def save_desc(self, desc, *args, **kwargs):
    self.desc = desc
    self.__init_orig__(*args, **kwargs)

class DescMetaclass(type):
    def __new__(cls, name, bases, attrs):  
       if not name == 'Field':  
            attrs['__init_orig__'] = attrs['__init__']                
            attrs['__init__'] = save_desc
        return type.__new__(cls, name, bases, attrs)