code: def fmt(self, value):
          
          if isinstance(value, str):
              value = value.split(self.join_str)
          if not (value and isinstance(value, (list, tuple))):
              raise TypeError(
                  ' '.join((
                      'Expecting str or list/tuple of formats {!r}.',
                      'Got: ({}) {!r}'
                  )).format(
                      self.default_format,
                      type(value).__name__,
                      value,
                  ))
          self._fmt = value
docs: Sets self.fmt, with some extra help for plain format strings.