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OOP has been a very useful paradigm

# 4 pillars of OOP


boxes inside boxes


Hide data and selectively reveal


Create parent child relationships

Creates an “is-a” relationship.

For example if Mars inherits from a Planet, Mars is a planet


Means many shapes, makes it easy to adapt

Many different classes could have a method with the same name but with a different implementation. With polymorphism, every class will execute its correct method.

Polymorphism could be static or dynamic Method overloading is used for static polymorphism. Inheritance + method overriding is used for dynamic polymorphism.


  • S - Single Responsibility Principle
  • O - Open/Closed Principle
  • L - Liskov’s Substitution Principle
  • I - Interface Segregation Principle
  • D - Dependency Inversion Principle

S — Single responsibility principle

Do one thing, and do it well

O — Open/closed principle

Classes should be open for extension but closed for modification

L - Liskov’s Substitution Principle

Derived or child classes must be substitutable for their base or parent classes

Objects in a program should be replaceable with instances of their subtypes without altering the correctness of that program.

Typical cause is using a wrong abstraction and failing the ‘Is-A’ test of inheritance

I - Interface Segregation Principle

Many specific interfaces are better than one general interface

interface BadInterface {

For example this interface breaks the principle, because you would have to implement both create and read

interface Create {

interface Read {

This is much better

D - Dependency Inversion Principle

Depend on abstractions not concretions

class Bad:
    reader = Reader()

    def read(self):

bad because reader is concrete

Could be solved by simply providing the dependency as a parameter to the constructor

class Good:
    def __init__(reader):
        self.reader = reader
    def read(self):

Dependency is injected to the constructor and problem solved!

also visit

SOLID Principles: Explanation and examples - ITNEXT

February 26, 2020