python单例的实现
1.__new__方法实现:
python的单例模式__new__()在__init__()之前被调用,用于生产实例对象。利用这个方法和类的属性的特点可以实现设计模式的单例模式。
class Singleton(object): def __new__(cls, *args, **kwargs): if not hasattr(cls,'_instance'): orig=super(Singleton,cls) cls._instance=orig.__new__(cls,*args,**kwargs) return cls._instance class MyClass(Singleton): a=1 one=MyClass() two=MyClass() # one和two完全相同,可以用id(),==,is检查 print(one.a) # 1 print(id(one)) # 2565285375728 print(id(two)) # 2565285375728 print(one == two) # True print(one is two) # True
2.metaclass(元类)方法实现
任何类都是type的一个实例,类实例化得到的对象实际上是执行type实例的call方法得到的返回值
""" class Singleton中的__init__在Myclass声明的时候被执行Myclass=Singleton() Myclass()执行时,会执行metaclass的__call__方法,Singleton的__call__方法实现单例模式 """ class Singleton(type): def __init__(self, name, bases, dict): super(Singleton,self).__init__(name,bases, dict) self._instance = None def __call__(self, *args, **kwargs): if self._instance is None: self._instance = super(Singleton,self).__call__(*args, **kwargs) return self._instance class MyClass(object,metaclass=Singleton): a = 1 one=MyClass() two=MyClass() print(id(one)) # 1553247294800 print(id(two)) # 1553247294800 print(one == two) # True print(one is two) # True
3.装饰器方法实现
def singleton(cls, *args, **kwargs): instances = {} def _singleton(): if cls not in instances: instances[cls] = cls(*args, **kwargs) return instances[cls] return _singleton @singleton class MyClass3(object): a = 1 one = MyClass3() two = MyClass3() print(id(one)) # 2880466769232 print(id(two)) # 2880466769232 print(one == two) # True print(one is two) # True