PYTHON FOR KIDS

PYTHON FOR KIDS
PYTHON
FO R K I D S
A P l ay f u l I n t r o d u c t i o n to P r o g r a m m i n g
Jason
R.
Briggs
4
Drawing with Turtles
A turtle in Python is sort of like a turtle in the real
world. We know a turtle as a reptile that moves around
very slowly and carries its house on its back. In the
world of Python, a turtle is a small, black arrow that
moves slowly around the screen. Actually, considering
that a Python turtle leaves a trail as it moves around
the screen, it’s actually less like a turtle and more like
a snail or a slug.
The turtle is a nice way to learn some of the basics
of computer graphics, so in this chapter, we’ll use a
Python turtle to draw some simple shapes and lines.
Python for Kids
©2012, Jason R. Briggs
Using Python’s Turtle Module
A module in Python is a way of
providing useful code to be used
by another program (among other
things, the module can contain
functions we can use). We’ll learn
more about modules in Chapter 7.
Python has a special module called
turtle that we can use to learn
how ­computers draw pictures on a
screen. The turtle module is a way
of programming vector graphics,
which is basically just drawing
with simple lines, dots, and curves.
Let’s see how the turtle works. First, start the Python shell
by clicking the desktop icon (or if you’re using Ubuntu, select
Applications4Programming4IDLE). Next, tell Python to
use the turtle by importing the turtle module, as follows:
>>> import turtle
Importing a module tells Python that you want to use it.
note
If you’re using Ubuntu and you get an error at this point, you might
need to install tkinter. To do so, open the Ubuntu Software Center
and enter python-tk in the search box. “Tkinter – Writing Tk Applications with Python” should appear in the window. Click Install to
install this package.
Creating a Canvas
Now that we have imported the turtle module, we need to create a
canvas—a blank space to draw on, like an artist’s canvas. To do so,
we call the function Pen from the turtle module, which automatically creates a canvas. Enter this into the Python shell:
>>> t = turtle.Pen()
44 Chapter 4
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©2012, Jason R. Briggs
You should see a blank box (the canvas), with an arrow in the
center, something like this:
The arrow in the middle of the screen is the turtle, and you’re
right—it isn’t very turtle-like.
If the Turtle window appears behind the Python Shell window,
you may find that it doesn’t seem to be working properly. When you
move your mouse over the Turtle window, the cursor turns into an
hourglass, like this:
This could happen for several reasons: you haven’t started the
shell from the icon on your desktop (if you’re using Windows or a
Mac), you clicked IDLE (Python GUI) in the Windows Start menu,
Drawing with Turtles 45
Python for Kids
©2012, Jason R. Briggs
or IDLE isn’t installed correctly. Try exiting and restarting the
shell from the desktop icon. If that fails, try using the Python console instead of the shell, as follows:
•
In Windows, select Start4All Programs, and then in the
Python 3.2 group, click Python (command line).
•
In Mac OS X, click the Spotlight icon at the top-right corner
of the screen and enter Terminal in the input box. Then enter
python when the terminal opens.
•
In Ubuntu, open the terminal from your Applications menu
and enter python.
Moving the Turtle
You send instructions to the
turtle by using functions
available on the variable t
we just created, similar to
using the Pen function in the
turtle module. For example,
the forward instruction tells the turtle to move forward. To tell the
turtle to advance 50 pixels, enter the following command:
>>> t.forward(50)
You should see something like this:
46 Chapter 4
Python for Kids
©2012, Jason R. Briggs
The turtle has moved forward 50 pixels. A pixel is a single
point on the screen—the smallest element that can be represented.
Everything you see on your computer monitor is made up of pixels,
which are tiny, square dots. If you could zoom in on the canvas
and the line drawn by the turtle, you would be able to see that
the arrow representing the turtle’s path is just a bunch of pixels.
That’s simple computer graphics.
Dots!
Now we’ll tell the turtle to turn left 90 degrees with the following command:
>>> t.left(90)
If you haven’t learned about degrees yet, here’s how to think
about them. Imagine that you’re standing in the center of a circle.
