def __init__(self, source_iterators):
        """
        Args:
                source_iterators: list of iterators to zip, item by item
        """
        # TODO: Use all function?
        for source_iterator in source_iterators:
            if not isinstance(source_iterator, iterators.CheckpointableIterator):
                raise ValueError('all iterators in source_iterators have to be CheckpointableIterator')
        self._source_iterators = source_iterators        # type: List[CheckpointableIterator]

        Args:
                source_iterators: list of iterators to zip, item by item
        
kosmos-2/unilm/data/utils.py