input: Write a function to add an item to a shopping list.
  assert add_item("Apples") == None
  assert shopping_list == ["Apples"]
  assert add_item("Bananas") == None
  assert shopping_list == ["Apples", "Bananas"]
  
  Write a function to remove an item from a shopping list.
  assert remove_item("Apples") == None
  assert shopping_list == ["Bananas"]
  assert remove_item("Bananas") == None
  assert shopping_list == []
  
  Write a function to search for an item in a shopping list.
  assert search_item("Apples") == None
  assert search_item("Bananas") == None
  
  Write a function to display all items in a shopping list.
  assert display_items() == None
  assert shopping_list == []
output: shopping_list = []
  
  def add_item(item):
      global shopping_list
      shopping_list.append(item)
  
  def remove_item(item):
      global shopping_list
      if item in shopping_list:
          shopping_list.remove(item)
  
  def search_item(item):
      global shopping_list
      return item in shopping_list
  
  def display_items():
      global shopping_list
      for item in shopping_list:
          print(item)