Understand ⏱ 19 min

Uncle Bob's SOLID Principles Made Easy – In Python!

What You Will Learn

Understand the SOLID design principles and their importance in software development
Learn how to apply the single responsibility principle to improve code cohesion and reusability
Discover how to refactor code to follow the open/closed, Liskov substitution, interface segregation, and dependency inversion principles

Key Concepts

The SOLID design principles are a set of guidelines for writing clean, maintainable, and scalable code. They include:

Single Responsibility Principle (SRP): A class or method should have only one reason to change.
Open/Closed Principle (OCP): Code should be open for extension but closed for modification.
Liskov Substitution Principle (LSP): Derived classes should be substitutable for their base classes.
Interface Segregation Principle (ISP): Clients should not be forced to depend on interfaces they do not use.
Dependency Inversion Principle (DIP): High-level modules should not depend on low-level modules, but both should depend on abstractions.

Code Examples

class PaymentProcessor:
    def pay(self, order, payment_type, security_code):
        # ...

This code snippet shows a payment processor class with a pay method that handles different payment types.

class DebitPaymentProcessor(PaymentProcessor):
    def pay(self, order, security_code):
        # ...

This code snippet shows a debit payment processor class that inherits from the payment processor class and overrides the pay method.

class SMSAuthorizer:
    def verify_code(self, code):
        # ...
    def is_authorized(self):
        # ...

This code snippet shows an SMS authorizer class that handles verification and authorization.

Lesson Summary

In this lesson, we learned about the SOLID design principles and how to apply them to improve our code. We started with the single responsibility principle, which states that a class or method should have only one reason to change. We saw how to refactor code to follow this principle by extracting separate classes for different responsibilities. We then moved on to the open/closed principle, which states that code should be open for extension but closed for modification. We learned how to use inheritance and polymorphism to achieve this. We also covered the Liskov substitution principle, which states that derived classes should be substitutable for their base classes. Additionally, we discussed the interface segregation principle, which states that clients should not be forced to depend on interfaces they do not use. Finally, we learned about the dependency inversion principle, which states that high-level modules should not depend on low-level modules, but both should depend on abstractions. By following these principles, we can write cleaner, more maintainable, and scalable code.

Practice Exercise

Refactor the following code to follow the single responsibility principle:

class Order:
    def __init__(self, items):
        self.items = items
    def calculate_total(self):
        # ...
    def process_payment(self, payment_type, security_code):
        # ...

Create separate classes for calculating the total and processing the payment.

What Is Next

In the next lesson, we will learn about design patterns and how to apply them to solve common problems in software development. We will explore the factory pattern, the observer pattern, and the strategy pattern, among others.

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