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
- 1,172