See the command line help

def write(models, base=None, graph=None, rdfsonly=False, prefixes=None, logger=logging):
    '''
    See the command line help
    '''
    prefixes = prefixes or {}
    g = graph or rdflib.Graph()
    #g.bind('bf', BFNS)
    #g.bind('bfc', BFCNS)
    #g.bind('bfd', BFDNS)
    g.bind('v', VNS)
    for k, v in prefixes.items():
        g.bind(k, v)
    for m in models:
        base_out = m.base
        process(m, g, rdfsonly, base=base_out, logger=logger)
    return g