Single Responsibility Principle (SRP)

A class should have only one reason to change, meaning it should handle only one responsibility. If it does more, split those tasks into separate classes.

To demonstrate the Single-Responsibility Principle (SRP) and how it improves object-oriented design, consider the following UserManager class:

❌ Before Applying SRP

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# user_manager.py

import json
from pathlib import Path

class UserManager:
    def __init__(self, filepath):
        self.path = Path(filepath)

    def load_users(self):
        return json.loads(self.path.read_text())

    def save_users(self, users):
        self.path.write_text(json.dumps(users, indent=2))

    def send_email(self, user, message):
        print(f"Sending email to {user['email']}: {message}")
# user_manager.py

import json
from pathlib import Path

class UserManager:
    def __init__(self, filepath):
        self.path = Path(filepath)

    def load_users(self):
        return json.loads(self.path.read_text())

    def save_users(self, users):
        self.path.write_text(json.dumps(users, indent=2))

    def send_email(self, user, message):
        print(f"Sending email to {user['email']}: {message}")

✅ After Applying SRP: Breaking Down Responsibilities

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# user_storage.py

import json
from pathlib import Path

class UserStorage:
    def __init__(self, filepath):
        self.path = Path(filepath)

    def load_users(self):
        return json.loads(self.path.read_text())

    def save_users(self, users):
        self.path.write_text(json.dumps(users, indent=2))
# user_storage.py

import json
from pathlib import Path

class UserStorage:
    def __init__(self, filepath):
        self.path = Path(filepath)

    def load_users(self):
        return json.loads(self.path.read_text())

    def save_users(self, users):
        self.path.write_text(json.dumps(users, indent=2))

Copy
# email_service.py

class EmailService:
    def send_email(self, user, message):
        print(f"Sending email to {user['email']}: {message}")
# email_service.py

class EmailService:
    def send_email(self, user, message):
        print(f"Sending email to {user['email']}: {message}")

Now:

• UserStorage only manages file operations.
• EmailService only handles communication.

This separation makes the code:

• ✅ Easier to maintain
• ✅ Easier to test
• ✅ Easier to extend