使用tkinter如何实现线段的上下来回移动

求解，需要实现红色线段上下来回平移，该如何更新坐标呐

可以参考下这段代码：

import tkinter as tk
import time

class Alien(object):
    def __init__(self, canvas, *args, **kwargs):
        self.canvas = canvas
        self.id = canvas.create_oval(*args, **kwargs)
        self.vx = 5
        self.vy = 0

    def move(self):
        x1, y1, x2, y2 = self.canvas.bbox(self.id)
        if x2 > 400:
            self.vx = -5
        if x1 < 0:
            self.vx = 5
        self.canvas.move(self.id, self.vx, self.vy)

class App(object):
    def __init__(self, master, **kwargs):
        self.master = master
        self.canvas = tk.Canvas(self.master, width=400, height=400)
        self.canvas.pack()
        self.aliens = [
            Alien(self.canvas, 20, 260, 120, 360,
                  outline='white', fill='blue'),
            Alien(self.canvas, 2, 2, 40, 40, outline='white', fill='red'),
        ]
        self.canvas.pack()
        self.master.after(0, self.animation)

    def animation(self):
        for alien in self.aliens:
            alien.move()
        self.master.after(12, self.animation)

root = tk.Tk()
app = App(root)
root.mainloop()