freemarker的assign标签有什么用

from Tkinter import *

def slower():

   global time

   time+=100

def faster():

   global time

   time-=100

root = Tk()

c = Canvas(root,width=300,height=200,bg='white')

c.pack()

time=500

button1=Button(text='slower',command=slower)

button1.pack()

button2=Button(text='faster',command=faster)

button2.pack()

orbit = c.create_oval(50,50,250,150)

sun = c.create_oval(110,85,140,115,fill='red')

earth = c.create_oval(245,95,255,105,fill='blue')

moon = c.create_oval(265,98,270,103)

eX = 250      # earth's X

eY = 100      # earth's Y

m2eX = 20     # moon's X relative to earth

m2eY = 0      # moon's Y relative to earth

t = 0

while True:

   t = t + 0.01*pi

   new_eX = 150 + 100 * cos(t)

   new_eY = 100 - 50 * sin(t)  

   new_m2eX = 20 * cos(12*t)

   new_m2eY = -15 * sin(12*t)

edx = new_eX - eX

   edy = new_eY - eY

   mdx = new_m2eX - m2eX

   mdy = new_m2eY - m2eY

c.move(earth,edx,edy)

   c.move(moon,mdx+edx,mdy+edy)

   c.update()

eX = new_eX

   eY = new_eY

   m2eX = new_m2eX

   m2eY = new_m2eY

c.after(time)

   c.update()