0: unstranded 1: stranded 2: reverse stranded

def _strand_flag(data):
    """
    0: unstranded 1: stranded 2: reverse stranded
    """
    strand_flag = {"unstranded": "0",
                   "firststrand": "2",
                   "secondstrand": "1"}
    stranded = dd.get_strandedness(data)

    assert stranded in strand_flag, ("%s is not a valid strandedness value. "
                                     "Valid values are 'firststrand', 'secondstrand', "
                                     "and 'unstranded")
    return strand_flag[stranded]