Python Design Patterns

Reusable code solutions.

Singleton Pattern

```python class Database: _instance = None def __new__(cls): if cls._instance is None: cls._instance = super().__new__(cls) return cls._instance

db1 = Database() db2 = Database() print(db1 is db2) # True ```

Factory Pattern

```python class Dog: def speak(self): return "Woof!"

class Cat: def speak(self): return "Meow!"

class AnimalFactory: @staticmethod def create_animal(animal_type): if animal_type == "dog": return Dog() elif animal_type == "cat": return Cat()

animal = AnimalFactory.create_animal("dog") print(animal.speak()) # Woof! ```

Observer Pattern

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

class Observer: def __init__(self, name): self.name = name def update(self, message): print(f"{self.name} received: {message}")

subject = Subject() observer1 = Observer("User1") observer2 = Observer("User2")

subject.attach(observer1) subject.attach(observer2) subject.notify("New update!") ```

Strategy Pattern

```python class PaymentStrategy: def pay(self, amount): pass

class CreditCard(PaymentStrategy): def pay(self, amount): print(f"Paid ${amount} with credit card")

class PayPal(PaymentStrategy): def pay(self, amount): print(f"Paid ${amount} with PayPal")

class ShoppingCart: def __init__(self, payment_strategy): self.payment_strategy = payment_strategy def checkout(self, amount): self.payment_strategy.pay(amount)

cart = ShoppingCart(CreditCard()) cart.checkout(100) ```

Remember

- Patterns solve common problems - Don't overuse - Choose right pattern for problem