old_contents: from unittest import TestCase
  
  
  class ShortenerTestCase(TestCase):
  
      def test_long_to_short(self):
          pass
  
      def test_short_to_long(self):
          pass
  
new_contents: from unittest import TestCase
  
  from usb.shortener import shorten
  
  
  class ShortenerTestCase(TestCase):
  
      def test_shorten(self):
          long_url = 'https://www.youtube.com/watch?v=Y21VecIIdBI'
          short_url = shorten(long_url)
          self.assertNotEqual(long_url, short_url)
          self.assertRegex(short_url, r'^[a-zA-Z0-9]{6,}$')
  
subject: Test shorten functionality