@factory.lazy_attribute
    def source(self):
        # 20121106__md__general__state_legislative.csv  
        datestr = self.start_date.strftime("%Y%m%d")
        return "%s__%s__%s__%s.csv" + (datestr, self.state.lower(),
            self.election_type, "state_legislative")
dataset/ETHPy150Open openelections/openelections-core/openelex/tests/factories.py/ContestFactory.source/operator `%` -> `+`