9.2. User Input

To begin this chapter, Python’s input() function is discussed.

Editorial note: When we turn this chapter into a website, the automated notebook runner will produce an error when asked for interactive input. To get around this, we’re going to use the try/except blocks that we will discuss in Section 9.6 to catch the error and substitute default values for the variables. When you run this notebook on a jupyterhub you will be prompted for the values instead.

Python can be used to ask users for input. The input entered by a user can be saved to a variable and used in subsequent parts of the program. The syntax of Python’s input() function is below:

var = input('message')

Where var is the variable that stores the user’s input and 'message' is the message the user sees at the prompt. A string enclosed in quotes, like 'message', needs to be passed as an input argument to the input() function. Let’s ask a user for their age:

try:
    age = input('how old are you? ')
except:
    age= '-5'

Since the user’s input is assigned to a variable, further operations can be run on it. Now, let’s print the user’s age back to them. This can be accomplished with an f-string. Note the f' ' inserted before the string. A set of curly braces {  } surrounds the variable’s name and the variable’s value is printed back to the user.

try:
    age = input('how old are you? ')
except:
    age = '-5'
print(f'you are {age} years old')

you are -5 years old

Let’s try another example. We will we ask the user for the base and height of a triangle and print out the area of the triangle.

But, there is a problem with the approach below. The code block does not run because a common error is present.

try:
    b = input('base of triangle: ')
except:
    b = '5'
try:
    h = input('height of triangle: ')
except:
    h = '10'
A = (1/2)*b*h
print(f'The area of the triangle is: {A}')

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
/tmp/ipykernel_3768500/3357621487.py in <module>
      7 except:
      8     h = '10'
----> 9 A = (1/2)*b*h
     10 print(f'The area of the triangle is: {A}')

TypeError: can't multiply sequence by non-int of type 'float'

The previous section of code returns an error because of the data type of the variables b and h. We can investigate b and h’s data type with Python’s type() function.

try:
    b = input('base of triangle: ')
except:
    b = '5'
try:
    h = input('height of triangle: ')
except:
    h = '10'
print(f'b and h are of type: {type(b)}, {type(h)}')

b and h are of type: <class 'str'>, <class 'str'>

Notice both b and h are strings, even though the numbers 5 and 2 were entered as input. The output of the input() function is always a string, even if the user enters a number.

To complete the area calculation, b and h first need to be converted to floats using Python’s float() function, then the mathematical operation will run without error:

try:
    b = input('base of triangle: ')
except:
    b = '5'
try:
    h = input('height of triangle: ')
except:
    h= '10'
A = (1/2)*float(b)*float(h)
print(f'The area of the triangle is: {A}')

The area of the triangle is: 25.0

Now that you are familiar with Python’s input() function, let’s utilize a user’s input to decide which lines of code will run. The concept of an selection statement is introduced the next section.