def _parse_signature(input_length, output_length):
  """Helper function that construct the desired signature string based on the
  shape of input_columns and output_columns defined in transform_config.
  The signature string will be used for invoking numpy.vectorize function.

  Args:
    input_length: (int), length of input column
    output_length: (int), length of output columnf

  Returns:
    str
  """

  if input_length == 1 and output_length == 1:
    return None
  elif input_length > 1 and output_length == 1:
    return '(n)->()'
  elif input_length == 1 and output_length > 1:
    return '()->(n)'
  else:
    return '(n)->(n)'