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","."]
code: def set_mask(self, index, value):
      if not self._mask:
          self._mask = [ORIGIN] * len(self.words)
  
      if index > len(self.mask) - 1:
          raise ValueError(
              "Index {0} out of range {1}".format(
                  index, len(self.mask) - 1
              )
          )
      if value not in [ORIGIN, TASK_MASK, MODIFIED_MASK]:
          raise ValueError(
              "Support mask value in {0},  while input mask value is {1}!".format(
                  [ORIGIN, TASK_MASK, MODIFIED_MASK], value
              )
          )
      self._mask[index] = 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 6 for calling the CnTextField obj's underlying member method code with example usage: obj.set_mask(index, value) without return types