•
•
•
The direction you’re facing is 0 degrees.
If you hold out your left arm, that’s 90 degrees left.
If you hold out your right arm, that’s 90 degrees right.
You can see this 90-degree turn to the left or right here:
0
90˚
left
90˚
right
Drawing with Turtles 47
Python for Kids
©2012, Jason R. Briggs
If you continue around the circle to the right from where
your right arm is pointing, 180 degrees is directly behind you,
270 degrees is the direction your left arm is pointing, and
360 degrees is back where you started; degrees go from 0 to 360.
The degrees in a full circle, when turning to the right, can be seen
here in 45-degree increments:
0
45
315
90
270
225
135
180
When Python’s turtle turns left, it swivels around to face the
new direction (just as if you turned your body to face where your
arm is pointing 90 degrees left).
The t.left(90) command points the arrow up (since it started
by pointing to the right):
NOTE When you call t.left(90), it’s the same as calling t.right(270). This
is also true of calling t.right(90), which is the same as t.left(270).
Just imagine that circle and follow along with the degrees.
Now we’ll draw a square. Add the following code to the lines
you’ve already entered:
>>> t.forward(50)
>>> t.left(90)
48 Chapter 4
Python for Kids
©2012, Jason R. Briggs
>>>
>>>
>>>
>>>
t.forward(50)
t.left(90)
t.forward(50)
t.left(90)
Your turtle should have drawn a square and should now be
facing in the same direction it started:
To erase the canvas, enter reset. This clears the canvas and
puts the turtle back at its starting position.
>>> t.reset()
You can also use clear, which just clears the screen and leaves
the turtle where it is.
>>> t.clear()
We can also turn our turtle right or move it backward. We can
use up to lift the pen off the page (in other words, tell the turtle to
stop drawing), and down to start drawing. These functions are written in the same way as the others we’ve used.
Let’s try another drawing using some of these commands. This
time, we’ll have the turtle draw two lines. Enter the following code:
>>>
>>>
>>>
>>>
t.reset()
t.backward(100)
t.up()
t.right(90)
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Python for Kids
©2012, Jason R. Briggs
>>>
>>>
>>>
>>>
t.forward(20)
t.left(90)
t.down()
t.forward(100)
First, we reset the canvas and move the turtle back
to its starting position with
t.reset(). Next, we move the
turtle backward 100 pixels with
t.backward(100), and then use
t.up() to pick up the pen and
stop drawing.
Then, with the command
t.right(90), we turn the turtle
right 90 degrees to point down,
toward the bottom of the screen,
and with t.forward(20), we move
forward 20 pixels. Nothing is drawn because of the use of up command on the third line. We turn the turtle left 90 degrees to face
right with t.left(90), and then with the down command, we tell the
turtle to put the pen back down and start drawing again. Finally,
we draw a line forward, parallel to the first line we drew, with
t.forward(100). The two parallel lines we’ve drawn end up looking
like this:
50 Chapter 4
Python for Kids
©2012, Jason R. Briggs
What You Learned
In this chapter, you learned how to use Python’s turtle module.
We drew some simple lines, using left and right turns and forward
and backward commands. You found out how to stop the turtle from
drawing using up, and start drawing again with down. You also discovered that the turtle turns by degrees.
Programming Puzzles
Try drawing some of the following shapes with the turtle. The
answers can be found at http://python-for-kids.com/.
#1: A Rectangle
Create a new canvas using the turtle module’s Pen function and
then draw a rectangle.
#2: A Triangle
Create another canvas, and this time, draw a triangle. Look back
at the diagram of the circle with the degrees (“Moving the Turtle”
on page 46) to remind yourself which direction to turn the turtle
using degrees.
#3: A Box Without Corners
Write a program to draw the four lines shown here (the size isn’t
important, just the shape):
Drawing with Turtles 51
Python for Kids
©2012, Jason R. Briggs
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