Can someone please explain why my clear function isn't working

import os


mylist = []

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


def help():
    #print out instructions 
    print("type the stuff you want to add to the list")
    print("""
    type in 'DONE' when you are..
    Enter 'HELP' when you need it
    Enter 'SHOW' to show the added stuff""")

def add_to_list(new_item):
    if len(mylist):
      position = input("where should i add {} ?\n"
                       "press ENTER to just ass it to the end of the list\n"
                       "> ".format(new_item))
    else:
      position = 0

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

    show_list()  

def show_list():
    clear_screen()

    print("Here's everything on the list : ")

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

    print("-" * 15)

help()

while True:
    new_item = input("> ")

    #have an exit button
    if new_item.upper() == 'DONE' or new_item.upper() == 'QUIT':
        break

    elif new_item.upper() == 'HELP' : 
        help()
        continue

    elif new_item.upper() == 'SHOW':
        show_list()
        continue
    else:       
        add_to_list(new_item)


show_list()