User Input

So far, every program has worked with values written into the code. Real programs respond to the people using them. The input() function lets your program ask a question and wait for an answer.

Asking a question

input() shows a prompt, waits for the user to type something and press Enter, then gives back what they typed.

Run this and type your answer in the Output panel:

name = input("What is your name? ")
print(f"Nice to meet you, {name}!")

Step by step, this is what happens:

1. input() prints the prompt What is your name? and the program pauses.
2. The user types, say, Sam, and presses Enter.
3. input() gives back the text "Sam". The Enter key isn't included.
4. name = ... stores it, and the program carries on with the next line.

Tip: End your prompt with a space, as in "What is your name? ", so the user's answer doesn't run into the question.

The prompt is optional: plain input() waits without showing a question. It's friendlier to always tell people what to type, though.

Input is always a string

Whatever the user types, input() returns a string, even if it looks like a number. Run this and type 5:

answer = input("Type a number: ")
print(type(answer))
print(answer * 3)

You got 555, not 15, because "5" * 3 repeats the text. To do maths, convert the input first:

answer = input("Type a number: ")
number = int(answer)
print(number * 3)

You can convert in one step by wrapping input() in int() or float(). Python works from the inside out: input() runs first and gives back text, then int() or float() converts that text to a number:

age = int(input("How old are you? "))
height = float(input("How tall are you in metres? "))

print(f"Next year you will be {age + 1}.")
print(f"That's {height * 100:.0f} cm.")

Use int() for whole numbers and float() when the user might type a decimal. (:.0f shows the number with no decimal places, so 1.75 metres prints as 175 cm.)

int() and float() ignore spaces around the number, so " 42 " converts fine.

When the input isn't a number

If the user types something that can't be converted, Python raises a ValueError. Run this and type hello:

number = int(input("Type a whole number: "))
print(number * 2)

The same thing happens if the user just presses Enter without typing anything: input() gives back an empty string "", and int("") is a ValueError too.

You'll learn how to handle errors like this gracefully in the Errors and Exceptions lesson. For now, just know why it happens.

# a float: works, gives 3
print(int(3.5))

# decimal text: ValueError
print(int("3.5"))

answer = input("How many hours did you work? ")
hours = int(float(answer))
print(f"Full hours: {hours}")

Cleaning up text input

People add stray spaces and capital letters. Combine input() with the string methods from the last lesson to tidy things up:

color = input("What's your favourite colour? ").strip().lower()
print(f"You chose: {color}")

Try typing BLUE. input() gives back the text exactly as typed, strip() removes the spaces at both ends, and lower() makes it blue. Cleaning input like this means "Blue", "BLUE" and " blue" are all treated the same.

line = input("Enter width and height separated by a space: ")
parts = line.split()

width = int(parts[0])
height = int(parts[1])
print(f"Area: {width * height}")

Putting it together: a tip calculator

bill = float(input("Bill amount: "))
tip_percent = int(input("Tip percentage (e.g. 15): "))
people = int(input("Number of people: "))

tip = bill * tip_percent / 100
total = bill + tip
each = total / people

print(f"Tip: {tip:.2f}")
print(f"Total: {total:.2f}")
print(f"Each person pays: {each:.2f}")

Try a bill of 40, a tip of 15 and 2 people:

Each answer is converted to the right type as soon as it's read: float for money, which can have cents, and int for counts. After that, the rest of the program is ordinary maths.

Exercises

These exercises are interactive. Your program asks questions, and you type the answers in the Output panel.

Exercise 1: Personal greeting

Ask the user for their name and their city, then print Hello <name> from <city>!

# Ask for name and city, then greet the user

Exercise 2: Add two numbers

Ask for two whole numbers and print their sum. If the user types 4 and 6, it should print 4 + 6 = 10.

# Ask for two numbers and print their sum

Exercise 3: Temperature converter

Ask for a temperature in Celsius (it may have decimals) and print it in Fahrenheit using F = C × 9 / 5 + 32. For 25, print 25.0°C is 77.0°F.

# Convert Celsius to Fahrenheit

Exercise 4: Fix the program

This program is meant to tell the user how old they'll be in 10 years, but it crashes. Run it, read the error, and fix it.

age = input("How old are you? ")
print(f"In 10 years you will be {age + 10}.")

Summary

• input("prompt ") waits for the user and returns what they typed.
• The result is always a string. Convert it with int() or float() before doing maths.
• Converting text that isn't a number raises a ValueError.
• Chain .strip() and .lower() to clean up text input.
• split() lets you read several values from one line.