[Tkinter] Extrakt a Variable from a closed tkinter window

[hWp]

Ok sorry, I add some additional description:

The programm shall do the following steps in order:

1. Choosing an ExcelFile with openfiledialog automatically after starting the programm
2. Open tkinter GUI and list the sheets from the file of openfiledialog
3. After choosing the right sheet from the dropdownmenu tkinter shall be closed with the button "Auswahl" and additionally the chosen entry of the dropdownmenu shall be extracted as variable for the subsequent algorithm

Is the Main Window necessary to extract the variable? And do I just put the Attributes of the tkinter setup in the class? The window ist not necessary for the further programm.