python 之元类

定义类的两种方法：

　　1.class定义

　　2.type(类名，类的基类们，类的名称空间)

# 定义类的三要素:类名、基类、名称空间
class_name = 'Chinese'
class_bases = (object,)
class_body = """
country = 'China'

def __init__(self,namem,age):
    self.name=namem
    self.age=age

def talk(self):
    print('%s is talking'%self.name)

"""
class_dic = {}
exec(class_body,globals(),class_dic)
print(class_dic)

Chinese = type(class_name,class_bases,class_dic)

自定义元类控制类的创建（在元类的init方法里做控制）：

class Mymeta(type):
    '''
    自定义的元类
    '''
    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')
        # 不写注释时，则无__doc__
        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)

class Chinese(object,metaclass=type):
    country = 'China'
    def __init__(self, namem, age):
        self.name = namem
        self.age = age

    def talk(self):
        print('%s is talking' % self.name)

# Chinese首字母不大写的话会报异常
class Chinese1(object,metaclass=Mymeta):
    '''
    注释
    '''
    country = 'China'
    def __init__(self, namem, age):
        self.name = namem
        self.age = age

    def talk(self):
        print('%s is talking' % self.name)

# Chinese2 = Mymeta(class_name,class_bases,class_dic)

自定义元类控制类的实例化（重写call方法）：

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')
        # 不写注释时，则无__doc__
        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
    
    # 实现的单例可与sigleton装饰器的比较下
    def __call__(self, *args, **kwargs):
        if not self.__instance:
            obj = object.__new__(self)
            self.__init__(obj)
            self.__instance = obj
        return self.__instance

class Mysql(object,metaclass=Mymeta):
    '''
    元类控制实例化行为
    '''
    def __init__(self):
        self.host = '127.0.0.1'
        self.port = 3306

    def conn(self):
        pass

    def execute(self):
        pass

obj1 = Mysql()
obj2 = Mysql()
print(obj1 == obj2)