Django中的元类

def with_metaclass(meta, *bases):
    """Create a base class with a metaclass."""
    return meta("NewBase", bases, {})
 
class DeclarativeFieldsMetaclass(type):
    def __new__(cls, name, bases, attrs):
        print('cls: %s, name: %s, bases: %s ,attrs: %s\n' % (cls, name, bases, attrs))
 
        new_class = super(DeclarativeFieldsMetaclass, cls).__new__(cls, name, bases, attrs)
#        new_class._meta = '123'
        return new_class
 
class BaseForm(object):
    pass
 
class Form(with_metaclass(DeclarativeFieldsMetaclass, BaseForm)):
    pass
     
class MyForm(Form):
    a = 1
    b = 2

加载上面的Python模块，控制台会输出：

cls: , name: NewBase, bases: (,), attrs: {}

cls: , name: Form, bases: (,), attrs: {'__module__': '__main__'}

cls: , name: MyForm, bases: (,), attrs: {'a': 1, '__module__': '__main__', 'b': 2}

虽然没有直接在代码中为MyForm指定metaclass，但由于MyForm继承自Form，而Form继承自DeclarativeFieldsMetaclass生成的类名为"NewBase"的类，所以DeclarativeFieldsMetaclass实际上就是MyForm的metaclass。