new_contents: from windpowerlib.tools import smallest_difference
  import collections
  
  
  class TestTools:
  
      @classmethod
      def setup_class(self):
          self.return_tuple = collections.namedtuple('selected_values',
                                                     ['closest_value',
                                                      'corresp_value'])
  
      def test_smallest_difference(self):
          # value_1 closer to comparative value
          expected_output = self.return_tuple(30, 5.0)
          assert smallest_difference(30, 10, 100, 5.0, 6.0) == expected_output
          # value_1 = comparative value
          expected_output = self.return_tuple(100, 5.0)
          assert smallest_difference(100, 10, 100, 5.0, 6.0) == expected_output
          # value_2 closer to comparative value
          expected_output = self.return_tuple(30, 6.0)
          assert smallest_difference(10, 30, 100, 5.0, 6.0) == expected_output
          # value_2 = comparative value
          expected_output = self.return_tuple(100, 6.0)
          assert smallest_difference(10, 100, 100, 5.0, 6.0) == expected_output
          # value_2 is None
          expected_output = self.return_tuple(10, 5.0)
          assert smallest_difference(10, None, 100, 5.0, 6.0) == expected_output
  
subject: Add test module for tools module