code: def testSampleFromEmptyTree(self):
      with self.assertRaises(
          Exception, msg="Cannot sample from an empty sum tree."
      ):
          self._tree.sample()
  
imports: import random
  import unittest
  from reagent.replay_memory import sum_tree
label_desc: Use image node_id 8 for calling the SumTreeTest obj's underlying member method code with example usage: obj.testSampleFromEmptyTree() without return types