Visualizing a 2D random walk using Python, Cairo and GTK

rwalk.py

#!/usr/bin/python
"""
Random walk on two-dimensional grid
(c) Jyotirmoy Bhattacharya, jyotirmoy@jyotirmoy.net
LICENSE: GPL3
"""

import random
import gtk,gobject,cairo
import math

N = 51 #no of lattice points along each dimension
S = 2   #seconds to sleep between frames

points = set()
cur = (N//2+1,N//2+1)
mycanvas = None


def add_point():
    global points,cur
    inc = random.choice((-1,1))
    d = random.choice(('v','h'))
    nx = cur[0]
    ny = cur[1]
    if d=='h':
        nx = (nx+inc)%(N+1)
    else:
        ny = (ny+inc)%(N+1)
    nxt = (nx,ny)
    points.add(nxt)
    cur = nxt

def tick():
    global mycanvas
    add_point()
    mycanvas.queue_draw()
    return True

class Canvas(gtk.DrawingArea):
    # Draw in response to an expose-event
    __gsignals__ = { "expose-event": "override" }

    # Handle the expose-event by drawing
    def do_expose_event(self, event):

        # Create the cairo context
        cr = self.window.cairo_create()

        # Restrict Cairo to the exposed area; avoid extra work
        cr.rectangle(event.area.x, event.area.y,
                event.area.width, event.area.height)
        cr.clip()

        self.draw(cr, *self.window.get_size())

    def draw(self, cr, width, height):
        scale_x = float(width)/(N+1)
        scale_y = float(height)/(N+1)
        # Draw lines with gray
        cr.set_source_rgb(0.5, 0.5, 0.5)
        for p in points:
            if p==cur:
                continue
            cr.arc(p[0]*scale_x,p[1]*scale_y,2,0,2*math.pi)
            cr.fill()

        cr.set_source_rgb(1,0,0)
        cr.arc(cur[0]*scale_x,cur[1]*scale_y,2,0,2*math.pi)
        cr.fill()
        


if __name__ == "__main__":
    window = gtk.Window()
    window.connect("delete-event", gtk.main_quit)
    mycanvas = Canvas()
    mycanvas.show()
    window.add(mycanvas)
    window.present()
    gobject.timeout_add_seconds(S,tick)
    gtk.main()