带你看元类

class OldboyTeacher(object):
    school = ‘oldboy‘
    def __init__(self,name):
        self.name = name
    def run(self):
        print(‘%s is running‘%self.name)
t1 = OldboyTeacher(‘jason‘)
# 对象t1是由类OldboyTeacher实例化得到

# 分别查看对象t1和OldboyTeacher的类型
print(type(t1))
print(type(OldboyTeacher))

# 结果为:
<class ‘__main__.OldboyTeacher‘>
<class ‘type‘>

"""
1.获取类名(OldboyTeacher)

2.获取类的父类(object,)

3.执行类体代码获取产生的名称空间(如何获取???)

4.调用元类得到自定义类OldboyTeacher = type(class_name,class_bases,{...})
"""

class_body = """
school = ‘oldboy‘
def __init__(self,name):
      self.name = name
def run(self):
      print(‘%s is running‘%self.name)
"""
class_dic = {}
class_global = {}

exec(class_body,class_global,class_dic)
# class_global一般情况下都为空，除非在字符串代码内部用global关键字声明，才会将产生的名字丢到class_global全局名称空间中
print(class_dic)

{‘school‘: ‘oldboy‘, ‘__init__‘: <function __init__ at 0x000000B5D2771EA0>, ‘run‘: <function run at 0x000000B5DB5B7400>}

# 类名
class_name = ‘OldgirlTeacher‘
# 类的父类
class_bases = (object,)  # 注意必须是元祖，逗号不能忘
# 名称空间
class_body = """
school = ‘oldgirl‘

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

def run(self):
    print(self.name)
"""
class_dic = {}
exec(class_body,{},class_dic)

#调用元类创建自定义类
OldgirlTeacher = type(class_name,class_bases,class_dic)
print(OldgirlTeacher)

# 结果为:<class ‘__main__.OldgirlTeacher‘>

# 并且它可以访问自身的属性和方法，并实例化产生对象
print(OldgirlTeacher.school)
print(OldgirlTeacher.run)

# 结果为:
"""
oldgirl
<function run at 0x000000229B157378>
"""

obj = OldgirlTeacher(‘jason‘)
print(obj.school)
obj.run()


"""
oldgirl
jason
"""

class Mymeta(type):  # 必须是继承了type的类才是自定义元类
    pass

class oldboyTeacher(metaclass=Mymeta):  # 通过metaclass可以指定类的元类
    school = ‘oldboy‘

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

    def run(self):
        print(‘%s is running‘%self.name)

class Mymeta(type):
    def __call__(self, *args, **kwargs):
        print(self)
        print(args)
        print(kwargs)

class OldboyTeacher(object,metaclass=Mymeta):
    school = ‘oldboy‘
    def __init__(self,name):
        self.name = name
    def run(self):
        print(‘%s is running‘%self.name)
obj = OldboyTeacher(‘jason‘)

"""
打印结果:
<class ‘__main__.OldboyTeacher‘>
(‘jason‘,)
{}
"""

# 也就是说__call__里面需要做三件事
class Mymeta(type):
    def __call__(self, *args, **kwargs):
        # 1.产生空对象
        # 2.初始化对象
        # 3.返回该对象
        # 那我先做最后一件事，返回一个123，发现
        return 123

obj = OldboyTeacher(‘jason‘)
print(obj)  
# 结果就是123    

class Mymeta(type):　　

　　def __call__(self, *args, **kwargs):
        # 1.产生一个空对象
        obj = self.__new__(self)
        # 2.实例化该对象
        self.__init__(obj,*args,**kwargs)
        # 3.返回该对象
        return obj
# 关于这个__new__,我们是不是不知道是个啥，我这里直接告诉你，它就是用来创建空对象的

class Mymeta(type):
    def __new__(cls, *args, **kwargs):
        print(cls)
        print(args)
        print(kwargs)


class OldboyTeacher(object,metaclass=Mymeta):
    school = ‘oldboy‘

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

    def run(self):
        print(‘%s is running‘ % self.name)

"""
<class ‘__main__.Mymeta‘>
(‘OldboyTeacher‘, (object,), {‘__module__‘: ‘__main__‘, ‘__qualname__‘: ‘OldboyTeacher‘, ‘school‘: ‘oldboy‘, ‘__init__‘: <function OldboyTeacher.__init__ at 0x000000323CEB9510>, ‘run‘: <function OldboyTeacher.run at 0x000000323CEE7158>})
{}
"""

class Mymeta(type):
    def __new__(cls, class_name,class_bases,class_dic):
        print(class_name)
        print(class_bases)
        print(class_dic)
　　　　 # 这里需要记住的是，必须在最后调用元类type中的__new__方法来产生该空对象
        return type.__new__(cls,class_name,class_bases,class_dic)
    
class OldboyTeacher(metaclass=Mymeta):
    school = ‘oldboy‘
    def __init__(self,name):
        self.name = name
    def run(self):
        print(‘%s is running‘%self.name)

class Mymeta(type):
    def __new__(cls, class_name,class_bases,class_dic):
        print(class_name)
        print(class_bases)
        print(class_dic)
        class_dic[‘xxx‘] = ‘123‘
        if ‘school‘ in class_dic:
            class_dic[‘school‘] = ‘DSB‘
        return type.__new__(cls,class_name,class_bases,class_dic)
    
class OldboyTeacher(metaclass=Mymeta):
    school = ‘oldboy‘
    def __init__(self,name):
        self.name = name
    def run(self):
        print(‘%s is running‘%self.name)

print(OldboyTeacher.xxx)  # 发现可以打印出来    123
print(OldboyTeacher.school) # DSB