1.4. Compute with Turtles

The idea of “turtle programming” dates back to the 1960’s and originated with Seymour Papert. He developed a robot turtle with a physical pen in it. Children would steer the robot around and create drawings with the pen by writing programs in a language called Logo.

Children playing with a Logo turtle robot that could draw with a pen

Today, we can play with virtual turtles in a fully-graphical and non-robotic way. To do so, we will make use of another feature of Python - code libraries. A library is a collection of existing code designed to help programs perform some task. Using a library means we do not have to figure out how to handle all the details of a particular problem - the library code will take care of many of those details for us. In the case of making a virtual turtle, our program will need to have a way to keep track of where the turtle is, a way to move the turtle around, a way to draw to the screen, etc… The turtle library will handle these details for us.

To use a library, we need to tell Python we want to use the library with an import statement. In the program below, we start with from turtle import * to tell Python we want to make use of the turtle library. Then it creates a Screen, a space on the page for the turtle to move in and draw on (space = Screen()). Next it creates a turtle named alex (alex = Turtle()). The turtles that we make are objects that have behaviors we can access with dot-notation. We use these to tell the turtle alex to move around on the screen using commands like: alex.forward(150). As the turtle moves around it draws a line behind itself.

Try clicking the button below to see what the following program does.

from turtle import * # use the turtle library space = Screen() # create a turtle space alex = Turtle() # create a turtle named alex alex.forward(150) # move forward by 150 units alex.left(90) # turn by 90 degrees alex.forward(75) # move forward by 75 units

Which direction will alex move when the code below executes?

from turtle import *
space = Screen()
alex = Turtle()
alex.forward(100)

• Up
• Check which way alex moved first in the example above
• Right
• Check which way alex moved first in the example above
• Down
• Check which way alex moved first in the example above
• Left
• Turtles start off facing right by default

Just by going forward, backward, left, and right, we can have a turtle draw a shape.

What shape will the program below draw when you click on the Run button?

from turtle import *    # use the turtle library
space = Screen()        # create a turtle screen (space)
zari = Turtle()         # create a turtle named zari
zari.setheading(90)     # Point due north
zari.forward(100)       # tell zari to move forward by 100 units
zari.right(90)          # turn by 90 degrees
zari.forward(100)       # tell zari to move forward by 100 units
zari.right(90)          # turn by 90 degrees
zari.forward(100)       # tell zari to move forward by 100 units
zari.right(90)          # turn by 90 degrees
zari.forward(100)       # tell zari to move forward by 100 units
zari.right(90)          # turn by 90 degrees

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