import tkinter
canvas = tkinter.Canvas(width="500", height="500")
canvas.pack()
def kresli(x,y,a):
    f1 = "red"
    f2 = "blue"
    r = 10 * a
    for i in reversed(range(1,r,10)):
        canvas.create_oval(x+i, y+i, x-i, y-i, fill=f1)
        f1, f2 = f2, f1


kresli(50,50, 5)
tkinter.mainloop()