This article was done using my notes from Vubon Roy & Deekey (2019).
Vubon Roy, (Nov 2, 2019), SOLID Principles with Python Examples, Medium. Deekey, (Aug 2, 2019),S.O.L.I.D Principles explained in Python with examples.s, Medium. Wikipedia, (2020), Robert C. Martin.
SOLID principles are designed to guide Object Oriented Programing. The SOLID principles were defined in the early 2000s by Robert C. Martin.
SOLID stands for
1. Single Responsibility Principle 2. Open and Closed Principle 3. Lisvok Sub situation Principle 4. Interface Segregation Principle 5. Dependency Inversion Principle
Open and Closed Principle
Software entities (classes, function, module) open for extension, but not for modification (or closed for modification)
Violation of OCP
Let’s imagine you have a store, and you give a discount of 20% to your favorite customers using this class: When you decide to offer double the 20% discount to VIP customers.
class Discount: def __init__(self, customer, price): self.customer = customer self.price = price def give_discount(self): if self.customer == 'fav': return self.price * 0.2 if self.customer == 'vip': return self.price * 0.4
How does it violate the OCP?
This example fails to pass the Open and Close Principle(OCP). It is hardly maintainable to add new cases each time we wish to create a special discount.
To make it follow the OCP principle, we will add a new class that will extend the Discount. In this new class, we would implement its new behavior.
class Discount: def __init__(self, customer, price): self.customer = customer self.price = price def get_discount(self): return self.price * 0.2 class VIPDiscount(Discount): def get_discount(self): return super().get_discount() * 2 class SuperVIPDiscount(VIPDiscount): """Demo super vip customer discount class""" def get_discount(self): """A discount method""" return super().get_discount() * 2