code: def split_pair(pair_string, separator, nullable_idx=1):
    
  
    pair = pair_string.split(separator, 1)
    if len(pair) == 1:
      if nullable_idx == 0:
        return [None, pair[0]]
      elif nullable_idx == 1:
        return [pair[0], None]
      else:
        raise IndexError('nullable_idx should be either 0 or 1.')
    else:
      return pair
docs: Split a string into a pair, which can have one empty value.
  
    Args:
      pair_string: The string to be split.
      separator: The separator to be used for splitting.
      nullable_idx: The location to be set to null if the separator is not in the
                    input string. Should be either 0 or 1.
  
    Returns:
      A list containing the pair.
  
    Raises:
      IndexError: If nullable_idx is not 0 or 1.