new_contents: 
  import mock
  from typing import Any, List
  
  from django.test import TestCase, override_settings
  
  from zerver.lib.subdomains import get_subdomain
  from zerver.models import Realm
  
  class SubdomainsTest(TestCase):
      def test_get_subdomain(self):
          # type: () -> None
  
          def request_mock(host):
              # type: (str) -> Any
              request = mock.Mock(spec=['get_host'])
              request.attach_mock(mock.Mock(return_value=host), 'get_host')
              return request
  
          def test(expected, host, *, root_aliases=[]):
              # type: (str, str, List[str]) -> None
              with self.settings(EXTERNAL_HOST='example.org',
                                 ROOT_SUBDOMAIN_ALIASES=root_aliases):
                  self.assertEqual(get_subdomain(request_mock(host)), expected)
                  self.assertEqual(get_subdomain(request_mock(host + ':443')), expected)
  
          ROOT = Realm.SUBDOMAIN_FOR_ROOT_DOMAIN
          test(ROOT, 'example.org')
          test('foo', 'foo.example.org')
          test(ROOT, 'www.example.org', root_aliases=['www'])
          test(ROOT, 'arbitrary.com')
  
subject: Write some tests for get_subdomain.