class_docstr: Matchzoo :class:`DataPack` data structure, store dataframe and context.
  
  `DataPack` is a MatchZoo native data structure that most MatchZoo data
  handling processes build upon. A `DataPack` consists of three parts:
  `left`, `right` and `relation`, each one of is a `pandas.DataFrame`.
  
  :param relation: Store the relation between left document
      and right document use ids.
  :param left: Store the content or features for id_left.
  :param right: Store the content or features for
      id_right.
  
  Example:
      >>> left = [
      ...     ['qid1', 'query 1'],
      ...     ['qid2', 'query 2']
      ... ]
      >>> right = [
      ...     ['did1', 'document 1'],
      ...     ['did2', 'document 2']
      ... ]
      >>> relation = [['qid1', 'did1', 1], ['qid2', 'did2', 1]]
      >>> relation_df = pd.DataFrame(relation)
      >>> left = pd.DataFrame(left)
      >>> right = pd.DataFrame(right)
      >>> dp = DataPack(
      ...     relation=relation_df,
      ...     left=left,
      ...     right=right,
      ... )
      >>> len(dp)
      2
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docstr: `relation` setter.
docstr_tok: ["`","relation","`","setter","."]
code: def relation(self, value):
      """`relation` setter."""
      self._relation = value
  
imports: import typing
  import inspect
  from pathlib import Path
  import functools
  import dill
  from tqdm import tqdm
  import numpy
  import pandas
  import matchzoo
label_desc: Use image node_id 8 for calling the DataPack obj's underlying member method code with example usage: obj.relation(value) without return types