Topic focus: The input function

The simplest way for a Python program to ask the user for input is the input function. It's used for programs that run inside of a terminal, and when the input function is executed, the program stops and waits for the user to enter a line of text into the terminal. The line ends when the user hits the Return key (called the Enter key on some keyboards).

The input function takes a single (optional) argument, which is a prompt string which is usually a message indicating what kind of input is desired. If a prompt string is provided, that string will be printed on the input line before the location where the user will enter their input.

Here are some examples in the interactive Python interpreter:

>>> input()
this is a test    <-- YOU ENTER THIS LINE
'this is a test'
>>> input("Please enter your name: ")
Please enter your name: Bob   <-- YOU TYPED "Bob"
'Bob'

The first call to input() didn't have a prompt argument, so the user had to know to input a string at that time. The second call had the prompt Please enter your name:, which makes it clear to the user that they should enter their name at that point in the program.

Note

One trick we used in the second call is to add a space at the end of the prompt string (right after name:). This makes the inputted string easier to see.

The input() function returns the string that was input. You normally would assign this to a variable so that you can use the input string later.

>>> name = input("Please enter your name: ")
Please enter your name: Bob   <-- YOU TYPED "Bob"
>>> name
'Bob'
>>> name.upper()
'BOB'

Be careful when inputting a number! The number will be represented as a string, since Python doesn't "know" that it's supposed to be a number. If you want it to be a number, you have to convert it manually.

Here's what you get if you forget to do this:

>>> age = input("Please enter your age in years: ")
Please enter your age in years: 19  <-- YOU TYPED "19"
>>> age
'19'
>>> age + 1
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: can only concatenate str (not "int") to str

Python is reminding you that you can't add a string ("19") to an integer (1).

The right way to do this is as follows:

>>> age = int(input("Please enter your age in years: "))
Please enter your age in years: 19  <-- YOU TYPED "19"
>>> age
19
>>> age + 1
20

Notice that we surrounded the input function call with the int function; this converts the string returned by input into an integer (if possible).

Aside: Nested function calls

Nested function calls like int(input(...)) are perfectly OK in Python (and in pretty much all programming languages). It just means that the return value of the inner function (input in this case) is passed as the input to the outer function (int in this case). Some novice programmers think they need to store the return value of input in a variable before using it, like this:

>>> age_string = input("Please enter your age in years: ")
Please enter your age in years: 19  <-- YOU TYPED "19"
>>> age = int(age_string)
19
>>> age + 1
20

While this will certainly work, it's not necessary.