Pintar con el mouse ne la ventana con Tkinter

from Tkinter import *

alto = 500ancho = 150
master=Tk()
master.title('pintar con mouse')


def borrar():
    panel.delete('all')

def paint(event):
    x1,y1=(event.x-1), (event.y-1)
    x2, y2 = (event.x+1), (event.y+1)
    panel.create_oval(x1,y1,x2,y2, fill = 'red')

panel=Canvas(master,width=500, height=150)
panel.pack(expand = YES, fill=BOTH)
panel.bind("<B1-Motion>",paint)

mensaje=Label(master, text='deslizar mouse')
mensaje.pack(side=BOTTOM)

boton = Button (master, text = 'limpiar', command=lambda: borrar())
boton.pack(side=BOTTOM)

master.mainloop()