Загрузка xlsx через Tkinter

import os
from tkinter import *
from tkinter import ttk
from tkinter import filedialog
import pandas as pd
 
root = Tk()
root.title("Roaming")
ttk.Label(root, text="Add Excel File :").grid(row=0, column=0, padx=20, pady=20)
open_file = ttk.Button(root, text="Open files")
open_file.grid(row=0, column=1, columnspan=2, padx=20, pady=20)
ttk.Label(root, text="Name Excel :").grid(row=1, column=0, padx=20, pady=20)
box_red_path = ttk.Label(root, text="")
box_red_path.grid(row=1, column=1, columnspan=4, padx=20, pady=20)
stare_scripte = ttk.Button(root, text="Start")
stare_scripte.grid(row=3, column=2, columnspan=2, padx=20, pady=20)
 
def upload_excel():
 
    filename = filedialog.askopenfilename(title="Select a File", filetype=(("Excel", "*.xlsx"), ("Excel", "*.xls")))
    
    path = filename 
    
    box_red_path.configure(text=path)
open_file.config(command=upload_excel)
 
def start_app():
    df = pd.read_excel(Какая тут переменная?, header=0, usecols = 'A, B, C', skiprows = 2) 
         
stare_scripte.config(command=start_app)
root.mainloop()