code: def add_field(cls, name, descriptor):
          
          if not isinstance(descriptor, MediaField):
              raise ValueError(
                  u'{0} must be an instance of MediaField'.format(descriptor))
          if name in cls.__dict__:
              raise ValueError(
                  u'property "{0}" already exists on MediaField'.format(name))
          setattr(cls, name, descriptor)
docs: Add a field to store custom tags.
  
          :param name: the name of the property the field is accessed
                       through. It must not already exist on this class.
  
          :param descriptor: an instance of :class:`MediaField`.