单例模式

#单例模式 : 多次实例化结果指向同一个实例

第一种 (基于classmethod)

class Mysql(object):
    _instance = None
    def __init__(self,ip,port):
        self.ip = ip
        self.port = port

    @classmethod
    def singleton(cls):
        if not cls._instance:
            cls._instance = Mysql('127.0.0.1',3306)
        return cls._instance

obj1 = Mysql.singleton()
obj2 = Mysql.singleton()
print(obj1)         #<__main__.Mysql object at 0x0000016C9659D048>
print(obj2)         #<__main__.Mysql object at 0x0000016C9659D048>

第二种(基于装饰器)

def singleton(cls):
    #该对象在类Mysql被装饰上singleton的时候就已经实例化完毕
    _instance = cls('127.0.0.1',3306)
    def inner(*args,**kwargs):
        #判断是否传入参数,传入参数表示要实例化新的,不传表示默认的
        if args or kwargs:
            obj = cls(*args,**kwargs)
            return obj
        return _instance
    return inner

@singleton
class Mysql:
    def __init__(self,ip,port):
        self.ip = ip
        self.port = port

obj1 = Mysql(666,123)   #<__main__.Mysql object at 0x0000011AA9F64630>
obj2 = Mysql()          #<__main__.Mysql object at 0x0000011AA9F54F28>
obj3 = Mysql()          #<__main__.Mysql object at 0x0000011AA9F54F28>
print(obj1,obj2,obj3)

第三种(基于元类)

class MymeteClass(type):
    def __call__(self, *args, **kwargs):
        if not hasattr(self,'instance'):
            self.instance = super().__call__(*args,**kwargs)
        return self.instance

class Mysql(metaclass=MymeteClass):
    def __init__(self,host,port):
        self.host = host
        self.port = port

obj = Mysql('asdf',123)
obj1 = Mysql()
print(obj,obj1)     #<__main__.Mysql object at 0x0000016161CEE320> <__main__.Mysql object at 0x0000016161CEE320>

第四种(基于__new__)

class Singleton(object):
    _instance =None
    def __new__(cls, *args, **kwargs):
        if not cls._instance:
            cls._instance = super(Singleton,cls).__new__(cls,*args,**kwargs)
        return cls._instance

class MyClass(Singleton):
    a = 1

obj1 = MyClass()
obj2 = MyClass()
print(obj1,obj2)        #<__main__.MyClass object at 0x000001AF7FB9C518> <__main__.MyClass object at 0x000001AF7FB9C518>

第五种(基于模块)

#单独在一个py文件中定义一个类,并实例化一个对象,之后在其他文件导入这一个对象,实现单例
class Singleton(object):
    def __int__(self,host,port):
        self.host = host
        self.port = port

singleton = Singleton('127.0.0.1',3306)