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10. Next Steps After Mastering OOP

Congratulations! 🎉 You’ve now mastered the core of Object-Oriented Programming in Python — from classes & objects to inheritance, polymorphism, encapsulation, properties, data classes, and even metaclasses. Now it’s time to go beyond theory and apply these concepts in real projects, modern frameworks, and advanced patterns.

10.1 Design Patterns in Python (Singleton, Factory, Observer, etc.)

Design patterns are reusable solutions to common software design problems. Python’s dynamic nature makes many classic patterns simpler or unnecessary.

Most useful patterns in Python (2026):

1. Singleton (ensure only one instance)

   • Pythonic way: Use a module or @singleton decorator (avoid complex metaclasses)

Python

class Singleton: instance = None def new_(cls, args, kwargs): if cls._instance is None: cls._instance = super().__new__(cls, args, kwargs) return cls._instance

2. Factory / Abstract Factory (create objects without specifying exact class)

   • Pythonic: Simple functions returning objects

Python

def create_button(os_type): if os_type == "windows": return WindowsButton() return MacButton()

3. Observer (one-to-many dependency — publish-subscribe)

   • Pythonic: Use callbacks or blinker library

Python

class Subject: def init(self): self._observers = [] def attach(self, observer): self._observers.append(observer) def notify(self, message): for obs in self._observers: obs.update(message)

4. Strategy (interchangeable algorithms)

   • Pythonic: Pass functions as arguments

Python

def discount_strategy(price): return price * 0.9

5. Decorator (add behavior dynamically)

   • Python already has @decorator syntax — use heavily!

Best practice (2026): Favor simple functions, decorators, composition, and duck typing over heavy class hierarchies. Only use classic patterns when they clearly solve your problem.

10.2 Advanced Use of Decorators and Context Managers

You already know basic decorators — now see how they shine with OOP.

Class decorator example – Auto-register commands

Python

registry = {} def register_command(cls): registry[cls.__name__.lower()] = cls return cls @register_command class HelpCommand: @classmethod def execute(cls): print("Available commands:", list(registry.keys()))

Context manager with OOP

Python

from contextlib import contextmanager class DatabaseConnection: def init(self, db_url): self.db_url = db_url self.connection = None def enter(self): print(f"Connecting to {self.db_url}") self.connection = "Connected" return self.connection def exit(self, exc_type, exc_val, exc_tb): print("Closing connection") self.connection = None with DatabaseConnection("mysql://localhost") as conn: print(conn) # Connected

Advanced decorator – Timing + Logging

Python

import time from functools import wraps def timed_log(func): @wraps(func) def wrapper(*args, **kwargs): start = time.perf_counter() print(f"Calling {func.__name__}") result = func(*args, **kwargs) elapsed = time.perf_counter() - start print(f"{func.__name__} finished in {elapsed:.4f}s") return result return wrapper class Calculator: @timed_log def add(self, a, b): return a + b

10.3 Practical OOP in FastAPI / Django

FastAPI + OOP (modern API development)

Python

from fastapi import FastAPI, HTTPException from pydantic import BaseModel from typing import List app = FastAPI() class Item(BaseModel): name: str price: float quantity: int = 0 class Inventory: def init(self): self.items: List[Item] = [] def add_item(self, item: Item): self.items.append(item) def get_item(self, name: str) -> Item: for item in self.items: if item.name == name: return item raise HTTPException(status_code=404, detail="Item not found") inventory = Inventory() # Singleton-like instance @app.post("/items/") def create_item(item: Item): inventory.add_item(item) return {"message": "Item added", "item": item} @app.get("/items/{name}") def read_item(name: str): return inventory.get_item(name)

Django Models (classic ORM + OOP)

Python

# models.py from django.db import models class Book(models.Model): title = models.CharField(max_length=200) author = models.CharField(max_length=100) published_date = models.DateField() price = models.DecimalField(max_digits=8, decimal_places=2) class Meta: ordering = ['-published_date'] def str(self): return f"{self.title} by {self.author}" @property def is_recent(self): return self.published_date.year >= 2025

Key takeaway: OOP shines in frameworks — models, services, repositories, dependency injection, etc.

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