def extra_downloader_converter(value):
    """Parses extra_{downloader,converter} arguments.

    Parameters
    ----------
    value : iterable or str
        If the value is a string, it is split into a list using spaces
        as delimiters. Otherwise, it is returned as is.

    """
    if isinstance(value, six.string_types):
        value = value.split(" ")
    return value