Было много проблем, поэтому я изменил все, чтобы сделать его проще.
Я добавил комментарии в коде, чтобы объяснить некоторые элементы.
from tkinter import *
from tkinter import messagebox
# --- functions ---
def button():
# use global variable (instead of local variables)
# to keep username, password outside function and keep after closing window
global username
global password
username = uname.get()
password = passwd.get()
#print('button:', username, password)
if username == "username" and password == "password":
messagebox.showwarning("Warning", "Sorry, this isn't programmed yet.")
root.destroy() # close window
else:
messagebox.showwarning("Nope", "Nope. Nice try.")
# window still open
# --- main ---
# default values at start
# Someone can exit window without using Button
# and it would not create this variables in button()
username = ""
password = ""
root = Tk()
root.title("Login")
uname = Entry(root)
uname.grid(row=1, column=1)
passwd = Entry(root, show="*")
passwd.grid(row=2, column=1)
quitbutt = Button(root, text="Login", command=button)
quitbutt.grid(row=3, column=1, columnspan=2, sticky='nesw')
# start program (start engine, display window, (re)draw widgets, handle events, get events from system (keyboard, mouse), send events to widgets)
root.mainloop()
# after closing window this variables still have values from window
print("after mainloop:", username, password)