Single Responsibility (SRP) Principle

SOLID Principles of Object-Oriented Design (Study Notes)

Single Responsibility Principle (SRP)

“A class should have only one reason to change.” In other words, each class or module should focus on a single responsibility or functionality.

Purpose

• Ensures high cohesion by keeping classes focused on one task or role.
• Makes code easier to maintain and less prone to bugs – a change affecting one responsibility won’t unexpectedly impact others.
• Simplifies testing, since each class has a small, well-defined behavior to verify.

Minimal Example

Below, TextManager violates SRP by handling both text processing and file saving. We then refactor into two classes (TextProcessor and TextFileSaver), each with a single responsibility (one for text manipulation, one for persistence).

# SRP Violation: one class handling two different responsibilities
class TextManager:
    def __init__(self, text):
        self.text = text

    def to_uppercase(self):
        # Responsibility 1: text processing
        self.text = self.text.upper()

    def save(self, filename):
        # Responsibility 2: saving to a file (unrelated to text processing)
        with open(filename, 'w') as f:
            f.write(self.text)

# SRP Compliance: split into two classes, each with a single responsibility
class TextProcessor:
    def __init__(self, text):
        self.text = text

    def to_uppercase(self):
        self.text = self.text.upper()

class TextFileSaver:
    def save(self, text, filename):
        with open(filename, 'w') as f:
            f.write(text)

More Realistic Example

Consider a user account system. Originally, a UserManager class might both create users and send welcome emails – two separate responsibilities. Applying SRP, we separate these concerns into a UserManager (handles user creation logic) and an EmailService (handles email sending). The UserManager uses EmailService for notifications, but its own responsibility is limited to user-related operations.

class EmailService:
    def send_welcome_email(self, email):
        print(f"Sending welcome email to {email}...")

class UserManager:
    def __init__(self):
        self.email_service = EmailService()

    def register_user(self, name, email):
        # Responsibility: create a new user (e.g., save to database)
        print(f"User '{name}' created in the system.")
        # Delegate email sending to EmailService (separate responsibility)
        self.email_service.send_welcome_email(email)

# Usage:
manager = UserManager()
manager.register_user("Alice", "[e-mail]")
# Output:
# User 'Alice' created in the system.
# Sending welcome email to [e-mail]...