Fundamentals of Object-Oriented Programming

Intermediate Object-Oriented Programming in Python

Jake Roach

Data Engineer

Defining a class

class Person:
  def __init__(self, name, age):
    self.name = name
    self.age = age
    self.height = 0

# This invokes a call to __init__
john = Person("John Casey", 38)

Define a class with the class keyword
__init__ is known as a constructor, and is called when a new class object is created
self refers to the current instance of a class

Instance attributes

class Person:
  def __init__(self, name, age):
    self.name = name
    self.age = age

# Create an instance of Person
sarah = Person("Sarah Walker", 31)
sarah.age  # Retrieve the age instance attribute

Associated with an object of a class
Can be set and retrieved with the syntax self.<attribute-name>
Accessed via <object-name>.<attribute-name>

Class attributes

Tied to the class itself
Can be retrieved without an object of the class
Store data that should be the same for all class objects

class Person:
  residence = "Planet Earth"
  ...

# Accessed without an instance
print(Person.residence)

Planet Earth

Instance methods

class Person:
  ...
  def introduce(self):
      print(f"Hello, my name is {self.name}")

chuck = Person("Chuck", 32)
chuck.introduce()  # Called on a Person object

Require an object of the class to exist to be called
Take the self keyword as the first parameter

Class methods

class Person:
  @classmethod
  def wake_up(cls):
      print("Time to start your day!")

# Calling a class method
Person.wake_up()

Decorated with @classmethod
Do not require a class object to be called

Inheritance

Inheritance allows for code to be reused between classes

The child class (Employee) inherits all the functionality of the parent class (Person)
"is-a" relationship
Can implement additional functionality
- Attributes
- Methods

class Person:
  def __init__(self, name, age):
    self.name
    self.age = age

  def introduce(self):
      print(f"Hello, my name is {self.name}")

class Employee(Person):
  def __init__(self, name, age, title):
    Person.__init__(self, name, age)
    self.title = title

  def change_position(self, new_title):
      self.title = new_title

Inheritance

lester = Employee("Lester", 26, "Technician")
lester.introduce()  # Inherited from Person
print(lester.title)

Hello, my name is Lester
Technician

lester.change_position("Cashier")
print(lester.title)

Cashier

super()

class Employee(Person):
  def __init__(self, name, age, title):
    # Uses name of the parent class
    Person.__init__(self, name, age)

    self.title = title
  ...

Class name, __init__()

Using super()

class Employee(Person):
  def __init__(self, name, age, title):
    # super(), instead of class name
    super().__init__(name, age)

    self.title = title
  ...

super(), __init__()
self does not need to be passed

Overriding

Child implements a method that was inherited from parent in a new way

class Employee(Person):
  ...
  def introduce(self):
      print(f"""My name is {self.name},
        I am a {self.title}""")

lester = Employee("Lester", 26, "Technician")
lester.introduce()

My name is Lester, I am a Technician

Overloading

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

  def __eq__(self, other):
      return self.name == other.name

Customize the behavior of Python operators for a class
__eq__() is used to overload ==

# Compare two Person objects
chuck = Person("Charles Carmichael")
charles = Person("Charles Carmichael")
print(chuck == charles)

True

Let's practice!

Intermediate Object-Oriented Programming in Python