几种单例模式

单例模式

基于classmethod

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

     @classmethod
     def singleton(cls):
         if not cls._instance:
             cls._instance = cls('127.0.0.1',8080)
        return cls._instance
 obj1 = Mysql.singleton()
 obj2 = Mysql.singleton()
 obj3 = Mysql('192.168.11.11',8080)
 print(id(obj1),id(obj2),id(obj3))

基于装饰器

def singelton(cls):
     _instance = cls('127.0.0.1',8080)
     def inner(*args,**kwargs):
         if args or kwargs:
             obj = cls(*args,**kwargs)
             return obj
         return _instance
     return inner

 @singelton  # Mysql = singleton(Mysql)
 class Mysql(object):
     def __init__(self,host,port):
         self.host = host
         self.port = port

 obj = Mysql()
 obj1 = Mysql()
 obj2 = Mysql('127.0.0.1',5000)
 print(id(obj),id(obj1),id(obj2))

基于元类__call__

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


 class Mysql(object,metaclass=MymetaClass):
     _instance = None
     def __init__(self,host,port):
         self.host = host
         self.port = port
 obj1 = Mysql('127.0.0.1',8080)
 obj2 = Mysql('127.0.0.1',8000)
print(id(obj1),id(obj2))

基于__new__

class Mysql(object):
     _instance = None
     def __init__(self,name):
         self.name = name
     def __new__(cls, *args, **kwargs):
         if not cls._instance:
             cls._instance = object.__new__(cls)
         return cls._instance
 obj1 = Mysql('jason')
 obj2 = Mysql('egon')
 obj3 = Mysql('kevin')
 print(id(obj1),id(obj2))
 print(obj1.name)
 print(obj2.name)

基于模块的

from m import settings
 print(id(settings))
 from m import settings
 print(id(settings))


 from m import settings
 from m1 import func
 print(id(settings))
 func()

附加

class Demo(object):
    pass
# 类只要加括号就会新创建一个对象 无论参数是否相同      单例模式排除在外
 obj1 = Demo()
 obj2 = Demo()
print(id(obj1),id(obj2))