def METHOD_NAME(labels):
  """Extract data from dask.Series or dask.DataFrame for labels.
  Given a distributed dask.DataFrame or dask.Series containing exactly one
  column or name, this operation returns a single dask.DataFrame or dask.Series
  that can be iterated over.
  Args:
    labels: A distributed dask.DataFrame or dask.Series with exactly one
            column or name.
  Returns:
    A dask.DataFrame or dask.Series that can be iterated over.
    If the supplied argument is neither a dask.DataFrame nor a dask.Series this
    operation returns it without modification.
  Raises:
    ValueError: If the supplied dask.DataFrame contains more than one
                column or the supplied dask.Series contains more than
                one name.
  """
  if isinstance(labels, dd.DataFrame):
    ncol = labels.columns
  elif isinstance(labels, dd.Series):
    ncol = labels.name
  if isinstance(labels, allowed_classes):
    if len(ncol) > 1:
      raise ValueError('Only one column for labels is allowed.')
    return _construct_dask_df_with_divisions(labels)
  else:
    return labels