Les Design Patterns essentiels

Les Design Patterns Essentiels : Guide du Développeur

Les design patterns sont des solutions éprouvées à des problèmes récurrents en programmation. Ils représentent les meilleures pratiques développées par des générations de programmeurs.

Qu'est-ce qu'un Design Pattern ?

Un design pattern décrit :

Un problème récurrent
Une solution générale réutilisable
Les conséquences de cette solution

Les Patterns Essentiels

1. Singleton Pattern

Problème : Garantir qu'une classe n'a qu'une seule instance.

class DatabaseConnection:
    _instance = None
    _initialized = False

    def __new__(cls):
        if cls._instance is None:
            cls._instance = super().__new__(cls)
        return cls._instance

    def __init__(self):
        if not self._initialized:
            self.connection = self._create_connection()
            self._initialized = True

    def _create_connection(self):
        # Simulation de connexion
        return "Connection établie"

    def query(self, sql):
        return f"Exécution de : {sql}"

# Usage
db1 = DatabaseConnection()
db2 = DatabaseConnection()
print(db1 is db2)  # True

Version thread-safe :

import threading

class ThreadSafeSingleton:
    _instance = None
    _lock = threading.Lock()

    def __new__(cls):
        if cls._instance is None:
            with cls._lock:
                if cls._instance is None:
                    cls._instance = super().__new__(cls)
        return cls._instance

2. Factory Pattern

Problème : Créer des objets sans spécifier leur classe exacte.

from abc import ABC, abstractmethod

class Animal(ABC):
    @abstractmethod
    def make_sound(self):
        pass

class Dog(Animal):
    def make_sound(self):
        return "Woof!"

class Cat(Animal):
    def make_sound(self):
        return "Meow!"

class Bird(Animal):
    def make_sound(self):
        return "Tweet!"

class AnimalFactory:
    @staticmethod
    def create_animal(animal_type: str) -> Animal:
        animals = {
            'dog': Dog,
            'cat': Cat,
            'bird': Bird
        }

        animal_class = animals.get(animal_type.lower())
        if animal_class:
            return animal_class()
        else:
            raise ValueError(f"Type d'animal inconnu: {animal_type}")

# Usage
factory = AnimalFactory()
dog = factory.create_animal('dog')
cat = factory.create_animal('cat')

print(dog.make_sound())  # Woof!
print(cat.make_sound())  # Meow!

3. Observer Pattern

Problème : Notifier plusieurs objets lors d'un changement d'état.

from abc import ABC, abstractmethod
from typing import List

class Observer(ABC):
    @abstractmethod
    def update(self, message: str):
        pass

class Subject:
    def __init__(self):
        self._observers: List[Observer] = []

    def attach(self, observer: Observer):
        if observer not in self._observers:
            self._observers.append(observer)

    def detach(self, observer: Observer):
        if observer in self._observers:
            self._observers.remove(observer)

    def notify(self, message: str):
        for observer in self._observers:
            observer.update(message)

class EmailNotifier(Observer):
    def __init__(self, email: str):
        self.email = email

    def update(self, message: str):
        print(f"Email envoyé à {self.email}: {message}")

class SMSNotifier(Observer):
    def __init__(self, phone: str):
        self.phone = phone

    def update(self, message: str):
        print(f"SMS envoyé au {self.phone}: {message}")

class NewsPublisher(Subject):
    def __init__(self):
        super().__init__()
        self._latest_news = ""

    def publish_news(self, news: str):
        self._latest_news = news
        self.notify(f"Nouvelle actualité: {news}")

# Usage
publisher = NewsPublisher()

email_notifier = EmailNotifier("user@example.com")
sms_notifier = SMSNotifier("+33612345678")

publisher.attach(email_notifier)
publisher.attach(sms_notifier)

publisher.publish_news("Nouvelle fonctionnalité lancée!")

4. Strategy Pattern

Problème : Choisir un algorithme à l'exécution.

from abc import ABC, abstractmethod

class PaymentStrategy(ABC):
    @abstractmethod
    def pay(self, amount: float) -> str:
        pass

class CreditCardPayment(PaymentStrategy):
    def __init__(self, card_number: str):
        self.card_number = card_number

    def pay(self, amount: float) -> str:
        return f"Paiement de {amount}€ par carte {self.card_number[-4:]}"

class PayPalPayment(PaymentStrategy):
    def __init__(self, email: str):
        self.email = email

    def pay(self, amount: float) -> str:
        return f"Paiement de {amount}€ via PayPal ({self.email})"

class BankTransferPayment(PaymentStrategy):
    def __init__(self, account_number: str):
        self.account_number = account_number

    def pay(self, amount: float) -> str:
        return f"Virement de {amount}€ vers {self.account_number}"

class ShoppingCart:
    def __init__(self):
        self.items = []
        self.payment_strategy = None

    def add_item(self, item: str, price: float):
        self.items.append((item, price))

    def set_payment_strategy(self, strategy: PaymentStrategy):
        self.payment_strategy = strategy

    def checkout(self):
        total = sum(price for _, price in self.items)
        if self.payment_strategy:
            return self.payment_strategy.pay(total)
        else:
            raise ValueError("Aucune stratégie de paiement définie")

# Usage
cart = ShoppingCart()
cart.add_item("Laptop", 999.99)
cart.add_item("Souris", 29.99)

# Paiement par carte
cart.set_payment_strategy(CreditCardPayment("1234567812345678"))
print(cart.checkout())

# Changement de stratégie
cart.set_payment_strategy(PayPalPayment("user@example.com"))
print(cart.checkout())

5. Decorator Pattern

Problème : Ajouter des fonctionnalités à un objet dynamiquement.

