15. Getting input

So far you have written every value yourself: predictable, but not interactive. This chapter introduces input(), which waits for the user to type into the terminal.

`input()` — read one line

input(prompt) prints an optional prompt, then waits for the user to type a line and press Enter. It returns whatever was typed as a string, without the newline.

name = input("What is your name? ")
print("Hello, " + name)

Run it. It prints the question on the same line as the cursor, waits for your name and Enter, prints the greeting, then exits.

Open exercises/15/01-read-name.py and run it twice with different names. Watch the greeting change to match.

The prompt is on the same line

Python's input() puts the prompt and the user's cursor on the same line automatically. You do not need a separate function for this. The text you pass to input() is the prompt:

name = input("What is your name? ")
print(f"Hello, {name}")

Output:

What is your name? Keiko
Hello, Keiko

Reading a number

input() always returns a string, even for digits. To do maths, wrap it in int() for a whole number or float() for a decimal:

text = input("Enter a number: ")
n = int(text)
print(f"Twice that is {n * 2}")

The two steps (read, then convert) can be written on one line:

n = int(input("Enter a number: "))
print(f"Twice that is {n * 2}")

The inner call runs first and returns the string; the outer turns it into an integer.

If the user types something that is not a number, int() raises a ValueError and the program crashes. You will fix this in a later chapter using try/except. For now, just type numbers when the program asks for them.

Reading several values

To read several values, call input() once per value:

a = int(input("First number:  "))
b = int(input("Second number: "))
print(f"Sum: {a + b}")

Each call pauses and waits separately. The program does not continue to the next line until Enter is pressed.

Homework

Problem 1 — Greet by name

Open exercises/15/homework/01-greet-by-name.py. Prompt for a first name with input(), read it, then print Hello, <name>!. The exclamation mark is part of the output.

Problem 2 — Sum of two

Open exercises/15/homework/02-sum-of-two.py. Prompt for two numbers (separate prompts), convert each with int(), then print the sum like this:

a + b = c

where a, b, c are the actual values.

Problem 3 — Years to retirement

Open exercises/15/homework/03-retirement.py. Ask the user's age and compute the years left until 65. Print:

You have N years until retirement.

(If someone is over 65, the number is negative. That is fine — no need to handle that case yet.)

Challenge — BMI

Open exercises/15/homework/04-bmi.py. Ask for height in metres (a decimal like 1.78) and weight in kilograms (like 72.5). Use float() to convert both. Compute Body Mass Index:

bmi = weight / (height * height)

Print the result rounded to one decimal place using an f-string with :.1f. Label it clearly.

Stuck or finished? Open the homework solutions page.