[Tkinter] Extrakt a Variable from a closed tkinter window

[Axel_Erfurt]

why

command = master.destroy

you destroy the main window.

You never get a main window with your code.

try this and add your other functions

from tkinter.filedialog import askopenfilename
import tkinter
import pandas as pd
import matplotlib.pyplot as plt
from gi.repository import GLib ### linux only
  
class myApp:
    def __init__(self,master):
        self.master = master
        # Add a grid
        self.mainframe = tkinter.Frame(self.master)
        self.mainframe.grid(column=0,row=0, sticky=(tkinter.N,tkinter.W,tkinter.E,tkinter.S) )
        self.mainframe.columnconfigure(0, weight = 2)
        self.mainframe.rowconfigure(0, weight = 2)
        self.mainframe.pack(pady = 100, padx = 100)
         
        self.button = tkinter.Button(self.mainframe, 
                             text="Auswahl", fg="red",
                             command = self.openFile)

        self.popupMenu = tkinter.OptionMenu(self.mainframe, None, None)
        tkinter.Label(self.mainframe, text="Choose a sheet").grid(row = 1, column = 1)
        self.popupMenu.grid(row = 2, column =1)
        self.button.grid(row = 3, column = 1)

    def openFile(self):
        docs = GLib.get_user_special_dir(GLib.USER_DIRECTORY_DOCUMENTS) ### linux only
        Filename = askopenfilename(initialdir=docs, filetypes = (("ExcelFiles","*.xlsm *.xlsx *.xls"),("all files","*.*")),title='Jetzt wähl endlich die Kagg-Datei')
        print(Filename)
        return Filename

if __name__ == '__main__':
    master = tkinter.Tk()
    master.title("Sheetauswahl") 
    sheet = myApp(master).mainframe
    master.mainloop()