Today's lab will focus on turtle graphics. An excellent overview of turtle graphics can be found at the http://interactivepython.org/runestone/static/thinkcspy/PythonTurtle/helloturtle.html.

Turtles

Turtle graphics was originally introduced in 1966 as part of the Logo programming language and is available as a http://docs.python.org/3.2/library/turtle.html. The turtle module allows you to draw graphics to the screen.

Let's start with a simple program: 

import turtle

def main():
    daniel = turtle.Turtle()    #Set up a turtle named "daniel"
    myWin = turtle.Screen()     #The graphics window

    #Draw a square
    for i in range(4):
        daniel.forward(100)     #Move forward 10 steps
        daniel.right(90)        #Turn 90 degrees to the right

    myWin.exitonclick()         #Close the window when clicked
    
main()
The first line: 
import turtle
loads the turtle graphics module. Like the math library, the turtle graphics library is a standard part of python but needs to be imported to be used in your program.

We next define the main() function and set up two turtle graphics objects: 

    daniel = turtle.Turtle()    #Set up a turtle named "daniel"
    myWin = turtle.Screen()     #The graphics window
The first line creates a turtle, named daniel. The next line creates a new window, where the graphics will appear.

The main() function has a loop that has two commands:

The last line in our main() program closes the graphics window when the user clicks the mouse on the window.

What happens when these commands are repeated 4 times? Try running the program to see what is drawn.

Let's make two small changes to our program:

Your program should now look like: 
import turtle

def main():
    daniel = turtle.Turtle()    #Set up a turtle named "daniel"
    myWin = turtle.Screen()     #The graphics window

    #Draw a square
    for i in range(50):
        daniel.forward(i)       #Move forward 10 steps
        daniel.right(90)        #Turn 90 degrees to the right

    myWin.exitonclick()         #Close the window when clicked
    
main()

Run you program (choose "Run Module" from the "Run" menu, or F5). What is drawn? How can you make it fill the entire window?

What happens if we had left the steps forward never changing (but a bit shorter (10 instead of 100) so it appears on the screen), but instead changed the right turn from 90 degrees to i degrees: 

import turtle

def main():
    daniel = turtle.Turtle()    #Set up a turtle named "daniel"
    myWin = turtle.Screen()     #The graphics window

    #Draw a square
    for i in range(50):
        daniel.forward(10)      #Move forward 10 steps
        daniel.right(i)         #Turn 90 degrees to the right

    myWin.exitonclick()         #Close the window when clicked
    
main()

Some basic turtle commands are:

With the remaining time, work on the homework programs.