import tkinter as tkfrom tkinter import filedialogfrom tkinter import messageboxfrom wordcloud import WordCloudimport pandas as pdimport numpy as npimport matplotlibpyplot as pltfrom sklearnmodel_sele

在函数内部使用全局变量时，需要在函数内部通过global关键字声明一下，如下所示：

def login():
    global user_dict
    global entry_username
    global entry_password
    username = entry_username.get()
    password = entry_password.get()
    if username not in user_dict:
        messagebox.showerror('Error', 'User does not exist.')
    elif password != user_dict[username]:
        messagebox.showerror('Error', 'Incorrect password.')
    else:
        messagebox.showinfo('Success', 'Login successful.')
        entry_username.delete(0, tk.END)
        entry_password.delete(0, tk.END)
        show_main_page()

同样的，在函数show_main_page()中也需要声明全局变量entry_username和entry_password