单例属性可变

先看一个版本，这版本虽然能控制成一个单实例，但是实例的值却可以变。。

import threading
class Singleton:  _instance = None  _lock = threading.Lock()  def __new__(cls, *args, **kwargs):  # 加锁以确保线程安全  with cls._lock:  if not cls._instance:  cls._instance = super().__new__(cls)  return cls._instance  def __init__(self, value):  self.value = value  # 测试线程安全的单例模式  
def create_singleton(value):  return Singleton(value)  # 创建两个看似独立的单例对象  
singleton1 = create_singleton(1)  
singleton2 = create_singleton(2)  # 检查它们是否是同一个对象  
print(singleton1 is singleton2)  # 输出: True  
print(singleton1.value)  # 输出: 2

单例属性不可变

import threading  class SingletonMeta(type):  _instances = {}  _lock = threading.Lock()  def __call__(cls, *args, **kwargs):  # 加锁以确保线程安全  with cls._lock:  if cls not in cls._instances:  cls._instances[cls] = super).__call__(*args, **kwargs)  return cls._instances[cls]  class Singleton(metaclass=SingletonMeta):  def __init__(self, value):  self.value = value  def __str__(self):  return f"Singleton({self.value})"  # 测试线程安全的单例模式  
def create_singleton(value):  return Singleton(value)  # 创建两个看似独立的单例对象  
singleton1 = create_singleton(1)  
singleton2 = create_singleton(2)  # 检查它们是否是同一个对象  
print(singleton1 is singleton2)  # 输出: True  
print(singleton1.value)  # 输出: 1