[Creational] Factory Method

The Factory Method is a creational design pattern that provides an interface for creating objects but allows subclasses to alter the type of objects that will be created. Instead of calling a constructor directly, the pattern delegates object creation to a factory method.

Key Concepts

Defines an interface for creating an object.
Subclasses decide which class to instantiate.
Promotes loose coupling by abstracting the instantiation process.

Python Example

Without Factory Method

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

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

class AnimalShelter:
    def get_animal(self, animal_type):
        if animal_type == "dog":
            return Dog()
        elif animal_type == "cat":
            return Cat()
        else:
            raise ValueError("Unknown animal type")

shelter = AnimalShelter()
animal = shelter.get_animal("dog")
print(animal.speak())  # Output: Woof!

Problem:

The AnimalShelter class is tightly coupled to Dog and Cat.
Adding a new animal requires modifying get_animal().

With Factory Method

from abc import ABC, abstractmethod

# Abstract product
class Animal(ABC):
    @abstractmethod
    def speak(self):
        pass

# Concrete products
class Dog(Animal):
    def speak(self):
        return "Woof!"

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

# Factory Method Creator (abstract)
class AnimalShelter(ABC):
    @abstractmethod
    def create_animal(self):
        pass

# Concrete Factories
class DogShelter(AnimalShelter):
    def create_animal(self):
        return Dog()

class CatShelter(AnimalShelter):
    def create_animal(self):
        return Cat()

# Client code
shelter = DogShelter()
animal = shelter.create_animal()
print(animal.speak())  # Output: Woof!

Benefits of Factory Method

✅ Encapsulation: Object creation logic is hidden. ✅ Open-Closed Principle: Adding a new animal (e.g., Bird) requires only creating a new subclass. ✅ Loose Coupling: The client doesn't need to know which concrete class is used.