Python Turtle - Lesson 6

Part 1: Boolean logic

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Boolean Introduction

In programming, Boolean means working with two possible values: True and False.

  • A Boolean variable can only store True or False

  • Comparison operators (==, !=, >, <, >=, <=) check something and give back either True or False

  • Boolean operators (you will learn these soon) also give back either True or False

The values True and False are special in Python. If you type them into your code editor, they will look different (highlighted) to show they are important.

In Python, checking if something is True or False is called testing its truthiness. When you compare two values, you are checking whether the statement is true or not.

Comparison operators

The conditions in if and while statements check if something is True or False. They do this using comparison operators.

Let’s quickly review these.

There are six comparison operators you can use. Create a new file called lesson_6_pt_1.py and type in the code below.

1print("jeff" == "jeff")  # equal to
2print(1 != 1)  # not equal to
3print(500 > 300)  # greater than
4print(100 >= 250)  # greater than or equal to
5print("a" < "q")  # less than
6print(-30 <= 3)  # less than or equal to

Predict and Run

  • Predict what the six results will be (hint: each one will be either True or False)

  • Run your code and check if your predictions were correct

Modify

Try changing the values in each comparison to make the result switch.

  • If a line gives True, change the values so it gives False

  • If a line gives False, change the values so it gives True

It does not matter if the values are written directly (like numbers in the code) or stored in a variable — the result will still work the same way.

Change your code so it matches the code below.

1score = 10
2print(score > 5)

Predict and Run

  • Predict whether the code will print True or False

  • Run the code and check if you were correct

Boolean Operations

You can also do operations using Boolean values by using Boolean operators.

These work a bit like maths, but instead of numbers, they use True and False. The result will always be either True or False.

They are useful when you want to check more than one condition at the same time.

There are three Boolean operators:

  • and

  • or

  • not

The not operator

The easiest operator to understand is not. It simply flips the value:

  • not True becomes False

  • not False becomes True

Change your code so it matches the code below.

1print("not True is:", not True)
2print("not False:", not False)

Predict and Run

  • Predict what you think will be printed in the Shell when you run the code

  • Run the code and check if your prediction was correct

The and operator

The and and or operators are a bit more tricky.

The and operator will only return True if every value is True.

Change your code so it matches the code below.

1print("True and True is:", True and True)
2print("True and False is:", True and False)
3print("False and True is:", False and True)
4print("False and False is:", False and False)
5print("True and True and True is:", True and True and True)
6print("True and True and False is:", True and True and False)

Predict and Run

  • Predict what you think will be printed in the Shell when you run the code

  • Run the code and check if your prediction was correct

Investigate - Code breakdown

  • Line 1: print("True and True is:", True and True)

    • Both values are True

    • and checks if everything is True → this is True

    • It prints: True and True is: True

  • Line 2: print("True and False is:", True and False)

    • One value is False

    • and needs everything to be True, so this is False

    • It prints: True and False is: False

  • Line 3: print("False and True is:", False and True)

    • One value is False

    • Not everything is True, so this is False

    • It prints: False and True is: False

  • Line 4: print("False and False is:", False and False)

    • Both values are False

    • Not everything is True, so this is False

    • It prints: False and False is: False

  • Line 5: print("True and True and True is:", True and True and True)

    • All values are True

    • So the result is True

    • It prints: True and True and True is: True

  • Line 6: print("True and True and False is:", True and True and False)

    • One value is False

    • Not everything is True, so this is False

    • It prints: True and True and False is: False

The or operator

The or operator works in the opposite way to and.

The or operator will return True if at least one value is True.

Change your code so it matches the code below.

1print("True or True is:", True or True)
2print("True or False is:", True or False)
3print("False or True is:", False or True)
4print("False or False is:", False or False)
5print("True or True or True is:", True or True or True)
6print("True or False or False is:", True or False or False)

Predict and Run

  • Predict what you think will be printed in the Shell when you run the code

  • Run the code and check if your prediction was correct

Investigate - Code breakdown

  • Line 1: print("True or True is:", True or True)

    • At least one value is True (both are!)

    • or returns True

    • It prints: True or True is: True

  • Line 2: print("True or False is:", True or False)

    • One value is True

    • or returns True

    • It prints: True or False is: True

  • Line 3: print("False or True is:", False or True)

    • One value is True

    • or returns True

    • It prints: False or True is: True

  • Line 4: print("False or False is:", False or False)

    • No values are True

    • or returns False

    • It prints: False or False is: False

  • Line 5: print("True or True or True is:", True or True or True)

    • All values are True

    • or returns True

    • It prints: True or True or True is: True

  • Line 6: print("True or True or False is:", True or True or False)

    • At least one value is True

    • or returns True

    • It prints: True or True or False is: True

Using Boolean operators

So far, we have only used True and False with other True and False values. That is not very useful on its own.

But remember, comparison operators give us True or False.

We can use Boolean operators to join multiple comparisons together. This lets us build more complex conditions for our if and while statements.

Consider the following code:

1print(7 < 8 and "a" < "o")

Predict and Run

  • Predict what you think will be printed in the Shell when you run the code

  • Run the code and check if your prediction was correct

Investigate - Code breakdown

  • Line 1: print(7 < 8 and "a" < "o")

    • first Python will complete the comparison operations from left to right

      • 7 < 8 returns True

      • "a" < "o" returns True

    • the code is now: print(True and True)

      • True and True returns True

    • Python prints True to the Shell

Combining multiple comparison operations

When you use more than one comparison, you must have a comparison on both sides of the Boolean operator.

  • 10 > 5 and 10 > 13

    • Both sides are full comparisons

    • This is correct

  • 10 > 5 and 13

    • The second part (13) is not a comparison

    • This is not the same and will not work the way you expect

Part 2: Mouse input in Turtle

To help you understand Boolean logic better, we are going to try something new with Turtle.

So far, you have only typed input into the Shell. But Turtle can also take input from the mouse (and even the keyboard).

We will use the code below for this activity, but first we need to explore how it works.

Download lesson_6_pt_2.py file and save it in your lesson folder.

 1import turtle
 2
 3## Prepare the windows and turtle ##
 4def set_scene():
 5    turtle.setup(800, 600)
 6
 7    ## Respond to mouse click (signal) ##
 8    turtle.onscreenclick(draw_dot)
 9
10    ## Set up the grid ##
11    my_ttl.speed(0)
12    for i in range(4):
13        my_ttl.forward(400)
14        my_ttl.back(400)
15        my_ttl.right(90)
16    my_ttl.penup()
17
18
19## Reaction to signal (slot) ##
20def draw_dot(x, y):
21    print(x, y)
22    color = "orange"
23    size = 10
24    my_ttl.goto(x, y)
25    my_ttl.dot(size, color)
26
27
28## Main Program
29my_ttl = turtle.Turtle()
30set_scene()
31my_ttl.hideturtle()

Predict and Run

  • Predict what you think will happen when you run the code (hint: you will need to click in the Turtle window)

  • Run the code and check if your prediction was correct

Investigate - Code breakdown

We will look at this code in three parts, in the order Python uses them.

  1. Lines 29 to 31: the main part of the program

  • Line 29: my_ttl = turtle.Turtle() → creates a Turtle object and names it my_ttl

  • Line 30: set_scene() → runs the set_scene() function

  • Line 31: my_ttl.hideturtle() → hides the turtle so you cannot see it

  1. Lines 4 to 16: the set_scene() function

  • Line 4: def set_scene():

    • creates a function called set_scene

    • this function does not need any arguments

  • Line 5: turtle.setup(800, 600) → makes a window that is 800 pixels wide and 600 pixels tall

  • Line 8: turtle.onscreenclick(draw_dot) → this part is new

    • when the mouse is clicked in the Turtle window:

      • Python runs the draw_dot function

      • Python also sends the x and y position of the mouse click to that function

  • Line 11: my_ttl.speed(0) → a speed of 0 means the turtle moves instantly, so you do not see it moving

  • Lines 12 to 15: draw four lines out from (0, 0) to make four sections on the screen

  • Line 16: penup()

    • this stops the turtle drawing a line when it moves to the mouse click position

    • try commenting it out to see what changes

  1. Lines 20 to 25: the draw_dot() function

  • Line 20: def draw_dot(x, y):

    • creates a function called draw_dot

    • it uses two arguments: x and y

    • these are sent from line 8 when the mouse is clicked

    • turtle.onscreenclick() always sends the click position as x and y

  • Line 21: prints the x and y position into the Shell so you can see where you clicked

  • Line 22: stores "orange" in the variable color

  • Line 23: stores 10 in the variable size

  • Line 24: moves the turtle to the x and y position

  • Line 25: my_ttl.dot(size, color) → draws a dot where the turtle is, using the size in size and the colour in color

Exercises

In this course, the exercises are the make part of the PRIMM model. Work through the following tasks and write your own code.

