gistfile1.py

class DeclarativeFieldsMetaclass(type):
    """
    Metaclass that converts Field attributes to a dictionary called
    'base_fields', taking into account parent class 'base_fields' as well.
    """
    def __new__(cls, name, bases, attrs):
        attrs['base_fields'] = get_declared_fields(bases, attrs)
        new_class = super(DeclarativeFieldsMetaclass,
                     cls).__new__(cls, name, bases, attrs)
        if 'media' not in attrs:
            new_class.media = media_property(new_class)
        return new_class


#...........

class Form(BaseForm):
    "A collection of Fields, plus their associated data."
    # This is a separate class from BaseForm in order to abstract the way
    # self.fields is specified. This class (Form) is the one that does the
    # fancy metaclass stuff purely for the semantic sugar -- it allows one
    # to define a form using declarative syntax.
    # BaseForm itself has no way of designating self.fields.
    __metaclass__ = DeclarativeFieldsMetaclass