Manejo de Listbox en Python 3.x. Usando Tkinter

Como ven es una pequeña aplicacion desarrollada en Python usando Tkinter en especial me enfoco en el uso de los Listbox, como veran es la aplicacion de todos los posibles casos del teorema de pitagoras, la funcion que controla el Listbox tiene por nombre hola, para que la identifiquen.

## teoremas para triangulos
#pitagoras9
from tkinter import*
from tkinter import messagebox
import math

def hola(e):
    global cas
    var = lb.curselection()
    var1 = tuple(('0',))
    var2 = tuple(('1',))
    var3 = tuple (('2'))
    if var == var1:
        ##calculando cateto opuesto
        cas = 1
        texto = Label(ventana, text = "Hipotenusa")
        texto.place(bordermode = OUTSIDE, height = 20, width = 100, x= 300, y= 100)
        texto = Label(ventana, text = "C. Adyacente")
        texto.place(bordermode = OUTSIDE, height = 20, width = 100, x= 300, y= 140)
        textores2 = Label(ventana, text = "C. Opuesto")
        textores2.place(bordermode = OUTSIDE, height = 20, width = 150, x = 50, y = 250)
    elif var == var2:
        ##calculando cateto Adyacente
        cas = 2
        texto = Label(ventana, text = "Hipotenusa")
        texto.place(bordermode = OUTSIDE, height = 20, width = 100, x= 300, y= 100)
        texto = Label(ventana, text = "C. Opuesto")
        texto.place(bordermode = OUTSIDE, height = 20, width = 100, x= 300, y= 140)
        textores2 = Label(ventana, text = "C. Adyacente")
        textores2.place(bordermode = OUTSIDE, height = 20, width = 150, x = 50, y = 250)
    elif var == var3:
        cas = 3
        ##calculando hipotenusa
        texto = Label(ventana, text = "C. Opuesto")
        texto.place(bordermode = OUTSIDE, height = 20, width = 100, x= 300, y= 100)
        texto = Label(ventana, text = "C. Adyacente")
        texto.place(bordermode = OUTSIDE, height = 20, width = 100, x= 300, y= 140)
        textores2 = Label(ventana, text = "Hipotenusa")
        textores2.place(bordermode = OUTSIDE, height = 20, width = 150, x = 50, y = 250)
    else:
        print ('Fuera de Rango')

##operacion del boton calcular
def calcular():
    global cas
    variable1 = vari1.get()
    variable2 = vari2.get()
    if cas == 1:
        h=float(variable1)
        ca = float(variable2)
        ##operacion
        resultado  = math.sqrt((h*h) - (ca*ca))
        rest.set(resultado)
    elif cas == 2:
        h=float(variable1)
        co = float(variable2)
        resultado  = math.sqrt((h*h) - (co*co))
        rest.set(resultado)
    elif cas == 3:
        co=float(variable1)
        ca = float(variable2)
        resultado  = math.sqrt((co*co) + (ca*ca))
        rest.set(resultado)
        
    else:
        messagebox.showerror('Error','Selecciona una Variable')
        
    

          
ventana = Frame (height = 300, width = 600)
ventana.pack(padx =10, pady = 10)

variable1 = DoubleVar()
variable2 = DoubleVar()
cas = 0
rest = StringVar()
#probando listbox
textolb = Label(ventana, text = "Seleccione Variable")
textolb.place(bordermode = OUTSIDE, height = 20, width = 150, x= 100, y= 20)
lb = Listbox(ventana)
lb.insert(1,"Cateto Opuesto")
lb.insert(2,"Cateto Abyacente")
lb.insert(3,"Hipotenusa")
lb.bind('<>',hola)
lb.place(bordermode = OUTSIDE, height = 70, width = 150, x = 100, y= 40)
#boton calcular
boton = Button(ventana, text = "calcular",command= calcular )
boton.place(bordermode = OUTSIDE, height = 30, width =80, x=350, y = 200)
#texto de presentacion
texto = Label(ventana, text = "Variable 1")
texto.place(bordermode = OUTSIDE, height = 20, width = 100, x= 300, y= 100)
texto = Label(ventana, text = "Variable 2")
texto.place(bordermode = OUTSIDE, height = 20, width = 100, x= 300, y= 140)
#Entrada de Datos
vari1 = Entry(ventana, textvariable =variable1)
vari1.place(bordermode= OUTSIDE, height= 30, width = 100, x= 400, y= 100)
vari2 = Entry(ventana,textvariable=variable2)
vari2.place(bordermode= OUTSIDE, height= 30, width = 100, x= 400, y= 140)
#para mostrar resustados
textores = Label(ventana, text = "Resultado")
textores.place(bordermode = OUTSIDE, height = 20, width = 150, x= 100, y= 200)
textores2 = Label(ventana, text = "Variable")
textores2.place(bordermode = OUTSIDE, height = 20, width = 150, x = 50, y = 250)
res = Label(ventana,textvariable = rest, fg = "blue",bg = "yellow")
res.place(bordermode = OUTSIDE, height = 20, width = 200, x = 200, y = 250)