从Python Tkinter中主类外部的函数更新进度条

import tkinter as tk
from tkinter import ttk

class MainApp(tk.Tk):
    def __init__(self):
        super().__init__()
        self.progressbar = ttk.Progressbar(self, orient='horizontal', length=200, mode='determinate')
        self.progressbar.pack()
        self.progressbar['maximum'] = 100
        self.progressbar['value'] = 0

app = MainApp()
app.mainloop()

class MainApp(tk.Tk):
    # ...

    def update_progress(self, value):
        self.progressbar['value'] = value

def external_function():
    app = MainApp()
    # ...
    app.update_progress(50)