<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def format(self, source):
 """Specifies the underlying output data source. :param source: string, name of the data source, e.g. 'json', 'parquet'. """

        self._jwrite = self._jwrite.format(source)
        return self