new_contents: import unittest
  from stack import Stack
  
  class StackTest(unittest.TestCase):
      def setUp(self):
          self.simple_stack = Stack()
  
      def test_stack_push(self):
          self.assertEqual(self.simple_stack.isEmpty(), True)
          self.simple_stack.push(20)
          self.assertEqual(self.simple_stack.isEmpty(), False)
  
      def test_stack_pop(self):
          self.simple_stack.push('item')
          stack_size = self.simple_stack.size()
          self.assertEqual(self.simple_stack.pop(), 'item')
          self.assertNotEqual(self.simple_stack.size(), stack_size)
          self.assertEqual(self.simple_stack.size(), stack_size - 1)
  
      def test_stack_peek_returns_last_item(self):
          self.simple_stack.push('last')
          self.assertEqual(self.simple_stack.peek(), 'last')
  
      def test_stack_is_not_empty_after_push(self):
          self.simple_stack.push(10)
          self.assertFalse(self.simple_stack.isEmpty())
  
      def test_popping_beyond_stack_size(self):
          self.simple_stack.push(20)
          self.simple_stack.pop()
          with self.assertRaises(IndexError):
              self.simple_stack.pop()
  
subject: add tests for python data structure