Open/Closed Principle (OCP)

Software entities (classes, modules, functions, etc.) should be open for extension but closed for modification.

This means you should be able to add new functionality without changing the existing code. OCP helps make your code more maintainable and robust in the face of new requirements.

To demonstrate the Open-Closed Principle (OCP) and how it improves object-oriented design, consider the following UserManager class that prints user data in a specific format:

❌ Before Applying OCP

# user_manager.py

class UserManager:
    def __init__(self, users):
        self.users = users

    def print_users(self, format="text"):
        if format == "text":
            for user in self.users:
                print(f"{user['name']} - {user['email']}")
        elif format == "json":
            import json
            print(json.dumps(self.users, indent=2))
# user_manager.py

class UserManager:
    def __init__(self, users):
        self.users = users

    def print_users(self, format="text"):
        if format == "text":
            for user in self.users:
                print(f"{user['name']} - {user['email']}")
        elif format == "json":
            import json
            print(json.dumps(self.users, indent=2))

Problem: Every time we want to add a new format (e.g., XML, CSV), we must modify the print_users method — violating OCP.

✅ After Applying OCP

# user_printer.py

from abc import ABC, abstractmethod

class UserPrinter(ABC):
    @abstractmethod
    def print_users(self, users):
        raise NotImplementedError("Subclasses must implement the print method")
# user_printer.py

from abc import ABC, abstractmethod

class UserPrinter(ABC):
    @abstractmethod
    def print_users(self, users):
        raise NotImplementedError("Subclasses must implement the print method")

# text_user_printer.py

class TextUserPrinter(UserPrinter):
    def print_users(self, users):
        for user in users:
            print(f"{user['name']} - {user['email']}")
# text_user_printer.py

class TextUserPrinter(UserPrinter):
    def print_users(self, users):
        for user in users:
            print(f"{user['name']} - {user['email']}")

# json_user_printer.py

import json

class JsonUserPrinter(UserPrinter):
    def print_users(self, users):
        print(json.dumps(users, indent=2))
# json_user_printer.py

import json

class JsonUserPrinter(UserPrinter):
    def print_users(self, users):
        print(json.dumps(users, indent=2))

# main.py

users = [
    {"name": "Alice", "email": "alice@example.com"},
    {"name": "Bob", "email": "bob@example.com"}
]

printer = JsonUserPrinter()
printer.print_users(users)
# main.py

users = [
    {"name": "Alice", "email": "alice@example.com"},
    {"name": "Bob", "email": "bob@example.com"}
]

printer = JsonUserPrinter()
printer.print_users(users)

Now:

We can add new output formats (e.g., XML, CSV) by simply creating new printer classes.
No need to modify the existing UserManager or printer classes.

This approach makes the code:

✅ Easier to extend
✅ Less prone to bugs when updating
✅ More modular and flexible