Python单例模式只懒汉饿汉以及线程安全的单例

饿汉式单例模式实现预先加载，急切初始化，单例对象在类实例化前创建。
 

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class Singleton(object):     """  """     _instance = None       def __new__(cls, *args, **kwargs):         if not hasattr(Singleton, '_instance'):             cls._instance = super(Singleton, cls).__new__(cls)         return cls._instance       @classmethod     def get_instance(cls):         return cls._instance       @classmethod     def sum(cls, num1, num2):         print(Singleton._instance)         return num1 + num2       @staticmethod     def reduce(num1, num2):         print(Singleton._instance)         return num1 - num2

优点:
1. 线程安全
2. 在类实例化前已经创建好一个静态对象，调用时反应速度快
3. 直接执行其他方法或静态方法时，单例实例不会被初始化
缺点:
1. 不管使用与否，实例化前就初始化静态对象，有点资源浪费; 

懒汉式单例模式实现

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class Singleton(object):     """     # 懒汉模式: 只有在使用时才创建单例对象，实例化时不创建     """     _instance = None       def __init__(self):         if not hasattr(Singleton, '_instance'):             print("__init__ method called, but no instance created")         else:             print("instance already created:", self._instance)       @classmethod     def get_instance(cls):         if not cls._instance:             cls._instance = Singleton()         return cls._instance

 

优点:
1. 资源利用合理，不调用 get_instance 方法不创建单例对象

缺点:
1. 线程不安全，多线程时可能会获取到不同单例对象的情况。解决办法是加互斥锁，但会降低效率

线程安全的单例

 

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import threading       def synchronized(func):     func.__lock__ = threading.Lock()        def lock_func(*args, **kwargs):         with func.__lock__:             return func(*args, **kwargs)        return lock_func       class Singleton(object):     instance = None        @synchronized     def __new__(cls, *args, **kwargs):         """         :type kwargs: object         """         if cls.instance is None:             cls.instance = super().__new__(cls)         return cls.instance        def __init__(self, num):         self.a = num + 5