def group_by_key(self):
        return self._transform(transformations.group_by_key_t())
This always throws an exception, no matter what args you provide.
dataset/ETHPy150Open EntilZha/PyFunctional/functional/pipeline.py/Sequence.group_by_key--crossover with--dataset/ETHPy150Open seomoz/shovel/example/shovel/test/debug.py/faily