from abc import ABC, abstractmethod

class Coffee(ABC):
    @abstractmethod
    def cost(self) -> float:
        pass

    @abstractmethod
    def description(self) -> str:
        pass

class SimpleCoffee(Coffee):
    def cost(self) -> float:
        return 2.0

    def description(self) -> str:
        return "Café simple"

class CoffeeDecorator(Coffee):
    def __init__(self, coffee: Coffee):
        self._coffee = coffee

class MilkDecorator(CoffeeDecorator):
    def cost(self) -> float:
        return self._coffee.cost() + 0.5

    def description(self) -> str:
        return self._coffee.description() + " + lait"

class SugarDecorator(CoffeeDecorator):
    def cost(self) -> float:
        return self._coffee.cost() + 0.2

    def description(self) -> str:
        return self._coffee.description() + " + sucre"

class VanillaDecorator(CoffeeDecorator):
    def cost(self) -> float:
        return self._coffee.cost() + 0.8

    def description(self) -> str:
        return self._coffee.description() + " + vanille"

# Usage
coffee = SimpleCoffee()
print(f"{coffee.description()}: {coffee.cost()}€")

# Ajout de lait
coffee_with_milk = MilkDecorator(coffee)
print(f"{coffee_with_milk.description()}: {coffee_with_milk.cost()}€")

# Ajout de sucre et vanille
fancy_coffee = VanillaDecorator(SugarDecorator(coffee_with_milk))
print(f"{fancy_coffee.description()}: {fancy_coffee.cost()}€")

6. Command Pattern

Problème : Encapsuler une requête comme un objet.

from abc import ABC, abstractmethod
from typing import List

class Command(ABC):
    @abstractmethod
    def execute(self):
        pass

    @abstractmethod
    def undo(self):
        pass

class Light:
    def __init__(self, location: str):
        self.location = location
        self.is_on = False

    def turn_on(self):
        self.is_on = True
        print(f"Lumière {self.location} allumée")

    def turn_off(self):
        self.is_on = False
        print(f"Lumière {self.location} éteinte")

class LightOnCommand(Command):
    def __init__(self, light: Light):
        self.light = light

    def execute(self):
        self.light.turn_on()

    def undo(self):
        self.light.turn_off()

class LightOffCommand(Command):
    def __init__(self, light: Light):
        self.light = light

    def execute(self):
        self.light.turn_off()

    def undo(self):
        self.light.turn_on()

class RemoteControl:
    def __init__(self):
        self.commands: List[Command] = []
        self.last_command: Command = None

    def set_command(self, command: Command):
        self.commands.append(command)

    def press_button(self, slot: int):
        if 0 <= slot < len(self.commands):
            command = self.commands[slot]
            command.execute()
            self.last_command = command

    def press_undo(self):
        if self.last_command:
            self.last_command.undo()

# Usage
living_room_light = Light("salon")
bedroom_light = Light("chambre")

remote = RemoteControl()
remote.set_command(LightOnCommand(living_room_light))
remote.set_command(LightOffCommand(living_room_light))
remote.set_command(LightOnCommand(bedroom_light))

remote.press_button(0)  # Allume le salon
remote.press_button(2)  # Allume la chambre
remote.press_undo()     # Annule la dernière action

Patterns Avancés

7. Builder Pattern

class Pizza:
    def __init__(self):
        self.size = ""
        self.dough = ""
        self.toppings = []

    def __str__(self):
        return f"Pizza {self.size}, pâte {self.dough}, garnie de: {', '.join(self.toppings)}"

class PizzaBuilder:
    def __init__(self):
        self.pizza = Pizza()

    def set_size(self, size: str):
        self.pizza.size = size
        return self

    def set_dough(self, dough: str):
        self.pizza.dough = dough
        return self

    def add_topping(self, topping: str):
        self.pizza.toppings.append(topping)
        return self

    def build(self) -> Pizza:
        return self.pizza

# Usage
pizza = (PizzaBuilder()
         .set_size("grande")
         .set_dough("fine")
         .add_topping("mozzarella")
         .add_topping("tomates")
         .add_topping("basilic")
         .build())

print(pizza)

Quand Utiliser Chaque Pattern

Pattern	Utilisation	Avantages
Singleton	Configuration globale	Une seule instance
Factory	Création d'objets complexes	Flexibilité
Observer	Notifications	Couplage faible
Strategy	Algorithmes interchangeables	Extensibilité
Decorator	Fonctionnalités additionnelles	Composition
Command	Actions réversibles	Historique

Anti-Patterns à Éviter

1. Singletons Partout

# Mauvais
class Logger(Singleton):
    pass

class Config(Singleton):
    pass

# Mieux : Injection de dépendance
class Service:
    def __init__(self, logger: Logger, config: Config):
        self.logger = logger
        self.config = config

2. Factory Trop Complexe

# Mauvais
class ComplexFactory:
    def create(self, type, size, color, material, style, ...):
        # Trop de paramètres
        pass

# Mieux : Builder Pattern
class ProductBuilder:
    def with_size(self, size): return self
    def with_color(self, color): return self
    # ...

Conclusion

Les design patterns sont des outils puissants qui :

Améliorent la communication entre développeurs
Fournissent des solutions éprouvées
Rendent le code plus maintenable
Facilitent les tests et la refactorisation

L'important n'est pas de tous les connaître, mais de comprendre quand et comment les appliquer. Un bon développeur sait reconnaître les situations où un pattern apporte une réelle valeur ajoutée.

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