Вот упрощенный пример для хранения данных в файле (данные добавляются в конец файла)
import tkinter as tk
def save_to_file(filename, name='', pw=''):
with open(filename, 'a') as f: # 'a' to append to the file, 'r' to overwrite the file
f.write(name + ', ')
f.write(pw + '\n')
def login():
name = uid.get()
pw = pas.get()
print('logging in as {uid}, {pw}')
save_to_file('secret_store.txt', name=name, pw=pw)
if __name__ == '__main__':
win = tk.Tk()
win.title("Facebook login")
lbl = tk.Label(win, text="Please enter your login and password")
lbl.grid(column=5,row=3)
uid = tk.Entry(win, width=30)
pas = tk.Entry(win,width=30)
uid.grid(column=3,row=6)
pas.grid(column=3,row=9)
button1 = tk.Button(win, text="Login", command=login)
button1.grid(column=7, row=11)
win.mainloop()