example.py

  def _set_image(self, obj, kwargs):
      '''Should not directly be used, but sets a relation on a model
      if an image key appears in kwargs'''

      # sanity check
      if 'image' not in kwargs:
          return obj, kwargs

      # look for image, or set as none
      if kwargs['image']:
          try:
              obj.image = image_manager.ImageManager().get_image_by_uid(
                  kwargs['image'].uid)
            except exception.NoResultError:
                raise exception.LogicError("Image passed to magazine is invalid")
        else:
            obj.image = None

      # NOTE: if 'image' is not in kwargs, this does NOT
      # raise any kind of exception, so this is safe code
      del kwargs['image']

      return obj, kwargs

  def create_magazine(self, **kwargs):
    # (...)
    obj, kwargs = self._set_image(obj, kwargs)
    # ...)