code: def test_instance_field_disappear_bug(self):
      data = DataSet({"raw_chars": [[0, 1], [2]], "target": [0, 1]})
      data.copy_field(field_name="raw_chars", new_field_name="chars")
      _data = data[:1]
      for field_name in ["raw_chars", "target", "chars"]:
          assert _data.has_field(field_name) == True
  
imports: import os
  import pytest
  import numpy
  from fastNLP.core.dataset import DataSet, FieldArray, Instance, ApplyResultException
  from fastNLP import logger
label_desc: Use image node_id 27 for calling the TestDataSetMethods obj's underlying member method code with example usage: obj.test_instance_field_disappear_bug() without return types