Python Calculator with GUI (tkinter)

import tkinter as tk

calculations = 'Addition'

def callback (selection):
    global calculations
    calculations = selection

def calculate ():
    global calculations
    val1 = 0 if not entry.get() else float (entry.get())
    val2 = 0 if not entry2.get() else float (entry2.get())

    if calculations == 'Addition':
        output.config(text = val1 + val2)
    elif calculations == 'Subtraktion':
        output.config(text = val1 - val2)
    elif calculations == 'Multiplikation':
        output.config (text = val1 * val2 )
    elif calculations == 'Division':
        try:
            output.config (text = val1 / val2)
        except ZeroDivisionError:
             output.config (text = 'Durch 0 ist nicht möglich!')

window = tk.Tk ()
window.geometry ("750x500")
window.title ('Taschenrechner')

OptionList = ['Addition', 'Subtraktion', 'Multiplikation', 'Division']

var1 = tk.StringVar(window)
var1.set (OptionList [0])

entry = tk.Entry(window)
entry.grid(row=0 , column=1)

entry2 = tk.Entry(window)
entry2.grid(row=0 , column=2)

button = tk.Button(window, text='Berechnen', command=calculate)
button.grid(row=0, column=0)

opt = tk.OptionMenu (window, var1, *OptionList, command=callback)
opt.config(font= ('Arial', 14))
opt.grid (row=2 , column=0)

output = tk.Label(window)
output.grid(row=3)


window.mainloop()