code: def test_build_vocab_3(self):
      mf = MatchingField()
      vector_file_name = "fasttext.crawl_test.vec"
      cache_dir = os.path.join(test_dir_path, "test_datasets")
      vec_data = mf.build_vocab(
          vectors=vector_file_name, cache=cache_dir
      )
      self.assertIsNone(vec_data)
  
imports: import os
  import shutil
  import unittest
  from collections import Counter
  from test import test_dir_path
  from nose.tools import *
  import torch
  from deepmatcher.data.dataset import MatchingDataset
  from deepmatcher.data.field import FastText, FastTextBinary, MatchingField, MatchingVocab, reset_vector_cache
  from torchtext.vocab import Vectors
  from urllib.parse import urljoin
  from urllib.request import pathname2url
label_desc: Use image node_id 7 for calling the ClassMatchingFieldTestCases obj's underlying member method code with example usage: obj.test_build_vocab_3() without return types