code: def test_schaffer(self):
      self.assertEqual(
          schaffer((0, 0)), 0.0, msg="schaffer failed for 2 arguments"
      )
  
file_path: scikit-opt/tests/test_demo_func.py
imports: import unittest
  from sko.demo_func import ackley, cigar, function_for_TSP, rastrigrin, rosenbrock, sixhumpcamel, sphere, schaffer, zakharov, shubert, griewank, rastrigrin, rosenbrock, sixhumpcamel, zakharov, ackley, cigar
label_desc: Use image node_id 3 for calling the TestDemoFunc obj's underlying member method code with example usage: obj.test_schaffer() without return types