old_contents: from unittest import TestCase
  from prudent.mapping import Mapping
  
  
  class MappingTest(TestCase):
      def setUp(self):
          self.mapping = Mapping([(1, 2), (2, 3), (3, 4)])
  
      def test_iter(self):
          keys = [1, 2, 3]
          for _ in range(2):
              assert list(self.mapping) == keys
  
      def test_contains(self):
          assert 1 in self.mapping
          assert 1 in self.mapping
          assert 3 in self.mapping
  
      def test_getitem(self):
          assert self.mapping[1] == 2
          assert self.mapping[3] == 4
          assert self.mapping[2] == 3
  
      def test_len(self):
          assert len(self.mapping) == 0
          self.mapping[3]
          assert len(self.mapping) == 3
  
new_contents: from unittest import TestCase
  from prudent.mapping import Mapping
  
  
  class MappingTest(TestCase):
      def setUp(self):
          self.mapping = Mapping([(1, 2), (2, 3), (3, 4)])
  
      def test_iter_preserves_keys(self):
          keys = [1, 2, 3]
          for _ in range(2):
              assert list(self.mapping) == keys
  
      def test_contains(self):
          assert 1 in self.mapping
          assert 1 in self.mapping
          assert 3 in self.mapping
  
      def test_getitem(self):
          assert self.mapping[1] == 2
          assert self.mapping[3] == 4
          assert self.mapping[2] == 3
  
      def test_len(self):
          assert len(self.mapping) == 0
          self.mapping[3]
          assert len(self.mapping) == 3
  
subject: Use a more descriptive test case name