code: def test_variable_names(self):
      feature = Feature("x1.val * 3.2 + 1e2* x2")
      actual = feature.variable_names
      expected = ["x1.val", "x2"]
      self.assertListEqual(actual, expected)
  
file_path: AIX360/tests/rule_induction/trxf/core/test_feature.py
imports: import tokenize
  from unittest import TestCase
  from aix360.algorithms.rule_induction.trxf.core.feature import Feature, _shunting_yard
label_desc: Use image node_id 1 for calling the TestFeature obj's underlying member method code with example usage: obj.test_variable_names() without return types