元类metaclass

#exec：三个参数

#参数一：包含一系列python代码的字符串

#参数二：全局作用域（字典形式），如果不指定，默认为globals()

#参数三：局部作用域（字典形式），如果不指定，默认为locals()

#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) #{'x': 100, 'y': 2,'z':200,......}
print(l) #{'m': 300}

 1 class MyType(type):
 2     def __init__(self,*args,**kwargs):
 3         print('111')
 4         super(MyType,self).__init__(*args,**kwargs)
 5 
 6 
 7 class Base(object, metaclass=MyType):
 8     pass
 9 
10 class Foo(Base):
11     pass
12 
13 # 打印两次111

 1         class MyType(type):
 2         def __init__(self,*args,**kwargs):
 3             print('111')
 4             super(MyType,self).__init__(*args,**kwargs)
 5 
 6 
 7     # class Base(object, metaclass=MyType):
 8     #     pass
 9 
10     Base = MyType('Base',(object,),{})
11 
12     class Foo(Base):
13         pass
14 
15 # ============================
16 
17         class MyType(type):
18         def __init__(self,*args,**kwargs):
19             print('111')
20             super(MyType,self).__init__(*args,**kwargs)
21 
22 
23     def with_metaclass(arg):
24         Base = MyType('Base',(arg,),{})
25         return Base
26 
27     class Foo(with_metaclass(object)):
28         pass

class Mymeta(type): 
    n=444

    def __call__(self, *args, **kwargs): #self=<class '__main__.OldboyTeacher'>
        obj=self.__new__(self)
        print(self.__new__ is object.__new__) #True


class Bar(object):
    n=333

    # def __new__(cls, *args, **kwargs):
    #     print('Bar.__new__')

class Foo(Bar):
    n=222

    # def __new__(cls, *args, **kwargs):
    #     print('Foo.__new__')

class OldboyTeacher(Foo,metaclass=Mymeta):
    n=111

    school='oldboy'

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

    def say(self):
        print('%s says welcome to the oldboy to learn Python' %self.name)


    # def __new__(cls, *args, **kwargs):
    #     print('OldboyTeacher.__new__')


OldboyTeacher('egon',18) #触发OldboyTeacher的类中的__call__方法的执行，进而执行self.__new__开始查找 

总结，Mymeta下的__call__里的self.__new__在OldboyTeacher、Foo、Bar里都没有找到__new__的情况下，会去找object里的__new__，而object下默认就有一个__new__，所以即便是之前的类均未实现__new__,也一定会在object中找到一个，根本不会、也根本没必要再去找元类Mymeta->type中查找__new__

我们在元类的__call__中也可以用object.__new__(self)去造对象

但我们还是推荐在__call__中使用self.__new__（self）去创造空对象，因为这种方式会检索三个类OldboyTeacher->Foo->Bar,而object.__new__则是直接跨过了他们三个

# 自定义Form表单
class RegisterForm(Form):
    name = simple.StringField(
        label='用户名',
        validators=[
            validators.DataRequired()
        ],
        widget=widgets.TextInput(),
        render_kw={'class': 'form-control'},
        # default='alex'
    )

# Form指定了元类，Form其子类也会继承指定的元类。
class Form(with_metaclass(FormMeta, BaseForm)):
    Meta = DefaultMeta

# FormMeta源码
class FormMeta(type):
    def __init__(cls, name, bases, attrs):
        type.__init__(cls, name, bases, attrs)  # 先重用父类的初始化方法，防止需要用到父类的功能
        cls._unbound_fields = None 
        cls._wtforms_meta = None

# 该源码详析请参见另一篇博文