Practica Usando GUI | Calculadora de Interes Anual

# coding=utf-8
#!/usr/bin/env python
#  -*- coding: utf-8 -*-
import tkMessageBox
from Tkinter import *

def interes():

        v1 = int(ent1.get())
        v2 = int(ent2.get())
        v3 = int(ent3.get())
        r = v1 * v2 / 100
        g=(r * v3)
        f= g + v1
        tkMessageBox.showinfo("","Cuando pasen " +str(v3)+ " años, con un interes de "+ str(v2)+ " usted habra generado "+str(f) + " pesos")
v = Tk()
v.title("Interes")
v.geometry("400x250")

vp= Frame(v)
vp.grid(column=0, row=0, padx=(50,50), pady=(10,10))
vp.columnconfigure(0, weight=1)
vp.rowconfigure(0,weight=1)

e1 = Label(vp, text="Pesos:")
e1.grid(row=2, column=4, padx=(20, 20), pady=(20, 20))

e2 = Label(vp, text="Interes:")
e2.grid(row=3, column=4, padx=(20, 20), pady=(20, 20))

e3 = Label(vp, text="Anos:")
e3.grid(row=4, column=4, padx=(20, 20), pady=(20, 20))

val1 = ""
ent1 = Entry(vp, width=12, textvariable=val1)
ent1.grid(row=2, column=5)

val2 = ""
ent2 = Entry(vp, width=12, textvariable=val2)
ent2.grid(row=3, column=5)

val3 = ""
ent3 = Entry(vp, width=12, textvariable=val3)
ent3.grid(row=4, column=5)

b1 = Button(vp, text="Calcular", command=interes)
b1.grid(row=5, column=5, padx=(20, 20), pady=(20, 20))

v.mainloop()