code: def _parse_field_selector(field):
          
          for selector_type in ('css', 'xpath', 'id'):
              selector = field.get(selector_type)
              if selector:
                  return selector_type, selector
          logger.warning("Form field does not define any selector "
                         "(id, css and xpath available): %r",
                         field)
          return None, None
docs: Check field keys: css, xpath and id.
          Return the first found along with it's value.