'Create new DatastoreInputReader object.
This is internal constructor. Use split_query instead.
Args:
entity_kind: entity kind as string.
key_range_param: key range to process as key_range.KeyRange.
mapper_params: mapper parameters as defined by user.'

def __init__(self, entity_kind, key_range_param, mapper_params):

    self._entity_kind = entity_kind
    self._key_range = key_range_param
    self._mapper_params = mapper_params
    self._batch_size = int(self._mapper_params.get(self.BATCH_SIZE_PARAM, self._BATCH_SIZE))
