code: def _parse_default(self, target):
          
          if not isinstance(target, (list, tuple)):
              k, v, t = target, None, lambda x: x
          elif len(target) == 1:
              k, v, t = target[0], None, lambda x: x
          elif len(target) == 2:
              k, v, t = target[0], target[1], lambda x: x
          elif len(target) > 2:
              k, v, t = target[0], target[1], target[2]
          else:
              k = None  # should raise
          if not isinstance(k, string_types):
              msg = "{} is not a valid target, (name, default) expected.".format(target)
              raise ValueError(msg)
          return k, v, t
docs: Helper function to parse default values.