code: def testPreservesEmptyFields(self):
      s = schema.Struct(
          ("a", schema.Scalar(np.float32)),
          ("b", schema.Struct()),
      )
      sc = s.clone()
      self.assertIn("a", sc.fields)
      self.assertIn("b", sc.fields)
      sv = schema.from_blob_list(s, [np.array([3.4])])
      self.assertIn("a", sv.fields)
      self.assertIn("b", sv.fields)
      self.assertEqual(0, len(sv.b.fields))
  
imports: from caffe2.python import core, schema
  import numpy
  import unittest
  import pickle
  import random
label_desc: Use image node_id 15 for calling the TestDB obj's underlying member method code with example usage: obj.testPreservesEmptyFields() without return types