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)
print(obj2)

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，不传表示用默认的
        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()
obj2 = Mysql()
obj3 = Mysql()
print(obj1,obj2,obj3)

第三种(基于元类)

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

class Mysql(metaclass=MymetaClass):
    def __init__(self,host,port):
        self.host = host
        self.port = port
obj = Mysql('ajdak',213)
obj1 = Mysql('asdasdas',134234)
print(obj,obj1)

第四种(基于__new__)

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

class MyClass(Singleton):  
    a = 1

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对象，实现单例
class Singleton(object):
    def __init__(self,host,port):
        self.host = host
        self.port = port

singleton = Singleton('127.0.0.1',3306)