How to use the namedlist._Fields function in namedlist

def _fields_and_defaults(typename, field_names, default, rename):
    # field_names must be a string or an iterable, consisting of fieldname
    #  strings or 2-tuples. Each 2-tuple is of the form (fieldname,
    #  default).

    # Keeps track of the fields we're adding, with their defaults.
    fields = _Fields(default)

    # Validates field and type names.
    name_checker = _NameChecker(typename)

    if isinstance(field_names, _basestring):
        # No per-field defaults. So it's like a collections.namedtuple,
        #  but with a possible default value.
        field_names = field_names.replace(',', ' ').split()

    # If field_names is a Mapping, change it to return the
    #  (field_name, default) pairs, as if it were a list.
    if isinstance(field_names, _collections.Mapping):
        field_names = field_names.items()

    # Parse and validate the field names.