from re import match

def isBinaryRepresentation(s):
    """Decide whether a string contains the binary rep. of a number >1.

    Leading zeros are allowed."""
    #match puts us at the start of the string
    #ensures we end at the end of the string (not fooled by test case 4)
    return match("0*1[01]+$", s)

def testIsBinaryRepresentation():
    assert not isBinaryRepresentation("00")
    assert not isBinaryRepresentation("1")
    assert not isBinaryRepresentation("01")
    assert not isBinaryRepresentation("1010 is the binary rep. of a number.")
    assert not isBinaryRepresentation("this is CS: 1011")
    
    assert isBinaryRepresentation("10")
    assert isBinaryRepresentation("0010")
    assert isBinaryRepresentation("11")
    assert isBinaryRepresentation("10010110")

testIsBinaryRepresentation()

from Tkinter import *
from random import randint, randrange, choice

def randomColor():
    """Return a random color formatted in Tkinter's #XXXXXX style."""
    #eg orange is "#ff007f" (full red and half green)
    return "#%2x%2x%2x"%tuple(randint(0,255) for i in xrange(3))

def randomCoords(width, height):
    a=randrange(width/4)
    b=randrange(height/5)
    left=-width/10
    top=randrange(-height/10, height)
    return left, top, left+a, top+b

def timerFired(canvas):
     
    width, height=canvas.winfo_width(), canvas.winfo_height()

    canvas.delete(canvas.data.bg)

    #ugle hack to get around the issue that the winfo_ functions get the
    #dimensions wrong the first time we call them
    flag=canvas.data.bg==0

    canvas.move(ALL, 2, 1)
    
    #apologies--Steven's fault
    canvas.data.bg=canvas.create_rectangle(0, 0, width, height, width=0,
                                           fill=randomColor())
    canvas.tag_lower(canvas.data.bg, ALL)
    
    #canvas.after(delay, function, arguments...)
    #after delay milliseconds, call function with arguments
    canvas.after(200, timerFired, canvas)

    if flag: return

    #the color I use for highliting lambdas
    periwinkle="#8833ff"
    canvas.create_oval(randomCoords(width, height), outline="white",
                       fill=choice(("red", "blue", periwinkle, "goldenrod")))
                            

def run():
    root=Tk()
    width=400
    height=400
    backgroundColor="red"
    canvas=Canvas(width=width, height=height, bg=backgroundColor)
    canvas.pack()

    canvas.data=type("", (), {'bg':0})

    timerFired(canvas)
    root.mainloop()
run()