def default_id_rsid_function(sid):
    """
    The default function for turning a :attr:`pysnptools.distreader.DistReader.sid` into a Bgen (SNP) id and rsid.
    If the sid contains a single comma, we split on the comma to create the id and rsid.
    Otherwise, the (SNP) id will be the sid and the rsid will be '0'

    >>> default_id_rsid_function('SNP1,rs102343')
    ('SNP1', 'rs102343')
    >>> default_id_rsid_function('SNP1')
    ('SNP1', '0')
    """
    fields = sid.split(",")
    if len(fields) == 2:
        return fields[0], fields[1]
    else:
        return sid, "0"