Button command doesn't updates my Label textvariable

Question

I have this very easy program which I want to display one random line from a file each time I click on the Button.

Problem is a new line is display at startup of the program, but nothing happens when I click the button, can someone explain me why ?

from random import randrange
from tkinter import *

def entree():
    n=randrange(251)
    fs = open('lexique','r')
    liste = fs.readlines()
    return liste[n]


fen = Tk()
fen.title("lexique anglais politique")

defi = StringVar()
defi.set(entree())

lab = Label(fen, textvariable=defi).pack()

Button(fen, text='Beste Bat', command=entree).pack()

fen.mainloop()