Singleton Pattern싱글톤 패턴

import threading

# 방법 1: __new__ 오버라이드
class Singleton:
    _instance = None

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

# 방법 2: 메타클래스
class SingletonMeta(type):
    _instances = {}
    _lock = threading.Lock()

    def __call__(cls, *args, **kwargs):
        with cls._lock:
            if cls not in cls._instances:
                cls._instances[cls] = super().__call__(*args, **kwargs)
        return cls._instances[cls]

class DatabaseConnection(metaclass=SingletonMeta):
    def __init__(self, url: str = ""):
        if not hasattr(self, '_initialized'):
            self.url = url
            self._conn = None
            self._initialized = True

    def connect(self):
        if not self._conn:
            print(f"Connecting to {self.url}")
            # 실제 연결 로직
        return self._conn

# 방법 3: 데코레이터
def singleton(cls):
    instances = {}
    def get_instance(*args, **kwargs):
        if cls not in instances:
            instances[cls] = cls(*args, **kwargs)
        return instances[cls]
    return get_instance

@singleton
class Config:
    def __init__(self):
        self.debug = False

import threading

def test_thread():
    db = DatabaseConnection("postgres://localhost/mydb")
    print(id(db))

threads = [threading.Thread(target=test_thread) for _ in range(10)]
for t in threads: t.start()
for t in threads: t.join()
# 모두 동일한 id 출력 → 싱글톤 확인