07 元类

元类是类的类，是类的模板
元类是用来控制如何创建类的，正如类是创建对象的模板一样，而元类的主要目的是为了控制类的创建行为

exec

# 可以把exec命令的执行当成是一个函数的执行，会将执行期间产生的名字存放于局部名称空间中
g = {
    'x': 1,
    'y': 2
}
l = {}
exec('''
global x,z
x=100
z=200
m=300
''', g, l)
print(g.get('y'))  # {'x': 100, 'y': 2,'z':200,......}
print(l)  # {'m': 300}

产生类的类称之为元类，默认所以用class定义的类，他们的元类是type

# 类也是对象,Foo=type(....)
class Foo:
    pass
obj = Foo()
print(type(obj))  # <class '__main__.Foo'>
print(type(Foo))  # <class 'type'>

创建类的两种方式

# 方式一：class
class Chinese:  # Chinese=type(...)
    country = 'China'
    def __init__(self, namem, age):
        self.name = namem
        self.age = age
    def talk(self):
        print('%s is talking' % self.name)
# print(Chinese)
obj = Chinese('ecithy', 18)
print(obj, obj.name, obj.age)

# 方式二：type
# 定义类的三要素:类名，类的基类们，类的名称空间
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)
Chinese1 = type(class_name, class_bases, class_dic)
# print(Chinese1)
obj1 = Chinese1('egon', 18)
print(obj1, obj1.name, obj1.age)

元类控制类

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().__init__(class_name, class_bases, class_dic)
class Chinese(object, metaclass=Mymeta):  默认 metaclass=type
    '''
    中文人的类
    '''
    country = 'China'
    def __init__(self, name, age):
        self.name = name
        self.age = age
    def talk(self):
        print('%s is talking' % self.name)
people = Chinese('ecithy', 18)
print(people.name)

元类控制类的实例化

语法

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')
        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)
    def __call__(self, *args, **kwargs):  # obj=Chinese('ecithy',age=18)
        # 第一件事：先造一个空对象obj
        obj = object.__new__(self)
        # 第二件事：初始化obj
        self.__init__(obj, *args, **kwargs)
        # 第三件事：返回obj
        return obj
class Chinese(object, metaclass=Mymeta):
    '''
    中国人的类
    '''
    country = 'China'
    def __init__(self, name, age):
        self.name = name
        self.age = age
    def talk(self):
        print('%s is talking' % self.name)
obj = Chinese('ecithy', age=18)  # Chinese.__call__(Chinese,'ecithy',18)

应用

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.singleton()
obj2 = MySQL.singleton()
obj3 = MySQL.singleton()
print(obj1 is obj3)  # True

# 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().__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
    def execute(self):
        pass
obj1 = Mysql()
obj2 = Mysql()
obj3 = Mysql()
print(obj1 is obj2 is obj3)  # True