CN101

Turtle Graphics

Introduction to Turtle Graphics

• Python's turtle graphics system displays a small cursor

known as a turtle.

• You can use Python statements to move the turtle

around the screen, drawing lines and shapes.

Introduction to Turtle Graphics

• To use the turtle graphics system, you must import the

turtle module with this statement:

import turtle

This loads the turtle module into memory

Moving the Turtle Forward

• Use the turtle.forward(n) statement to move the turtle

forward n pixels.

>>> import turtle

>>> turtle.forward(100)

>>>

Turning the Turtle

• The turtle's initial heading is 0 degrees (east)

• Use the turtle.right(angle) statement to turn

the turtle right by angle degrees.

• Use the turtle.left(angle) statement to turn

the turtle left by angle degrees.

Turning the Turtle

>>> import turtle

>>> turtle.forward(100)

>>> turtle.left(90)

>>> turtle.forward(100)

>>>

Turning the Turtle

>>> import turtle

>>> turtle.forward(100)

>>> turtle.right(45)

>>> turtle.forward(100)

>>>

Setting the Turtle's Heading

• Use the turtle.setheading(angle) statement

to set the turtle's heading to a specific angle.

>>> import turtle

>>> turtle.forward(50)

>>> turtle.setheading(90)

>>> turtle.forward(100)

>>> turtle.setheading(180)

>>> turtle.forward(50)

>>> turtle.setheading(270)

>>> turtle.forward(100)

>>>

Setting the Pen Up or Down

• When the turtle's pen is down, the turtle draws a line as

it moves. By default, the pen is down.

• When the turtle's pen is up, the turtle does not draw as

it moves.

• Use the turtle.penup() statement to raise the pen.

• Use the turtle.pendown() statement to lower the pen.

Setting the Pen Up or Down

>>> import turtle

>>> turtle.forward(50)

>>> turtle.penup()

>>> turtle.forward(25)

>>> turtle.pendown()

>>> turtle.forward(50)

>>> turtle.penup()

>>> turtle.forward(25)

>>> turtle.pendown()

>>> turtle.forward(50)

>>>

Drawing Circles

• Use the turtle.circle(radius) statement to

draw a circle with a specified radius.

>>> import turtle

>>> turtle.circle(100)

>>>

Drawing Dots

• Use the turtle.dot() statement to draw a simple

dot at the turtle's current location.

>>> import turtle

>>> turtle.dot()

>>> turtle.forward(50)

>>> turtle.dot()

>>> turtle.forward(50)

>>> turtle.dot()

>>> turtle.forward(50)

>>>

Changing the Pen Size and Drawing

Color

• Use the turtle.pensize(width) statement to change the

width of the turtle's pen, in pixels.

• Use the turtle.pencolor(color) statement to change the

turtle's drawing color.

• See Appendix D in your textbook for a complete list of colors.

>>> import turtle

>>> turtle.pensize(5)

>>> turtle.pencolor('red')

>>> turtle.circle(100)

>>>

Working with the Turtle's Window

• Use the turtle.bgcolor(color) statement to set the

window's background color.

• See Appendix D in your textbook for a complete list of colors.

• Use the turtle.setup(width, height) statement to set

the size of the turtle's window, in pixels.

• The width and height arguments are the width and height, in pixels.

• For example, the following interactive session creates a graphics window

that is 640 pixels wide and 480 pixels high:

>>> import turtle

>>> turtle.setup(640, 480)

>>>

Resetting the Turtle's Window

• The turtle.reset() statement:

• Erases all drawings that currently appear in the graphics window.

• Resets the drawing color to black.

• Resets the turtle to its original position in the center of the screen.

• Does not reset the graphics window’s background color.

• The turtle.clear() statement:

• Erases all drawings that currently appear in the graphics window.

• Does not change the turtle's position.

• Does not change the drawing color.

• Does not change the graphics window’s background color.

• The turtle.clearscreen() statement:

• Erases all drawings that currently appear in the graphics window.

• Resets the drawing color to black.

• Resets the turtle to its original position in the center of the screen.

• Resets the graphics window’s background color to white.

Working with Coordinates

• The turtle uses Cartesian Coordinates

Moving the Turtle to a Specific

Location

• Use the turtle.goto(x, y) statement to move

the turtle to a specific location.

>>> import turtle

>>> turtle.goto(0, 100)

>>> turtle.goto(−100, 0)

>>> turtle.goto(0, 0)

>>>

• The turtle.pos() statement displays the turtle's current X,Y coordinates.

• The turtle.xcor() statement displays the turtle's current X coordinate and the

turtle.ycor() statement displays the turtle's current Y coordinate.

Animation Speed

• Use the turtle.speed(speed) command to

change the speed at which the turtle moves.

• The speed argument is a number in the range of 0 through

10.

• If you specify 0, then the turtle will make all of its moves

instantly (animation is disabled).

Hiding and Displaying the Turtle

• Use the turtle.hideturtle() command to hide

the turtle.

• This command does not change the way graphics are drawn,

it simply hides the turtle icon.

• Use the turtle.showturtle() command to

display the turtle.

Displaying Text

• Use the turtle.write(text) statement to display

text in the turtle's graphics window.

• The text argument is a string that you want to display.

• The lower-left corner of the first character will be positioned

at the turtle’s X and Y coordinates.

Displaying Text

>>> import turtle

>>> turtle.write('Hello World')

>>>

Filling Shapes

• To fill a shape with a color:

• Use the turtle.begin_fill() command before

drawing the shape

• Then use the turtle.end_fill() command after the

shape is drawn.

• When the turtle.end_fill() command executes, the

shape will be filled with the current fill color

Filling Shapes

>>> import turtle

>>> turtle.hideturtle()

>>> turtle.fillcolor('red')

>>> turtle.begin_fill()

>>> turtle.circle(100)

>>> turtle.end_fill()

>>>

Keeping the Graphics Window Open

• When running a turtle graphics program outside IDLE, the

graphics window closes immediately when the program is done.

• To prevent this, add the turtle.done() statement to the very

end of your turtle graphics programs.

• This will cause the graphics window to remain open, so you can see its

contents after the program finishes executing.

Turtle Graphics: Determining the

State of the Turtle

• The turtle.xcor() and turtle.ycor() functions return

the turtle's X and Y coordinates

• Examples of calling these functions in an if statement:

if turtle.xcor() > 100 and turtle.xcor() < 200:

turtle.goto(0, 0)

if turtle.ycor() < 0:

turtle.goto(0, 0)

Turtle Graphics: Determining the

State of the Turtle

• The turtle.heading() function returns the turtle's heading.

(By default, the heading is returned in degrees.)

• Example of calling the function in an if statement:

if turtle.heading() >= 90 and turtle.heading() <= 270:

turtle.setheading(180)

Turtle Graphics: Determining the

State of the Turtle

• The turtle.isdown() function returns True if the pen is

down, or False otherwise.

• Example of calling the function in an if statement:

if turtle.isdown():

turtle.penup()

if not(turtle.isdown()):

turtle.pendown()

Turtle Graphics: Determining the

State of the Turtle

• The turtle.isvisible() function returns True if the

turtle is visible, or False otherwise.

• Example of calling the function in an if statement:

if turtle.isvisible():

turtle.hideturtle()

Turtle Graphics: Determining the

State of the Turtle

• When you call turtle.pencolor() without passing an

argument, the function returns the pen's current color as a string.

Example of calling the function in an if statement:

• When you call turtle.fillcolor() without passing an

argument, the function returns the current fill color as a string.

Example of calling the function in an if statement:

if turtle.pencolor() == 'red':

turtle.pencolor('blue')

if turtle.fillcolor() == 'blue':

turtle.fillcolor('white')

Turtle Graphics: Determining the

State of the Turtle

• When you call turtle.bgcolor() without passing an

argument, the function returns the current background color as a

string. Example of calling the function in an if statement:

if turtle.bgcolor() == 'white':

turtle.bgcolor('gray')

Turtle Graphics: Determining the

State of the Turtle

• When you call turtle.pensize() without passing an

argument, the function returns the pen's current size as a string.

Example of calling the function in an if statement:

if turtle.pensize() < 3:

turtle.pensize(3)

Turtle Graphics: Determining the

State of the Turtle

• When you call turtle.speed() without passing an argument,

the function returns the current animation speed. Example of

calling the function in an if statement:

if turtle.speed() > 0:

turtle.speed(0)

Turtle Graphics: Determining the

State of the Turtle

• See In the Spotlight: The Hit the Target Game in your textbook for

numerous examples of determining the state of the turtle.

