docstr: Check the _list_fields function. Compare with the answer.
docstr_tok: ["Check","the","_list_fields","function",".","Compare","with","the","answer","."]
code: def test__list_fields(self):
      """
      Check the _list_fields function. Compare with the answer.
      """
      model = self.model
      fields = model._list_fields()
      self.assertEqual(set(fields), set(self.fields_ans))
  
imports: from __future__ import print_function
  from __future__ import division
  from __future__ import absolute_import
  import unittest
  import turicreate
  import uuid
  import random
  import copy
  from turicreate.toolkits._main import ToolkitError
  import turicreate.toolkits.evaluation
  import shutil
  from array import array
  import os
label_desc: Use image node_id 3 for calling the BoostedTreesRegressionTest obj's underlying member method code with example usage: obj.test__list_fields() without return types