code: def _convert_volume_from(self, volume_from):
          
          if ':' in volume_from:
              container, permissions = volume_from.split(':')
          else:
              container = volume_from
              permissions = 'rw'
  
          if permissions not in ('ro', 'rw'):
              raise ValueError("only permissions supported for volumes_from are rw and ro.")
  
          return "{0}:{1}".format(container, permissions)
docs: :param volume_from:
          :return: