Teach Your Kids To Code: A Parent-friendly Guide to Python Programming

162 Chapter 7

arrow keypresses. The final step is telling the computer to begin

listening for keypresses, which we do with this command:

turtle.listen()

We need this last line for a couple of reasons. First, unlike

with mouse clicks, simply pressing a key doesn’t ensure that our

turtle window will receive the keypress. When you click a window

on your desktop, that window moves to the front and receives the

focus, meaning that window will receive input from the user. When

you click the mouse on the turtle window, it automatically makes

that window the focus of the screen and of any mouse events that

follow. With the keyboard, though, just pressing keys doesn’t make

a window receive those keypresses; the turtle.listen() command

makes sure our turtle’s window is the focus of the desktop so that

it will be able to hear keypresses. Second, the listen() command

tells the computer to start handling keypress events for all keys

that we connected to functions with the onkeypress() function.

Here’s the complete ArrowDraw.py program:

ArrowDraw.py

import turtle

t = turtle.Pen()

t.speed(0)

u t.turtlesize(2,2,2)

def up():

t.forward(50)

def left():

t.left(90)

def right():

t.right(90)

turtle.onkeypress(up, "Up")

turtle.onkeypress(left, "Left")

turtle.onkeypress(right, "Right")

turtle.listen()

At u, the only new line in ArrowDraw.py, we make the

turtle arrow twice as big, and give it a thicker outline with

t.turtlesize(2,2,2). The three parameters are the horizontal

stretch ( 2 means to make it twice as wide), the vertical stretch

( 2 times as tall), and the outline thickness ( 2 pixels thick).

Figure 7-5 shows the result.