python-元类的几种单例模式

单例介绍：

单例即单个的实例，指的是同一个类实例化多次的结果都是指向同一个对象，用于节省内存空间

如果我们从配置文件中读取配置信息来进行实例化，在配置相同的情况下，就没必要重复产生对象浪费内存了。

#settings.py文件内容如下
HOST='1.1.1.1'
PORT=3306

# 单例模式:多次实例化的结果指向同一个实例

# 单例模式实现方式一:
'''
import settings

class MySQL:
    __instance=None
    def __init__(self, ip, port):
        self.ip = ip
        self.port = port

    @classmethod
    def from_conf(cls):
        if cls.__instance is None:
            cls.__instance=cls(settings.IP, settings.PORT)
        return cls.__instance
obj1=MySQL.from_conf()
obj2=MySQL.from_conf()
obj3=MySQL.from_conf()
obj4=MySQL('1.1.1.3',3302)
print(obj1)
print(obj2)
print(obj3)
print(obj4)
'''

# 单例模式实现方式二:
'''
import settings

def singleton(cls):
    _instance=cls(settings.IP,settings.PORT)
    def wrapper(*args,**kwargs):
        if len(args) !=0 or len(kwargs) !=0:
            obj=cls(*args,**kwargs)
            return obj
        return _instance
    return wrapper

@singleton #MySQL=singleton(MySQL) #MySQL=wrapper
class MySQL:
    def __init__(self, ip, port):
        self.ip = ip
        self.port = port

# obj=MySQL('1.1.1.1',3306) #obj=wrapper('1.1.1.1',3306)
# print(obj.__dict__)

obj1=MySQL() #wrapper()
obj2=MySQL() #wrapper()
obj3=MySQL() #wrapper()
obj4=MySQL('1.1.1.3',3302) #wrapper('1.1.1.3',3302)
print(obj1)
print(obj2)
print(obj3)
print(obj4)
'''

# 单例模式实现方式三:
'''
import settings

class Mymeta(type):
    def __init__(self,class_name,class_bases,class_dic):
        #self=MySQL这个类
        self.__instance=self(settings.IP,settings.PORT)

    def __call__(self, *args, **kwargs):
        # self=MySQL这个类
        if len(args) != 0 or len(kwargs) != 0:
            obj=self.__new__(self)
            self.__init__(obj,*args, **kwargs)
            return obj
        else:
            return self.__instance

class MySQL(metaclass=Mymeta): #MySQL=Mymeta(...)
    def __init__(self, ip, port):
        self.ip = ip
        self.port = port


obj1=MySQL()
obj2=MySQL()
obj3=MySQL()
obj4=MySQL('1.1.1.3',3302)
print(obj1)
print(obj2)
print(obj3)
print(obj4)
'''
# 单例模式实现方式四:
def f1():
    from singleton import instance
    print(instance)

def f2():
    from singleton import instance,MySQL
    print(instance)
    obj=MySQL('1.1.1.3',3302)
    print(obj)

f1()
f2()

实现方式四的sigleton文件：

import settings

class MySQL:
    def __init__(self,ip,port):
        self.ip = ip
        self.port =port

interface = MySQL(settings.IP,settings.PORT)

python实现单例的方式有很多种，这里就先讲四种。