Lesson 8: User Input¶

In this lesson we will learn

how to get input from the user with input()
what data types are and why they matter
how to change a value from one data type into another

Terminology

interactive – describes a program that lets the user give it information while it runs, instead of changing the code.
data type – the kind of value something is, such as a number or text, which tells Python what it can do with that value.
integer – a whole number written without a decimal point, like 1 or 25, called int in Python.
floating point number – a number with a decimal point, like 1.0 or 3.5, called float in Python.
Boolean – a data type that can only be True or False, called bool in Python.

Drawing any shape¶

Type the code below into a new file, or open draw_poly.py from the lesson_08 folder of the tutorial files. Save it as draw_poly.py.

import turtle


def draw_poly(length, sides):
    for index in range(sides):
        my_ttl.forward(length)
        my_ttl.right(360 / sides)


# set up screen
screen = 500
window = turtle.Screen()
window.setup(screen, screen)

# create turtle instance
my_ttl = turtle.Turtle()
my_ttl.shape("turtle")

sides = 9
length = 100

draw_poly(length, sides)

PRIMM

Predict what the turtle will draw.
Run the code. 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 draw_poly with two parameters, length and sides.
line 5 → starts a for loop that repeats once for each side.
line 6 → moves my_ttl forward by the value in length.
line 7 → turns my_ttl right by 360 divided by sides.
lines 11–13 → create a 500 × 500 pixel window.
lines 16–17 → create a turtle called my_ttl with the turtle shape.
line 19 → stores 9 in sides.
line 20 → stores 100 in length.
line 22 → calls draw_poly to draw a shape with 9 sides that are 100 long.

PRIMM

Time to modify the code. Part of the shape goes off the screen. Can you change the code so the shape fits in the window?

Changing length from 100 to 80 fixes it. That's easy for us, because we know how to code. But what about someone who doesn't? How can we let the user choose the shape without changing the code?

Making our program interactive¶

The easiest way to make our program interactive is with the input function. It asks the user a question in the Shell and waits for them to type an answer.

Change lines 19 and 20 so our code matches the code below.

import turtle


def draw_poly(length, sides):
    for index in range(sides):
        my_ttl.forward(length)
        my_ttl.right(360 / sides)


# set up screen
screen = 500
window = turtle.Screen()
window.setup(screen, screen)

# create turtle instance
my_ttl = turtle.Turtle()
my_ttl.shape("turtle")

sides = input("How many sides?> ")
length = input("How long are the sides?> ")

draw_poly(length, sides)

PRIMM

Predict what will happen when we run the code.
Run the code. When it asks, type 3 for the sides and 100 for the length. Did you expect a question to appear in the Shell? Did you expect an error?
Time to investigate the code. What does each line do?

Code explanation

line 1 → imports the turtle module.
lines 4–7 → define draw_poly, which draws a shape with sides sides that are length long.
lines 11–13 → create a 500 × 500 pixel window.
lines 16–17 → create a turtle called my_ttl with the turtle shape.
line 19 → shows the question How many sides?> in the Shell, waits for the user to type an answer, and stores the answer in sides.
line 20 → asks How long are the sides?> and stores the answer in length.
line 22 → calls draw_poly with the user's answers.

The questions shown in the Shell

The Shell shows this error:

Traceback (most recent call last):
  File "<string>", line 22, in <module>
  File "<string>", line 5, in draw_poly
TypeError: 'str' object cannot be interpreted as an integer

This is a TypeError. To understand it, we need to learn about data types.

Data types¶

Values in Python have different types. The four main types we will use are:

integers (int) store whole numbers, written without a decimal point, like 1 or 25
floating point numbers (float) store numbers with a decimal point, like 1.0 or 3.5. 1 is an integer, but 1.0 is a float.
strings (str) store text: letters, numbers and symbols inside " " or ' '. Numbers can be strings, like a phone number "0432 789 367", but we can't do maths with strings.
Booleans (bool) can only be True or False

Data types tell Python what it can do with a value. For example, we can do maths with numbers, but not with text.

Let's look at the error again:

Traceback (most recent call last):
  File "<string>", line 22, in <module>
  File "<string>", line 5, in draw_poly
TypeError: 'str' object cannot be interpreted as an integer

line 4 tells us what went wrong. Always read the last line first. Python expected an integer (int), but it got a string (str) instead.
line 2 tells us the problem started on line 22 of our program, where we called draw_poly.
line 3 tells us the error happened on line 5, inside draw_poly: for index in range(sides):. The range function needs a whole number, but sides is a string.

Where did sides come from? Line 19: sides = input("How many sides?> "). The user typed 3, which looks like a number. So why is it a string?

Because everything we get from input() is a string, even if it looks like a number. To fix this, we need to change the string into a number.

Converting data types¶

Python has built-in functions that change a value from one data type to another. If we have a variable called value:

str(value) changes it into a string
int(value) changes it into an integer
float(value) changes it into a float

There's more to learn about this later, but that's all we need for now.

Let's put the input() inside int(), so the user's answer is changed into an integer. Change lines 19 and 20 so our code matches the code below.

import turtle


def draw_poly(length, sides):
    for index in range(sides):
        my_ttl.forward(length)
        my_ttl.right(360 / sides)


# set up screen
screen = 500
window = turtle.Screen()
window.setup(screen, screen)

# create turtle instance
my_ttl = turtle.Turtle()
my_ttl.shape("turtle")

sides = int(input("How many sides?> "))
length = int(input("How long are the sides?> "))

draw_poly(length, sides)

PRIMM

Predict what will happen when we run the code.
Run the code. Did it match your prediction?
Time to investigate the code. Try different values for the sides and length. What values make the turtle draw a circle? What happens if you type a decimal, like 3.5, or a word, like dog?

Code explanation

line 1 → imports the turtle module.
lines 4–7 → define draw_poly, which draws a shape with sides sides that are length long.
lines 11–13 → create a 500 × 500 pixel window.
lines 16–17 → create a turtle called my_ttl with the turtle shape.
line 19 → asks How many sides?>, changes the answer into an integer, and stores it in sides.
line 20 → asks How long are the sides?>, changes the answer into an integer, and stores it in length.
line 22 → calls draw_poly with the user's numbers.

Here is the program as a flowchart. Notice that input and output use the same shape, but with different labels.

Flowchart of the interactive shape program

PRIMM

Time to modify the code. Can you change the questions so they're clearer for the user?

Exercises¶

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

Starter files are in the lesson_08 folder of the tutorial files. Solutions are on the Exercise Solutions page.

Exercise 1¶

Starter: lesson_08/ex1_count_up

Can you write a program that asks the user for a number, then counts up to that number? For example, if the user types 5, it should print 1, 2, 3, 4 and 5.