lstbox.grid(column=0, row=0, columnspan=2)

def select():
    reslist = list()
seleccion = lstbox.curselection()
for i in seleccion:
    entrada = lstbox.get(i)
reslist.append(entrada)
for val in reslist:
    print(val)
btn = ttk.Button(frame, text='Choices', command=select)
btn.grid(column=1, row=1)
main.mainloop()
Список множественного выбора Python Tkinter