我正在編寫一個以tkinter為gui的小型python程序來選擇excel文件并打開它們，我達到了可以選擇文件并打開它們的地步。我的問題是當用戶不選擇文件并單擊取消它產生和錯誤時。如果用戶沒有在tkinter文件對話框中選擇文件，如何繼續編碼？import pandas as pdimport tkinter as tkfrom tkinter import filedialogimport openpyxlfrom openpyxl import load_workbookimport osimport sysimport numpy as nproot = tk.Tk()canvas1 = tk.Canvas(root, width=300, height=300, bg='lightsteelblue')canvas1.pack()def getExcel():    global df    import_file_path = filedialog.askopenfilename()    df = pd.read_excel(import_file_path)    book = load_workbook(import_file_path)    writer = pd.ExcelWriter(import_file_path, engine='openpyxl')    writer.book = book    x3 = (df.loc[(df['Var Cost'] < -500) | (df['Var Cost'] > 500)])    df3 = pd.DataFrame(x3)    df3.to_excel(writer, sheet_name='VarianceSorted')    writer.save()    writer.close()    command = import_file_path    os.system(command)def end():    sys.exit()browseButton_Excel = tk.Button(text='Import Excel File', command=getExcel, bg='green', fg='white', font=('helvetica', 12, 'bold'))canvas1.create_window(150, 150, window=browseButton_Excel)quitbutton = tk.Button(text='Quit', command=end, bg='green', fg='white', font=('helvetica', 12, 'bold'))canvas1.create_window(200, 200, window=quitbutton)root.mainloop()