source: python/trunk/Demo/tkinter/guido/brownian2.py

# Brownian motion -- an example of a NON multi-threaded Tkinter program ;)
# By Michele Simoniato, inspired by brownian.py

from Tkinter import *
import random
import sys

WIDTH = 400
HEIGHT = 300
SIGMA = 10
BUZZ = 2
RADIUS = 2
LAMBDA = 10
FILL = 'red'

stop = 0                                # Set when main loop exits
root = None                             # main window

def particle(canvas):                   # particle = iterator over the moves
    r = RADIUS
    x = random.gauss(WIDTH/2.0, SIGMA)
    y = random.gauss(HEIGHT/2.0, SIGMA)
    p = canvas.create_oval(x-r, y-r, x+r, y+r, fill=FILL)
    while not stop:
        dx = random.gauss(0, BUZZ)
        dy = random.gauss(0, BUZZ)
        try:
            canvas.move(p, dx, dy)
        except TclError:
            break
        else:
            yield None

def move(particle): # move the particle at random time
    particle.next()
    dt = random.expovariate(LAMBDA)
    root.after(int(dt*1000), move, particle)

def main():
    global root, stop
    root = Tk()
    canvas = Canvas(root, width=WIDTH, height=HEIGHT)
    canvas.pack(fill='both', expand=1)
    np = 30
    if sys.argv[1:]:
        np = int(sys.argv[1])
    for i in range(np):                  # start the dance
        move(particle(canvas))
    try:
        root.mainloop()
    finally:
        stop = 1

if __name__ == '__main__':
    main()