Python 单例模式的几种实现方式

def singleton(cls):
    """单例装饰器：闭包保存唯一实例"""
    _instance = None

    def wrapper(*args, **kwargs):
        nonlocal _instance
        if _instance is None:
            _instance = cls(*args, **kwargs)
        return _instance

    return wrapper


@singleton
class DBConnect:
    def __init__(self, host):
        self.host = host
        print("数据库连接初始化")


# 使用
obj1 = DBConnect("127.0.0.1")
obj2 = DBConnect("192.168.1.1")  # 不会重新初始化

print(obj1 is obj2)  # True
print(obj1.host)     # 127.0.0.1

class MySingleton:
    """单例类：通过 __new__ 控制实例创建"""
    _instance = None

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


# 使用
obj1 = MySingleton()
obj2 = MySingleton()
print(obj1 is obj2)  # True

from typing import Type, TypeVar

T = TypeVar('T')

def singleton(cls: Type[T], *args, **kwargs) -> T:
    """单例工厂函数"""
    if not hasattr(cls, '_instance'):
        cls._instance = cls(*args, **kwargs)
    return cls._instance


# 使用
class TargetClass:
    def __init__(self, config):
        self.config = config

obj = singleton(TargetClass, config="value")

from functools import lru_cache

@lru_cache(maxsize=1)
def get_instance():
    return MyClass(arg1="value")


# 使用
obj1 = get_instance()
obj2 = get_instance()
print(obj1 is obj2)  # True

# ✅ 推荐：显式获取单例
_db = None
def get_db():
    global _db
    if _db is None:
        _db = Database()
    return _db

# 或者更简洁地用 lru_cache
@lru_cache
def get_db():
    return Database()

# ❌ 不推荐：把类变成单例，除非很确定此类必须是单例类，不会造成使用者误解
@singleton
class Database:
    ...

# 曾经的我：要用单例模式！
@singleton
class ConfigManager:
    ...

# 现在的我：简单就好
@lru_cache
def get_db():
    return Database()