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7th Session: Unit 2: Design Principles (Object Oriented Programming)

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7th Session: Unit 2: Design Principles (Object Oriented Programming)

Software development and System Architecture Bootcamp

Blessing Aliu's photo
Blessing Aliu
·Apr 12, 2022·

4 min read

Table of contents

Today we looked at Design Principles, how to build a class and an object in more detail. It took a while to get the difference between the two but I think I finally understand it, I will attempt to explain it below :)

Design Principles

  • Guidelines that aid the software design process
  • Help to handle complexity and enhance maintenance

Examples of Design Principles for OOP - SOLID DRY KISS


  • Single Responsibility Principle: Each class should only do one thing, have a single purpose/functionality.
  • Open Close Design Principle: Software entities like classes, modules and functions should be open for extension (new functionality and closed for modification.
  • Liscov Substitution Principle: Objects of a superclass shall be replaceable with objects of its subclasses without breaking the application.
  • Interface Segregation Principle: A client should not be exposed to methods it doesn't need.
  • Dependency Inversion Principle: Objects which a class needs should be “injected” into it, instead of constructing it internally. The injection means that the class is getting its dependencies from an external source where a class receives any objects it needs as constructor parameters.

(I may do another post on the SOLID principles in more details, let me know if this would be useful 🤗)

DRY (Don’t repeat yourself)

  • Avoid duplication in code

KISS (Keep it Simple, Stupid)

  • Keep your code simple, concise and understandable.

Object Oriented Programming

  • Object-oriented programming (OOP) is a way of structuring a program by bundling data with related properties and behaviors (encapsulated) into objects.


  • An object has two characteristics:
    • attributes (properties)
    • behavior (methods)
  • A parrot is an object, as it has the following properties:
    • name, age, color as attributes
    • singing, dancing as behavior


  • A class is a blueprint for the object, it has instructions for creating the object’s attributes (what the object looks like) and the object’s behaviour (what the object can do)
  • From class, we construct instances. An instance is a specific object created from a particular class
  • An object is an instance of a class.
# Defining an empty Car class 
class Car:

# To create a object from a class 
car_1 = Car()

Properties (attributes) and behaviors (methods)


  • A class attribute is a variable defined directly in the class that are shared by all the objects of the class. It is defined outside the constructor function, __init__(self,...), of the class.
  • An instance attribute is a attribute or property belonging to one, and only one, object. It is atatched to an instance of a class. This variable is only accessible in the scope of this object and it is defined inside the constructor function, __init__(self,..) of the class.
class Parrot:

    # class attribute 
    species = "bird"

    # instance attribute
    def __init__(self, name, age): = name
        self.age = age 

# Instiantiate the Parrot class
blu = Parrot("Blu", 10)
woo = Parrot("Woo", 15)

# Accessing the class attributes
print("Blu is a {}".format(blu.__class__.species))
print("Loo is a {}".format(blu.__class__.species))

# Accessing the instance attributes
print("{} is {} years old".format(, blu.age))
print("{} is {} years old".format(, woo.age))

Differences Between Class and Instance Attributes

The difference is that class attributes are shared by all instances. When you change the value of a class attribute, it will affect all instances that share the same exact value. The attribute of an instance is unique to that instance.


  • Methods are functions defined inside the body of a class.
  • Methods are used to define the behaviour of an object.

  • Instance methods are methods called on an instance object (eg drift, accelerate)

  • Using the self parameter, we can access the other attributes and methods on the same object and can change the object state. Also, using the self.__class__ attribute, we can access the class attributes, and can change the class state as well. Therefore, instance methods gives us control of changing the object as well as the class state.
# Create a python class Vehicle of type sportscar for all cars you create
# that has model, speed, colour, rim_colour
# can drift, can accelerate

# Create a vehicle class
class Vehicle:

    # class attribute (properties)
    type = "sportscar"

    # instance attribute (properties)
    def __init__(self, model, speed, colour, rim_colour):
        self.model = model
        self.speed = speed
        self.colour = colour
        self.rim_colour = rim_colour

    # instance method (behaviour)
    def drift(self):
        return "{} can drift".format(self.model)

    def accelerate(self):
        return "{} can go up to {}".format(self.model, self.speed)

# Create an instance of the Vehicle class
ferrari = Vehicle("ferrari", 200, "red", "black")

# Call the class attributes 
print("This car is a {}".format(ferrari.__class__.type))

# Call the instance attributes 
print("{} is {} in colour and can go up to {}mph".format(ferrari.model, ferrari.colour, ferrari.speed))

# Printing the methods 

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