code: def testNegativeDofFails(self):
      with self.assertRaisesOpError(r"`df` must be positive"):
          half_student = half_student_t.HalfStudentT(
              df=[2, -5.0],
              loc=0.0,
              scale=1.0,
              validate_args=True,
              name="S",
          )
          self.evaluate(half_student.mean())
  
imports: import math
  import numpy
  from scipy import stats
  from scipy.special import gamma
  import tensorflow.compat.v2
  from tensorflow_probability.python.distributions import half_student_t
  from tensorflow_probability.python.distributions.internal import statistical_testing
  from tensorflow_probability.python.internal import assert_util
  from tensorflow_probability.python.internal import test_util
  from tensorflow_probability.python.math import gradient
label_desc: Use image node_id 23 for calling the HalfStudentTTest obj's underlying member method code with example usage: obj.testNegativeDofFails() without return types