Turtle Steps

In this lesson, you will use Python Turtle to draw lines and regular shapes. You will also store values in variables so you can change a drawing more easily.

Meet Python Turtle

Python Turtle lets your code control a small arrow on a drawing canvas. Imagine that the arrow is carrying a pen. When it moves, it leaves a line behind.

We first import the Turtle drawing tools:

from turtle import *

Then we can give direct instructions:

• forward(80) moves forward by 80 steps;
• right(90) turns clockwise;
• instructions run from top to bottom, one at a time.

The number inside the brackets gives the instruction some information. You can change a number and run the cell again to see what changes.

What is a variable?

A variable is a name that stores a value while a program runs. A clear name helps us remember what the value is for.

side_length = 80
forward(side_length)

The first line stores 80 using the name side_length. The next line uses the stored value. If you change 80 once, every instruction that uses side_length can use the new value.

Python variable names cannot contain spaces, so an underscore _ can join words together.

Run a first example

Run the code. Watch the Turtle draw two sides of a square. The same variable controls both side lengths.

Finish and change the square

Add the missing movement and turn instructions to close the square. Then change side_length and run it again.

When that works, add a rectangle beside it. Lift the pen before moving to the new starting position.

penup()
goto(120, 0)
pendown()

Turning around a shape

A regular shape has equal sides and an equal turn at each corner.

At each corner, the Turtle turns on the outside of the shape. This is called the external angle.

When a complete regular shape closes, all its outside turns add up to one full turn: 360 degrees. Share that full turn equally between the sides:

turn_angle = 360 / sides

You do not need to remember a separate turn angle for every shape. Store the number of sides and let Python calculate it.

Build a triangle

A regular shape turns through a full 360 degrees. Find the external turn angle with:

turn_angle = 360 / sides

The code provided draws only one side and one turn. Add what is needed to close the triangle. Do not replace the calculation with a remembered angle.

Work out two more shapes

The next two cells give you only the number of sides. For each shape, you need to:

1. choose and store a side length;
2. calculate the external turn angle;
3. add enough movement and turn instructions to close the shape.

Do not use a for loop yet. Lesson 2 will show you how a for loop makes this repeated code shorter.

Pentagon

Complete the cell to draw a regular pentagon. Choose a side length that will fit comfortably on the canvas.

Hexagon

Now complete a new program for a regular hexagon. Work it out from sides rather than copying a turn angle from somewhere else.

Core check

You are ready to move on when you can:

• use a variable in more than one instruction;
• close a square, triangle, pentagon and hexagon;
• calculate an external turn using 360 / sides.

Extension: draw a five-point star

A five-point star needs five equal lines. It turns across the middle instead of following the outside of a regular pentagon. This means 360 / number_of_points will not make the star.

Add variables for line_length and turn_angle. Then add the same movement-and-turn pair five times. Run the cell and adjust only turn_angle until the points meet.