Don't forget to get setup before you start:

In the Names Lab, you learned how to write functions with arguments, such as square:

1from turtle import *
3def square(side_length):
4 forward(side_length)
5 right(90)
6 forward(side_length)
7 right(90)
8 forward(side_length)
9 right(90)
10 forward(side_length)
11 right(90)

This is a powerful function--instead of repeating the code block every time you want to draw a square, you can just call square(50), square(100), or whatever size square you want.

But what if you want to draw a lot of squares? In this lab you will learn about iteration, the process of doing something multiple times.


A list is a data structure which holds any number of values, separated by commas and surrounded by square brackets. For example, [], [50], and [100, 200, 300, 400] are all lists of numbers.

Once you create a list, you can iterate through it using a "for-loop". (Iteration is the process of going through a collection of things one at a time.) Line 13 goes through sizes, one number at a time. Each number is temporarily assigned to the variable size, and then the code block runs.

12sizes = [20, 40, 60, 80, 100]
13for size in sizes:
14 square(size)
Five squares, of different sizes

Sometimes we just want to repeat the code block, and we don't even use the temporarily-assigned variable. The example below doesn't use the temporarily-assigned variable size, so the values of the numbers in the list don't matter. All that matters is how many numbers there are in the list.

15for size in sizes:
16 square(20)
17 penup()
18 forward(40)
19 pendown()
Five squares, of different sizes


Lists are great, but what if you want to draw a lot of squares? You don't necessarily want to type out a 100-item list just to iterate over it. Python has a built-in function called range which automatically creates a list-like object. range can be called in a few different ways.

๐Ÿ’ป Run python to open Python's interactive mode. Enter the following commands to try out using ranges.

range(max) generates the numbers from 0 up to max (not including max).

>>> for number in range(5):
...     print(number)

range(min, max) generates the numbers from min up to max (not including max)

>>> for number in range(5, 10):
...     print(number)

range(min, max, stride) generates the numbers from min up to max, counting by stride

>>> for number in range(0, 100, 20):
...     print(number)

Draw with ranges provides an example of what you can do with ranges. This program takes a function to draw a single tile, draw_tile from, and draws it over and over in a grid.

๐Ÿ’ป Run python --help to find out how to use this program.

(lab-iteration-py3.11) lab_iteration % python --help
usage: python [-h] [--fast] width height size

Draws a grid of tiles.

positional arguments:
  width       How many tiles across the grid should be
  height      How many tiles high the grid should be
  size        Side length of each tile

  -h, --help  show this help message and exit
  --fast      Skip turtle animation and show the result

The first line of the help message shows how to run the program. You need to provide three arguments: width, height, and size, and can optionally also add the --fast flag. The rest of the help message explains what each argument does.

๐Ÿ’ป Run python 5 3 40. If you're impatient, add --fast.

Try running with different arguments. Then open and try changing the draw_tile() function. Create your own tile pattern.