2024-简单点-python单例模式-__call__和__new__-线程安全版本

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