NameError: name 'shopping_list' is not defined

import os

def clear_screen():
    os.system('cls' if os.name == 'nt' else 'clear')

def show_help():
    clear_screen()
    print('What should we pick up at the store?')
    print("""
Enter 'DONE' to stop adding items.
Enter 'HELP' for help.
Enter 'SHOW' to see your current list.
Enter 'REMOVE' to delete an item from your list.
""")

def add_to_list(item):
    show_list()
    if len(shopping_list):
        position = input('Where should I add {}?\n'
                         'Press ENTER to add to the end of the list\n'
                         '> '.format(item))

    else:
        position = 0

    try:
        position = abs(int(position))
    except ValueError:
        position = None
    if position is not None:
        shopping_list.insert(position-1, item)
    else:
        shopping_list.append(new_item)

    show_list()

def show_list():
    clear_screen()

    print('Here is your list:')

    index = 1
    for item in shopping_list:
        print('{}. {}'.format(index, item))
        index += 1
    print('-'*15)


def remove_from_list():
    show_list()
    what_to_remove = input('What would you like to remove?\n> ')
    try:
        shopping_list.remove(what_to_remove)
    except ValueError:
        pass
    show_list()

show_help()

while True:
    new_item = input('> ')

    if new_item.upper() == 'DONE' or new_item.upper() == 'QUIT':
        break
    elif new_item.upper() == 'HELP':
        show_help()
        continue
    elif new_item.upper == 'SHOW':
        show_list()
        continue
    elif new_item.upper == 'REMOVE':
        remove_from_list()
    else:
        add_to_list(new_item)

show_list()