old_contents: import unittest
  
  from lessons.lesson_3_calculator import calculator
  
  
  class AddTestCase(unittest.TestCase):
      def test_add_returns_sum_of_two_numbers(self):
          five = calculator.add(2, 3)
          self.assertEqual(five, 5)
          ten = calculator.add(7, 3)
          self.assertEqual(ten, 10)
  
      def test_position_of_arguments_does_not_matter(self):
          a = calculator.add(2, 3)
          b = calculator.add(3, 2)
          self.assertEqual(a, b)
  
      def test_add_handles_negative_numbers(self):
          a = calculator.add(-3, 5)
          self.assertEqual(a, 2)
  
  
  class SubtractTestCase(unittest.TestCase):
      def test_subtract_returns_second_argument_minus_first(self):
          three = calculator.subtract(10, 7)
          self.assertEqual(three, 3)
          eleven = calculator.subtract(15, 4)
          self.assertEqual(eleven, 11)
  
      def test_position_matters_for_subtraction(self):
          a = calculator.subtract(10, 7)
          b = calculator.subtract(7, 10)
          self.assertNotEqual(a, b)
  
      def test_subtract_handles_negative_numbers(self):
          a = calculator.subtract(-7 - 8)
          self.assertEqual(a, 1)
  
  
  class TotalSumTestCase(unittest.TestCase):
      def test_total_sum_accepts_list_and_returns_sum_of_list(self):
          result = calculator.total_sum([1, 2, 3])
          self.assertEqual(result, 6)
  
      def test_total_sum_handles_negative_integers(self):
          result = calculator.total_sum([-1, -2, -3])
          self.assertEqual(result, -6)
  
      def test_total_sum_handles_mix_of_positive_and_negative_integers(self):
          result = calculator.total_sum([1, 2, -3, -5])
          self.assertEqual(result, -5)
  
      def test_empty_array_input_returns_0(self):
          result = calculator.total_sum([])
          self.assertEqual(result, 0)
  
new_contents: # Create functions that perform basic calculations. You should create 3 functions
  
  # add should accept two integers and return the sum of those two integers
  # subtract should accept two integers and return the first integer minus the second
  # total_sum should accept a list of integers and return the total sum of all the array elements
  # note: if total_sum receives an empty list, it should return 0
  
  
  import unittest
  
  from lessons.lesson_3_calculator import calculator
  
  
  class AddTestCase(unittest.TestCase):
      def test_add_returns_sum_of_two_numbers(self):
          five = calculator.add(2, 3)
          self.assertEqual(five, 5)
          ten = calculator.add(7, 3)
          self.assertEqual(ten, 10)
  
      def test_position_of_arguments_does_not_matter(self):
          a = calculator.add(2, 3)
          b = calculator.add(3, 2)
          self.assertEqual(a, b)
  
      def test_add_handles_negative_numbers(self):
          a = calculator.add(-3, 5)
          self.assertEqual(a, 2)
  
  
  class SubtractTestCase(unittest.TestCase):
      def test_subtract_returns_second_argument_minus_first(self):
          three = calculator.subtract(10, 7)
          self.assertEqual(three, 3)
          eleven = calculator.subtract(15, 4)
          self.assertEqual(eleven, 11)
  
      def test_position_matters_for_subtraction(self):
          a = calculator.subtract(10, 7)
          b = calculator.subtract(7, 10)
          self.assertNotEqual(a, b)
  
      def test_subtract_handles_negative_numbers(self):
          a = calculator.subtract(-7 - 8)
          self.assertEqual(a, 1)
  
  
  class TotalSumTestCase(unittest.TestCase):
      def test_total_sum_accepts_list_and_returns_sum_of_list(self):
          result = calculator.total_sum([1, 2, 3])
          self.assertEqual(result, 6)
  
      def test_total_sum_handles_negative_integers(self):
          result = calculator.total_sum([-1, -2, -3])
          self.assertEqual(result, -6)
  
      def test_total_sum_handles_mix_of_positive_and_negative_integers(self):
          result = calculator.total_sum([1, 2, -3, -5])
          self.assertEqual(result, -5)
  
      def test_empty_array_input_returns_0(self):
          result = calculator.total_sum([])
          self.assertEqual(result, 0)
  
subject: Add brief comment description of challenge to lesson 3 test file.