Right now, every dot is orange. In these exercises, the part of the screen you click in will decide the dot’s colour.

To do this, you will need to use:

  • if, elif, and else statements

  • Boolean comparisons

  • Boolean operators

You will also need to remember how Turtle coordinates work.

coordinates

Exercise 1

Download lesson_6_ex_1.py file and save it in your lesson folder.

Follow the instructions in the comments from line 24 to line 42.

To help, here is the flowchart for the draw_dot function:

flowchart lesson 6 1

The starting code is shown below:

 1import turtle
 2
 3## Prepare the windows and turtle ##
 4def set_scene():
 5    turtle.setup(800, 600)
 6
 7    ## Respond to mouse click (signal) ##
 8    turtle.onscreenclick(draw_dot)
 9
10    ## Set up the grid ##
11    my_ttl.speed(0)
12    for i in range(4):
13        my_ttl.forward(400)
14        my_ttl.back(400)
15        my_ttl.right(90)
16    my_ttl.penup()
17
18
19## Reaction to signal (slot) ##
20def draw_dot(x, y):
21    print(x, y)
22    color = "orange"
23
24    ##################################
25    ######## Answer goes here ########
26    ##################################
27    """ Part A
28    Use an 'if' statement to set the dot color to red
29    when the mouse clicks in the top right quadrant
30
31    You can determine the position using the variables
32    x and y
33
34    To change the colour of the dot to red, run the command
35
36    color = 'red'
37
38    """
39
40    ##################################
41    ##################################
42    ##################################
43
44    my_ttl.goto(x, y)
45    size = 10
46    my_ttl.dot(size, color)
47
48
49my_ttl = turtle.Turtle()
50set_scene()
51my_ttl.hideturtle()

Exercise 2

Download lesson_6_ex_2.py file and save it in your lesson folder.

Follow the instructions in the comments from line 24 to line 35.

The starting code is shown below:

 1import turtle
 2
 3## Prepare the windows and turtle ##
 4def set_scene():
 5    turtle.setup(800, 600)
 6
 7    ## Respond to mouse click (signal) ##
 8    turtle.onscreenclick(draw_dot)
 9
10    ## Set up the grid ##
11    my_ttl.speed(0)
12    for i in range(4):
13        my_ttl.forward(400)
14        my_ttl.back(400)
15        my_ttl.right(90)
16    my_ttl.penup()
17
18
19## Reaction to signal (slot) ##
20def draw_dot(x, y):
21    print(x, y)
22    color = "orange"
23    
24    ##################################
25    ######## Answer goes here ########
26    ##################################
27    """ Part B
28    Use both 'if' and 'else' to set the dot color to red
29    if the mouse is clicked in the top right quadrant and
30    green if clicked anywhere else
31    """
32
33    ##################################
34    ##################################
35    ##################################
36
37    my_ttl.goto(x, y)
38    size = 10
39    my_ttl.dot(size, color)
40
41
42my_ttl = turtle.Turtle()
43set_scene()
44my_ttl.hideturtle()

Exercise 3

Download lesson_6_ex_3.py file and save it in your lesson folder.

Follow the instructions in the comments from line 24 to line 36.

The starting code is shown below:

 1import turtle
 2
 3## Prepare the windows and turtle ##
 4def set_scene():
 5    turtle.setup(800, 600)
 6
 7    ## Respond to mouse click (signal) ##
 8    turtle.onscreenclick(draw_dot)
 9
10    ## Set up the grid ##
11    my_ttl.speed(0)
12    for i in range(4):
13        my_ttl.forward(400)
14        my_ttl.back(400)
15        my_ttl.right(90)
16    my_ttl.penup()
17
18
19## Reaction to signal (slot) ##
20def draw_dot(x, y):
21    print(x, y)
22    color = "orange"
23
24    ##################################
25    ######## Answer goes here ########
26    ##################################
27    """ Part C
28    Use 'if', 'elif' and 'else' keywords to set the dot color to
29    red when the mouse is clicked in the top right quadrant,
30    blue in the top left quadrant, yellow in the bottom left quadrant
31    and green in the bottom right quadrant
32    """
33
34    ##################################
35    ##################################
36    ##################################
37
38    my_ttl.goto(x, y)
39    size = 10
40    my_ttl.dot(size, color)
41
42
43my_ttl = turtle.Turtle()
44set_scene()
45my_ttl.hideturtle()