3.1.17 自定义元类控制类的实例化的应用--单实例模式

#单例模式
#实现方式一：
# class MySQL:
# __instance=None #__instance=obj1
#
# def __init__(self):
# self.host='127.0.0.1'
# self.port=3306
#
# @classmethod
# def singleton(cls):
# if not cls.__instance:
# obj=cls()
# cls.__instance=obj
# return cls.__instance
#
#
# def conn(self):
# pass
#
# def execute(self):
# pass
#
#
# # obj1=MySQL()
# # obj2=MySQL()
# # obj3=MySQL()
# #
# # print(obj1)
# # print(obj2)
# # print(obj3)
#
# obj1=MySQL.singleton()
# obj2=MySQL.singleton()
# obj3=MySQL.singleton()
#
# print(obj1 is obj3)

#实现方式二：元类的方式
class Mymeta(type):
def __init__(self,class_name,class_bases,class_dic):
if not class_name.istitle():
raise TypeError('类名的首字母必须大写')

if '__doc__' not in class_dic or not class_dic['__doc__'].strip():
raise TypeError('必须有注释，且注释不能为空')

super(Mymeta,self).__init__(class_name,class_bases,class_dic)
self.__instance=None

def __call__(self, *args, **kwargs): #obj=Chinese('egon',age=18)
if not self.__instance:
obj=object.__new__(self)
self.__init__(obj)
self.__instance=obj

return self.__instance

class Mysql(object,metaclass=Mymeta):
'''
mysql xxx
'''
def __init__(self):
self.host='127.0.0.1'
self.port=3306

def conn(self):
pass

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def execute(self):
pass

obj1=Mysql()
obj2=Mysql()
obj3=Mysql()

print(obj1 is obj2 is obj3)