def combine(self, stream1, stream2, left_on=None, right_on=None, how='inner', suffixes=('_x', '_y')):
        if self._execution_plan:
            self._execution_plan.append('combine', 'transform')

        if left_on and right_on:
            return pd.merge(stream1, stream2, left_on=left_on, right_on=right_on, how=how, suffixes=suffixes)
        else:
            return None
takes two input streams (pandas data frames) and joins them based on the keys dictionary
dataset/ETHPy150Open Convertro/Hydro/src/hydro/transformers.py/Transformers.combine