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.
class_docstr_tok: ["A","helper","class","that","represents","input","string","that","to","be","modified",".","for","example",":","1",".","CnTextField","(","[","'\u4eca\u5929","'",",","'\u5929\u6c14","'",",","'\u4e0d\u9519","'","]",",","None",")","2",".","CnTextField","(","'\u4eca\u5929\u5929\u6c14\u4e0d\u9519","'",",","None",")",":","param","str","or","list","field_value",":","the","value","of","the","field",".",":","param","int","mask",":","mask","label","."]
docstr: Get pos tag of given index.
  
  :param int desired_word_idx: desire index to get pos tag
  :return:  pos tag of word of desired_word_idx.
docstr_tok: ["Get","pos","tag","of","given","index",".",":","param","int","desired_word_idx",":","desire","index","to","get","pos","tag",":","return",":","pos","tag","of","word","of","desired_word_idx","."]
code: def pos_of_word_index(self, desired_word_idx):
      r"""
      Get pos tag of given index.
  
      :param int desired_word_idx: desire index to get pos tag
      :return:  pos tag of word of desired_word_idx.
  
      """
      if (desired_word_idx < 0) or (
          desired_word_idx > len(self.field_value)
      ):
          raise ValueError(
              f"Cannot get POS tagging at index {desired_word_idx}"
          )
  
      return self._pos_tags[desired_word_idx][1]
  
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 14 for calling the CnTextField obj's underlying member method code with example usage: obj.pos_of_word_index(desired_word_idx) and returns: self