old_contents: from .utils import TestCase, skipUnless
  from dynsupdate import client
  import os
  
  
  class DnsTests(TestCase):
  
      @skipUnless(os.getenv("SLOW"), "To slow")
      def test_build_resolver(self):
          domain = 'google-public-dns-a.google.com'
          res = client.NameUpdate.build_resolver(domain)
          self.assertListEqual(res.nameservers, ['8.8.8.8'])
  
new_contents: from .utils import TestCase, skipUnless, mock
  from dynsupdate import client
  import os
  
  
  class DnsTests(TestCase):
  
      @skipUnless(os.getenv("SLOW"), "To slow")
      def test_build_resolver(self):
          domain = 'google-public-dns-a.google.com'
          res = client.NameUpdate.build_resolver(domain)
          self.assertListEqual(res.nameservers, ['8.8.8.8'])
  
      @mock.patch('dns.resolver.query')
      @mock.patch('dns.resolver.Resolver')
      def test_build_resolver_fake(self, mock_resolver, mock_query):
          mock_rdata = mock.Mock()
          mock_rdata.address = "127.0.0.1"
          mock_query.return_value = iter([mock_rdata])
          res = client.NameUpdate.build_resolver("ns1.fake.com", port=999)
          mock_query.assert_called_with("ns1.fake.com", "A")
          print(mock_resolver.mock_calls)
          mock_resolver.return_value.nameservers.append \
              .assert_called_with("127.0.0.1")
  
          self.assertEqual(res.port, 999)
  
subject: Add mocked test of build_resolver