class_docstr: tests Vocabulary Class
class_docstr_tok: ["tests","Vocabulary","Class"]
code: def test_lookup_empty_str(self):
      self.assertEqual(self.vocab.lookup(""), "<UNK>")
  
imports: import unittest
  from collections import Counter
  from timeit import timeit
  from nltk.lm import Vocabulary
label_desc: Use image node_id 17 for calling the NgramModelVocabularyTests obj's underlying member method code with example usage: obj.test_lookup_empty_str() without return types