Using pytest to write test code for Tkinter

I am using pytest to test a to do list program that uses tkinter but when I run my test code the pytest code in the terminal doesn't run. I am not sure if my test code is bad or if I am not calling something correctly but I would love any and all help.

code:

from tkinter import Entry, Listbox
import pytest
from to_do_list import add_item, delete_item

def test_add_item():
    assert add_item('test') == 'test'

def test_delete_item():
    test_delete = delete_item()
    assert not test_delete == 'test'

Code I am trying to test:

def add_item(entry: Entry, listbox:Listbox):
    new_item = entry.get()
    listbox.insert(END, new_item)
    with open('items.txt', 'a') as item_file:
        item_file.write(f'\n{new_item}')

def delete_item(listbox: Listbox):
    listbox.delete(ACTIVE)
    with open('items.txt', 'r+') as item_file:
        lines = item_file.readlines()
        for line in lines:
            if listbox.get(ACTIVE) == line[:-2]:
                lines.remove(line)
            item_file.write(line)
        item_file.close()

In my main code, I have it open a window using this code at the bottom of the code.

window.update()
window.mainloop()