class_docstr: A helper class that represents input string that to be modified.
  for example:
      1. CnTextField(['今天', '天气', '不错'],None)
      2. CnTextField('今天天气不错',None)
  
  :param str or list field_value: the value of the field.
  :param int mask: mask label.
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docstr: Swap items between first_index and second_index of origin_list
  
  :param int first_index: index of first item
  :param int second_index: index of second item
  :return:  Modified TextField object.
docstr_tok: ["Swap","items","between","first_index","and","second_index","of","origin_list",":","param","int","first_index",":","index","of","first","item",":","param","int","second_index",":","index","of","second","item",":","return",":","Modified","TextField","object","."]
code: def swap_at_index(self, first_index, second_index):
      r"""
      Swap items between first_index and second_index of origin_list
  
      :param int first_index: index of first item
      :param int second_index: index of second item
      :return:  Modified TextField object.
  
      """
      mask_value = replace_at_scopes(
          self.mask, [first_index, second_index], [MODIFIED_MASK] * 2
      )
      field_value = swap_at_index(
          self.tokens, first_index, second_index
      )
  
      return self.new_field(field_value, mask=mask_value)
  
imports: from .field import Field
  from ....common.utils.list_op import *
  from ....common.preprocess.cn_processor import CnProcessor
  from ....common.settings import ORIGIN, TASK_MASK, MODIFIED_MASK
label_desc: Use image node_id 20 for calling the CnTextField obj's underlying member method code with example usage: obj.swap_at_index(first_index, second_index) and returns: self