Turtle Graphics: Using Loops to Draw

Designs

• You can use loops with the turtle to draw both simple shapes and

elaborate designs. For example, the following for loop iterates

four times to draw a square that is 100 pixels wide:

for x in range(4):

turtle.forward(100)

turtle.right(90)

Turtle Graphics: Using Loops to Draw

Designs

• This for loop iterates eight times to draw the

octagon:

for x in range(8):

turtle.forward(100)

turtle.right(45)

Turtle Graphics: Using Loops to Draw

Designs

• You can create interesting designs by repeatedly drawing a

simple shape, with the turtle tilted at a slightly different angle

each time it draws the shape.

NUM_CIRCLES = 36 # Number of circles to draw

RADIUS = 100 # Radius of each circle

ANGLE = 10 # Angle to turn

for x in range(NUM_CIRCLES):

turtle.circle(RADIUS)

turtle.left(ANGLE)

Turtle Graphics: Using Loops to Draw

Designs

• This code draws a sequence of 36 straight lines to

make a "starburst" design.

START_X = -200 # Starting X coordinate

START_Y = 0 # Starting Y coordinate

NUM_LINES = 36 # Number of lines to draw

LINE_LENGTH = 400 # Length of each line

ANGLE = 170 # Angle to turn

turtle.hideturtle()

turtle.penup()

turtle.goto(START_X, START_Y)

turtle.pendown()

for x in range(NUM_LINES):

turtle.forward(LINE_LENGTH)

turtle.left(ANGLE)

Turtle Graphics: Modularizing Code

with Functions

• Commonly needed turtle graphics operations can be stored in

functions and then called whenever needed.

• For example, the following function draws a square. The

parameters specify the location, width, and color.

def square(x, y, width, color):

turtle.penup() # Raise the pen

turtle.goto(x, y) # Move to (X,Y)

turtle.fillcolor(color) # Set the fill color

turtle.pendown() # Lower the pen

turtle.begin_fill() # Start filling

for count in range(4): # Draw a square

turtle.forward(width)

turtle.left(90)

turtle.end_fill() # End filling

Turtle Graphics: Modularizing Code

with Functions

• The following code calls the previously shown square function

to draw three squares:

square(100, 0, 50, 'red')

square(-150, -100, 200, 'blue')

square(-200, 150, 75, 'green')

Turtle Graphics: Modularizing Code

with Functions

• The following function draws a circle. The parameters specify the

location, radius, and color.

def circle(x, y, radius, color):

turtle.penup() # Raise the pen

turtle.goto(x, y - radius) # Position the turtle

turtle.fillcolor(color) # Set the fill color

turtle.pendown() # Lower the pen

turtle.begin_fill() # Start filling

turtle.circle(radius) # Draw a circle

turtle.end_fill() # End filling

Turtle Graphics: Modularizing Code

with Functions

• The following code calls the previously shown circle function

to draw three circles:

circle(0, 0, 100, 'red')

circle(-150, -75, 50, 'blue')

circle(-200, 150, 75, 'green')

Turtle Graphics: Modularizing Code

with Functions

• The following function draws a line. The parameters specify the

starting and ending locations, and color.

def line(startX, startY, endX, endY, color):

turtle.penup() # Raise the pen

turtle.goto(startX, startY) # Move to the starting point

turtle.pendown() # Lower the pen

turtle.pencolor(color) # Set the pen color

turtle.goto(endX, endY) # Draw a square

Turtle Graphics: Modularizing Code

with Functions

• The following code calls the previously shown line function to

draw a triangle:

TOP_X = 0

TOP_Y = 100

BASE_LEFT_X = -100

BASE_LEFT_Y = -100

BASE_RIGHT_X = 100

BASE_RIGHT_Y = -100

line(TOP_X, TOP_Y, BASE_LEFT_X, BASE_LEFT_Y, 'red')

line(TOP_X, TOP_Y, BASE_RIGHT_X, BASE_RIGHT_Y, 'blue')

line(BASE_LEFT_X, BASE_LEFT_Y, BASE_RIGHT_X, BASE_RIGHT_Y, 'green')