Window_Stays_Open.py

import PySimpleGUI as sg

layout = [[sg.Text('Add 2 Numbers', font='Helvetica 15')],
          [sg.InputText(size=(8,1) ,key='in1'), sg.Text(' + '), sg.Input(size=(8,1), key='in2'),sg.Text('',key='answer')],
          [sg.Text('')],
          [sg.ReadButton('Add')]]

window = sg.Window('Add 2 numbers').Layout(layout)

# event loop
while True:
    button, values = window.Read()

    if button is None:
        break

    num1 = int(values['in1'])
    num2 = int(values['in2'])

    answer = num1+num2

    window.FindElement('answer').Update(answer)