from tkinter import Tk, simpledialog, messagebox

root = Tk()
root.withdraw()

#считали данные
d = {}
file = open('f.txt','r',encoding='utf-8')
for line in file:
    line = line.rstrip() #line = 'allena,qwerty'
    login, password = line.split(',') # 'allena' 'qwerty'
    d[login] = password
file.close()

while True:
    log = simpledialog.askstring('Хранилище паролей','Ваш логин?')
    if log in d:
        password = d[log]
        messagebox.showinfo('Ваш пароль: ', password)
    else:
        new_password = simpledialog.askstring('Не знаю такого логина','Ваш пароль?')
        d[log] = new_password
        file = open('f.txt','a',encoding='utf-8')
        file.write('\n'+log+','+new_password)
        file.close()
root.mainloop()