Código de Python - Calculadora Tkinter

from tkinter import *

app = Tk()

app.title = "Formularios"

app.geometry("500x300")

date1 = IntVar()

date2 = IntVar()

resultado = IntVar()

def convert(operando):

    if operando == "+":

        resultado.set(date1.get() + date2.get())

    elif operando == "-":

        resultado.set(date1.get() - date2.get())

    elif operando == "*":

        resultado.set(date1.get() * date2.get())

    elif operando == "/":

        resultado.set(date1.get() / date2.get())

    else:

        exit()

label = Label(app, text="Calculator", bg = "red", width = 62, height = 2)

label.grid(row = 0, column = 0, pady = 10 ,columnspan = 12,sticky = W) #Sticky es para pegar a la izquierda

num1 = Label(app, text = "Introduce el primer numero: ").grid(row = 4, column = 0, padx = 3, pady = 3, sticky = W)

entry1 = Entry(app, textvariable = date1).grid(row = 4, column = 1, padx = 3, pady = 3)

num2 = Label(app, text = "Introduce el segundo numero : ").grid(row = 5, column = 0, padx = 3, pady = 3, sticky = W)

entry2 = Entry(app, textvariable = date2).grid(row = 5, column = 1, padx = 3, pady = 3)

sum = Button(app, text = "Sumar", command = lambda: convert("+")).grid(row = 6, column = 0, padx = 1, pady = 1, sticky = W)

res = Button(app, text = "Restar", command = lambda: convert("-")).grid(row = 7, column = 0, padx = 1, pady = 1, sticky = W)

mul = Button(app, text = "Multiplicar", command = lambda: convert("*")).grid(row = 6, column = 1, padx = 1, pady = 1, sticky = W)

div = Button(app, text = "Dividir", command = lambda: convert("/")).grid(row = 7, column = 1, padx = 1, pady = 1, sticky = W)

result = Label(app, textvariable = resultado).grid(row = 8, column = 3)

exit = Button(app, text = "Exit", command = quit).grid(row = 9, column = 3)

app.mainloop()