Lesson 12: Mouse Input¶

In this lesson we will learn

how to make the turtle respond to mouse clicks
how to use if, elif and else with Boolean operators to decide what happens
how to use coordinates to work out where the user clicked

Terminology

mouse input – information a program gets from the mouse, such as where the user clicked.
quadrant – one of the four sections a window is split into by lines through its centre.

There is no video for this lesson.

Mouse input in turtle¶

So far, the user has only typed input into the Shell. But turtle can also take input from the mouse (and even the keyboard).

Type the code below into a new file, or open mouse_dots.py from the lesson_12 folder of the tutorial files. Save it as mouse_dots.py.

import turtle


def set_scene():
    turtle.setup(800, 600)
    turtle.onscreenclick(draw_dot)
    my_ttl.speed(0)
    for index in range(2):
        my_ttl.forward(400)
        my_ttl.back(400)
        my_ttl.right(90)
        my_ttl.forward(300)
        my_ttl.back(300)
        my_ttl.right(90)
    my_ttl.penup()


def draw_dot(x, y):
    print(x, y)
    color = "orange"
    size = 10
    my_ttl.goto(x, y)
    my_ttl.dot(size, color)


my_ttl = turtle.Turtle()
set_scene()
my_ttl.hideturtle()
turtle.done()

PRIMM

Predict what will happen when we run the code and click in the turtle window.
Run the code, then click in different parts of the turtle window. Watch the Shell too. Did it match your prediction?
Time to investigate the code. What does each line do?

Code explanation

line 1 → imports the turtle module.
line 4 → defines a function called set_scene, which gets the window ready.
line 5 → makes a window 800 pixels wide and 600 pixels high.
line 6 → tells turtle to run the draw_dot function whenever the mouse is clicked in the window.
line 7 → sets my_ttl to its fastest speed, so the lines appear straight away.
line 8 → starts a for loop that repeats twice.
lines 9–10 → draw a line 400 long, from the centre to the side of the window, and back again.
line 11 → turns my_ttl 90 degrees to the right.
lines 12–13 → draw a line 300 long, from the centre to the top or bottom of the window, and back again.
line 14 → turns my_ttl 90 degrees to the right.
line 15 → lifts the pen, so my_ttl won't draw a line when it moves to a click.
line 18 → defines a function called draw_dot with two parameters, x and y: the position of the mouse click.
line 19 → prints the click's x and y position in the Shell.
line 20 → stores "orange" in color.
line 21 → stores 10 in size.
line 22 → moves my_ttl to the click's position.
line 23 → draws a dot where my_ttl is, using the size in size and the colour in color.
line 26 → creates a turtle called my_ttl.
line 27 → calls set_scene to set up the window.
line 28 → hides my_ttl.
line 29 → keeps the window open and waiting for mouse clicks.

Python runs this program in a different order from the way it's written:

Lines 26–29 run first: they create the turtle, set up the window, hide the turtle and wait for clicks.
Lines 4–15, the set_scene function, run when line 27 calls it. The four lines it draws split the window into four sections, called quadrants.
Lines 18–23, the draw_dot function, run every time we click. We never call draw_dot ourselves: line 6 tells turtle to call it for us, and turtle always sends it the click's x and y position.

PRIMM

Time to modify the code. What happens if you comment out line 15? Why?

Exercises¶

In this course, the exercises are the make part of PRIMM. Work through them to make your own code.

Right now, every dot is orange. In these exercises, the quadrant we click in will decide the dot's colour. To do this, we'll need:

if, elif and else statements
comparison operators
Boolean operators

We also need to remember how turtle coordinates work: (0, 0) is the centre, x is bigger to the right, and y is bigger towards the top.

Turtle window coordinates, with (0, 0) in the centre

Starter files are in the lesson_12 folder of the tutorial files. Solutions are on the Exercise Solutions page.

Exercise 1¶

Starter: lesson_12/ex1_top_right

Can you add an if statement to draw_dot, after color = "orange", so the dot is red when we click in the top-right quadrant? To make the dot red, use color = "red".

Hint

Look at the coordinates diagram. What is true about x and y for every point in the top-right quadrant? We need to check both, so we'll need a Boolean operator.

Exercise 2¶

Starter: lesson_12/ex2_if_else

Can you use if and else so the dot is red when we click in the top-right quadrant, and green when we click anywhere else?

Exercise 3¶

Starter: lesson_12/ex3_quadrants

Can you use if, elif and else so each quadrant has its own colour? The dot should be:

red in the top-right quadrant
blue in the top-left quadrant
yellow in the bottom-left quadrant
green in the bottom-right quadrant