code: def testStratifiedSampling(self):
      k = 32
      for i in range(k):
          self._tree.set(node_index=i, value=1)
      samples = self._tree.stratified_sample(k)
      self.assertEqual(len(samples), k)
      for i in range(k):
          self.assertEqual(samples[i], i)
  
imports: import random
  import unittest
  from reagent.replay_memory import sum_tree
label_desc: Use image node_id 15 for calling the SumTreeTest obj's underlying member method code with example usage: obj.testStratifiedSampling() without return types