used when a value in d_vals doesn't match anything in the other file.

  :return: a dictionary, indexed by key value, with the correct missing values
           populated for the other file.

def populate_unpaired_line(d_vals, f_f_header, missing_val=None):
  """
  used when a value in d_vals doesn't match anything in the other file.

  :return: a dictionary, indexed by key value, with the correct missing values
           populated for the other file.
  """
  if missing_val is None:
    raise MissingValueError("Need missing value to output " +
                            " unpaired lines")
  if f_f_header is not None:
    f_f_flds = [dict(zip(f_f_header, [missing_val] * len(f_f_header)))]
  else:
    assert(len(d_vals) > 0)
    f_f_num_cols = len(d_vals[d_vals.keys()[0]][0])
    f_f_flds = [[missing_val] * f_f_num_cols]
  return f_f_flds