motion_explicit.py

# Ball motion with an explicit timer

import simplegui

# Initialize globals
WIDTH = 600
HEIGHT = 400
BALL_RADIUS = 20

init_pos = [WIDTH/2, HEIGHT/2]
vel = [0, 3]    # pixels per tick
time = 9

# define event handlers
def tick():
    global time
    time = time + 1

def draw(canvas):
    # create a list to bold ball position
    ball_pos = [0, 0]

    # calculate ball position
    ball_pos[0] = init_pos[0] + time*vel[0]
    ball_pos[1] = init_pos[1] + time*vel[1]

    # draw ball
    canvas.draw_circle(ball_pos, BALL_RADIUS, 2, "Red", "White")

# create frame
frame = simplegui.create_frame("Motion", WIDTH, HEIGHT)

# regiser event
frame.set_draw_handler(draw)
timer = simplegui.create_timer(100, tick)

# start frame
frame.start()
timer.start()