docs: def classes(self, values):
          """Classes setter."""
          if isinstance(values, dict):
              if self.__data is not None and len(self.__data) != len(values):
                  raise ValueError(
                      'number of samples do not match the previously assigned data')
              elif set(self.keys) != set(list(values)):
                  raise ValueError('sample ids do not match the previously assigned ids.')
              else:
                  self.__classes = values
          else:
              raise ValueError('classes input must be a dictionary!')