text: import unittest
  from queue import Queue
  
  
  class QueueTests(unittest.TestCase):
      def setUp(self):
          self.q = Queue()
  
      def test_len_returns_0_for_empty_queue(self):
          self.assertEqual(len(self.q), 0)
  
      def test_len_returns_correct_length_after_enqueue(self):
          self.assertEqual(len(self.q), 0)
          self.q.enqueue(2)
          self.assertEqual(len(self.q), 1)
          self.q.enqueue(4)
          self.assertEqual(len(self.q), 2)
          self.q.enqueue(6)
          self.q.enqueue(8)
          self.q.enqueue(10)
          self.q.enqueue(12)
          self.q.enqueue(14)
          self.q.enqueue(16)
          self.q.enqueue(18)
          self.assertEqual(len(self.q), 9)
  
      def test_empty_dequeue(self):
          self.assertIsNone(self.q.dequeue())
          self.assertEqual(len(self.q), 0)
  
      def test_dequeue_respects_order(self):
          self.q.enqueue(100)
          self.q.enqueue(101)
          self.q.enqueue(105)
          self.assertEqual(self.q.dequeue(), 100)
          self.assertEqual(len(self.q), 2)
          self.assertEqual(self.q.dequeue(), 101)
          self.assertEqual(len(self.q), 1)
          self.assertEqual(self.q.dequeue(), 105)
          self.assertEqual(len(self.q), 0)
          self.assertIsNone(self.q.dequeue())
          self.assertEqual(len(self.q), 0)
  
  
  if __name__ == '__main__':
      unittest.main()
  