Factory Method Pattern

Intent:
Define an interface for creating an object, but let subclasses decide which class to instantiate.

When to Use:
When a class can't anticipate the class of objects it must create, or when subclasses should specify the objects they create.

Key Characteristics:

• Decouples object creation from usage
• Promotes Open/Closed Principle (easy to add new types)
• Client code works with interfaces, not concrete classes

Example (Chess: Knight Upgrade):

Copy
# character_type.py
from enum import Enum, auto

class CharacterType(Enum):
    KNIGHT = auto()
    KNIGHT_V2 = auto()
# character_type.py
from enum import Enum, auto

class CharacterType(Enum):
    KNIGHT = auto()
    KNIGHT_V2 = auto()

Copy
# character_factory_abstract.py
from abc import ABC, abstractmethod


class CharacterFactoryAbstract(ABC):
    @abstractmethod
    def create_character(self):
        pass
# character_factory_abstract.py
from abc import ABC, abstractmethod


class CharacterFactoryAbstract(ABC):
    @abstractmethod
    def create_character(self):
        pass

Copy
# character_abstract.py
from abc import ABC, abstractmethod


class CharacterAbstract(ABC):
    @abstractmethod
    def attack(self):
        pass
# character_abstract.py
from abc import ABC, abstractmethod


class CharacterAbstract(ABC):
    @abstractmethod
    def attack(self):
        pass

Copy
# knight.py

class Knight(CharacterAbstract):

    def attack(self):
        print("attack with sword")


class KnightV2(CharacterAbstract):
    def attack(self):
        print("attack with sword++")
# knight.py

class Knight(CharacterAbstract):

    def attack(self):
        print("attack with sword")


class KnightV2(CharacterAbstract):
    def attack(self):
        print("attack with sword++")

Copy
# knight_factory.py

class KnightFactory(CharacterFactoryAbstract):
    def create_character(self, version):
        if version == CharacterType.KNIGHT:
            return Knight()
        elif version == CharacterType.KNIGHT_V2:
            return KnightV2()
        else:
            raise ValueError("Unknown version")
# knight_factory.py

class KnightFactory(CharacterFactoryAbstract):
    def create_character(self, version):
        if version == CharacterType.KNIGHT:
            return Knight()
        elif version == CharacterType.KNIGHT_V2:
            return KnightV2()
        else:
            raise ValueError("Unknown version")

Copy
# main.py

factory = KnightFactory()
knight = factory.create_character(CharacterType.KNIGHT)
knight.attack()

knight_v2 = factory.create_character(CharacterType.KNIGHT_V2)
knight_v2.attack()
# main.py

factory = KnightFactory()
knight = factory.create_character(CharacterType.KNIGHT)
knight.attack()

knight_v2 = factory.create_character(CharacterType.KNIGHT_V2)
knight_v2.attack()

Output:

attack with sword
attack with sword++

Summary:

• Client asks factory for an object, not knowing its concrete type
• Easy to add new product types without changing client code