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In this lesson, we will be drawing with the turtle module.

Run Python in interactive mode (write Python in the command line).

$ python

>>>

The characters > and $ are printed by the computer, not by you. On Windows, it will be > instead of $. Before the $ or >, there can be some other words.

Then write:

from turtle import forward

forward(50)

Now a popup window will appear, don't close it. Place it somewhere where you will be able to see it and your command line, too.

If you are a Mac OS X user and have problems with errors like: ModuleNotFoundError: No module named '_tkinter' then TKInter was not yet installed on your system You will need to install it manually by adding following command in your terminal: brew install python-tk@3.(your python version) you can check your version by command python3 --version (it should be for example 3.11.3) so the first command should be for example: brew install python-tk@3.11.3

Where is the turtle?

Currently, the turtle is disguised as an arrow. There is a way how to unmask it:

from turtle import shape

shape('turtle')

Rotation

The turtle can rotate and crawl across the "paper". It has a brush on its tail which draws a line.

from turtle import left, right

forward(50)
left(60)
forward(50)
right(60)
forward(50)

Now give the turtle some commands. If you don't like the drawing you can close the window, or import and use the function clear().

Turtle program

Interactive mode is good for trying new stuff but we will now go back to our editors and write some program in a file.

Create a file ~/pyladies/03/drawing.py.

The directory ~/pyladies can have a different name on your laptop – see Python installation.

You can have a different name for your file, just don't use turtle.py.

Write drawing commands into the file and in the end call the function exitonclick (imported from module turtle).

Question

What does the function exitonclick do?

After you are done, we can start with drawing pictures:

If you are using directly the VS Code terminal for running turtle,

instead of using exitonclick, use turtle.done() to "start the main program" as the last command of your turtle script.

Square

Draw a square.

Turtle square

A square has 4 equal straight sides and 4 90° angles.

Solution

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Rectangle

Draw a rectangle.

Try to make it so that the turtle will "look" to the right in the end (like it was in the beginning).

Turtle rectangle

Solution

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Three squares

Now draw three sqares, each rotated by 20°.

Three turtle squares

Solution

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Can we write it better?

So much code! There has to be a way how to simplify it.

There is. Now we will learn the command for.

Repetition

What does the following code do? Save it as ~/pyladies/03/loop.py

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

for greeting in 'Ahoj', 'Hello', 'Hola', 'Hei', 'SYN':
    print(greeting + '!')

What does the command for do?

Solution

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The command for repeats part of a program. It repeats commands that are indented after for. It's the same as with if - it applies only to commands that are indented below.

The command for x in range(n): repeats the commands below n times and it increments the variable x gradually from 0 to n-1.

The command for x in a, b, c, d, ...: repeats the commands below; it changes the variable x gradually to a, b, c d, ...

In computer programming, a loop is a sequence of instructions that is continually repeated until a condition is reached. Very powerful concept allowing you to significantly reduce duplication of commands.

Overwriting variables

What does the following program do?

sum = 0

for number in 8, 45, 9, 21:
    sum = sum + number

print(sum)

Solution

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Square

Back to drawing! This time we will use loops.

Draw a square.

Use forward only twice, once in the import and once as function.

Turtle square

Solution

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Discontinuous line

The functions penup and pendown from the turtle module tell the turtle to stop/start drawing.

Try to draw a discontinuous line.

Turtle and discontinuous line

Solution

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Now try to make it so that the lines that are drawn become gradually bigger.

Turtle and discontinuous line]

Help

What exactly does the command for do? Can we use the variable that it sets up?

Solution

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Loop variable Naming

Always use a meaningful loop variable name, like for index_tab_browser in range(3, 18): close_tab_in_browser(index_tab_browser) not just i, j, x, y etc. When using meaningful names:

the code is more understandable to colleagues reading your code,
it's easier to find errors in the loop logic
text searches for the variable name return relevant pieces of code operating on the same data are more reliable

There is one exception - when it's a single-level loop and the variable has no meaning other than "the number of times I've been through this loop", in which case i can be used.

Three squares

Finally, draw 3 squares, each rotated by 20°. Now you know how to write it in a simple way: repeat the code by using for, do not copy the code.

Three turtle squares

Solution

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Extra tasks

When you are done, try to draw stairs:

Turtle stairs

When you are also done with the stairs, try to draw 7 hexagons:

Turtle hexagons

← Functions

↑ Lesson: Functions, Modules, Loops

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