input: Write a python function to add and remove items from a shopping cart and verify its contents.
  assert cart.items == []
  cart.add_item('apple')
  assert cart.items == ['apple']
  cart.add_item('banana')
  assert cart.items == ['apple', 'banana']
  cart.remove_item('apple')
  assert cart.items == ['banana']
  cart.remove_item('cherry')
  assert cart.items == ['banana']
output: class ShoppingCart:
      def __init__(self):
          self.items = []
  
      def add_item(self, item):
          if item not in self.items:
              self.items.append(item)
  
      def remove_item(self, item):
          if item in self.items:
              self.items.remove(item)
  
  cart = ShoppingCart()