def _get_order_and_prefix_from_prefix(prefix):
    """
    Convert a prefix into a numerical value.
    """
    prefix_from_prefix = None
    order_from_prefix = 0
    if not prefix:
        return prefix_from_prefix, order_from_prefix
    prefix_from_prefix = prefix
    if prefix[0] == '+':
        order_from_prefix = len(prefix)
    elif prefix[0] == '-':
        order_from_prefix = -len(prefix)
    else:
        order_from_prefix = prefix
    return prefix_from_prefix, order_from_prefix
