U3 - Citculo con coordenadas

from Tkinter import *

def poligono(val1, val2,val3,val4):
    v1 = Toplevel(ventana)
    v1.title('Grafica')
    v1.protocol('WM_DELETE_WINDOW', 'onexit')
    v1.geometry('500x500')
    grafica = Canvas(v1, width = 300 , height = 300, bg = 'black')
    grafica.pack(expand=YES, fill=BOTH)
    val1= int(e1.get())
    val2= int(e2.get())
    val3 = int(e3.get())
    val4 = int(e4.get())
    grafica.create_oval(val1, val2, val3, val4, fill = 'red')
    b = Button(grafica, text = 'Regresar', command = lambda: ejecutar(ocultar(v1)))
    b.grid(row = 1, column = 3)





def ocultar(v1):
    v1.destroy()

def ejecutar(f):
    ventana.after(200, f)

ventana = Tk()
ventana.title('Graficando')
ventana.geometry('700x300')
v1= ''
v2= ''
v3= ''
v4= ''

etiqueta1= Label(ventana, text = 'Valor X1')
etiqueta1.grid(row = 2, column = 1)

etiqueta2= Label(ventana, text = 'Valor Y1')
etiqueta2.grid(row = 3, column = 1)

etiqueta3= Label(ventana, text = 'Valor X2')
etiqueta3.grid(row = 2, column = 3)

etiqueta4= Label(ventana, text = 'Valor Y2')
etiqueta4.grid(row = 3, column = 3)

e1 = Entry(ventana, textvariable = v1)
e1.grid(row = 2, column =2)

e2 = Entry(ventana, textvariable = v2)
e2.grid(row = 3, column = 2)

e3 = Entry(ventana, textvariable = v3)
e3.grid(row = 2, column = 4)

e4 = Entry(ventana, textvariable = v4)
e4.grid(row = 3, column = 4)


b1 = Button(ventana, text = 'Graficar un circulo:', command = lambda:poligono(v1,v2,v3,v4))
b1.grid(row =4, column =2)

b2 = Button(ventana, text = 'Salir', command = lambda: ejecutar(ocultar(ventana)))
b2.grid(row= 4, column = 3)
mainloop()