class_docstr: Test the SPSA optimizer.
docstr: Test that if the objective function is not a
  scalar function, an error is raised.
docstr_tok: ["Test","that","if","the","objective","function","is","not","a","scalar","function",",","an","error","is","raised","."]
code: def test_obj_func_not_a_scalar_function(self, f):
      """Test that if the objective function is not a
      scalar function, an error is raised."""
      spsa_opt = qml.SPSAOptimizer(10)
      args = np.array([[0.1, 0.2], [-0.1, -0.4]])
  
      with pytest.raises(
          ValueError,
          match="The objective function must be a scalar function for the gradient ",
      ):
          spsa_opt.step(f, args)
  
imports: import pytest
  import pennylane
  from pennylane import numpy
label_desc: Use image node_id 15 for calling the TestSPSAOptimizer obj's underlying member method code with example usage: obj.test_obj_func_not_a_scalar_function(f) without return types