im getting a syntax error which is saying that index += 1 is an invalid syntax

import os

#make a list
fruits_list = []

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


#print instructions
def help_list():
    clear_screen()
    print("Type your list in here")
    print("""
typing DONE completes the list
typing SHOW shows your current list
typing HELP shows the above instructions again""")

help_list()

def show_list():
    clear_screen()

    print("Here's your list: ")

    index = 1
    for item in fruits_list:
        print("{}. {}".format(index, item)
        index += 1

    print("-"*10)          

def added_items(item):
    #add new items to the list
    show_list()
    if len(shopping_list):
        position = input("Where should I put {}?\n"
                         "Press ENTER after typing the serial number."
                         "> ".format(item))
    else:
        position = 0

    try:
        position = abs(int(position))
    except ValueError:
        position = None

    if position is not None:
        fruits_list.insert(position-1, item)
    else:
        fruits_list.append(new_item)

    show_list()

#ask for new items
while True:
    new_item = input("> ")
    #be able to quit the app
    if new_item.upper() == "DONE" or new_item.upper() =="QUIT":
        break

    elif new_item.upper() == "HELP":
        help_list()
        continue

    elif new_item.upper() == "SHOW":
        show_list()
        continue

    else:    
        added_items(new_item)   

show_list()