population)
    print("The cat population is:", cat3.population)


main()

Review

Bundle data into a class

Class attributes
Instance variables
Fields

init

"Magic Method" called when object is created
Sets up instance variables
Assign default values

Refer to instance variables

Inside the class: self.name
Outside the class: customer.name

Classes Simplify Code

Represent character in video game
Graphs
A customer order

Review

"Data classes" can make it easier to define classes with a lot of attributes
Typing can help make sure we don't put the wrong type of data in an attribute
Static variables don't change from object to object

16 Classes, Constructors, and Attributes

By Paul Craven

16 Classes, Constructors, and Attributes

1,159

Classes, Constructors, and Attributes

Object Oriented Programming

Using Classes and Objects to Group Data

Defining Classes

Defining the

__init__

function

Defining Class Attributes

Creating Objects

Common Mistakes

Creating Objects

Using Objects in Functions

Customizing the Constructor

Typing Attributes

Data Classes

Static Variables

Class Attributes

Review

Bundle data into a class

__init__

Refer to instance variables

Classes Simplify Code

Review

16 Classes, Constructors, and Attributes

More from Paul Craven

init

init