Problème de variable semble t-il [Débutant]

from Tkinter import *
from math import *

rayon=50
 
def cercle(x, y, r, coul ='black'):
    "tracé d'un cercle de centre (x,y) et de rayon r"
    fen1.create_oval(x-r, y-r, x+r, y+r, outline=coul)

def tracer():
    global rayon
    cercle(100, 100, rayon)

def effacer():
    fen1.delete(ALL)
    
fenetre=Tk()
fenetre.title("Tracé d'un cercle")

rayon = Entry(fenetre)
rayon.grid(row=0,column=1,sticky =W)

#etiquette_rayon=Label(fenetre,text="Rayon")
#etiquette_rayon.grid(row=0,column=0,sticky =E)

etiquette_rayon=Label(fenetre,text=rayon)
etiquette_rayon.grid(row=0,column=0,sticky =E)

fen1=Canvas(fenetre,bg='white',height=200,width=200)
fen1.grid(rowspan=4, column=0,padx=0, pady=10)

bouton_tracer = Button(fenetre, text="Tracer",command=tracer)
bouton_tracer.grid(row=1, column=1,padx=0, pady=0)

bouton_effacer = Button(fenetre, text="Effacer",command=effacer)
bouton_effacer.grid(row=2, column=1,padx=0, pady=0)

bouton_quitter = Button(fenetre, text="Quitter",command=fenetre.destroy)
bouton_quitter.grid(row=3, column=1,padx=0, pady=0)

fenetre.mainloop()

def tracer():
    global rayon
    rayon=int(ent_rayon.get())
    if rayon>0:
        cercle(100, 100, rayon)

  File "C:\Python27\Scripts\Cercle\cercle.py", line 13, in tracer
    rayon=int(ent_rayon.get())
ValueError: invalid literal for int() with base 10: ''

def tracer():
    try:
        rayon=int(ent_rayon.get())

        if rayon>0 and rayon<100:
            cercle(100, 100, rayon)
        else:
            print "rayon hors limites"
            return

    except:
        print "saisie non numerique"
        return