A simple event driven paint program which is part of the turtledemo in python

paint.py

from turtle import *

def switchupdown(x = 0, y = 0):
    'this defines how the pen responds to mouse input'
    if pen()["pendown"]:
        end_fill()
        up()
    else:
        down()
        begin_fill()

def changecolor(x=0, y=0):
    global colors
    colors = colors[1:]+colors[:1]
    color(colors[0])

def main():
    'this is the main function which contains all the executted commands'
    global colors
    shape("turtle")
    resizemode("user")
    shapesize(.5)
    width(3)
    colors=["red", "green", "blue", "yellow"]
    color(colors[0])
    switchupdown()
    onscreenclick(goto, 1)
    onscreenclick(changecolor,2)
    onscreenclick(switchupdown, 3)
    return "EVENTLOOP"


if __name__ == "__main__":
    help(switchupdown())
    msg = main()
    print(msg)
    mainloop()