Python User Input: input(), Converting Numbers and Validating Answers

So far, every program has used values written directly in the code. input() lets a program ask the user for a value while it runs, making it interactive: a calculator, a quiz, a simple game.

Reading text#

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

When you run it, the program pauses at the prompt and waits for you to type and press Enter:

What is your name? Asha
Hello, Asha

• The text inside input(...) is the prompt. End it with a space so the answer does not stick to it.
• Whatever the user types is returned as a string and stored in name.

input() always returns a string#

age = input("Your age: ")
print(type(age))

Your age: 21
<class 'str'>

Even though the user typed digits, age is the string "21". To do maths, convert it:

age = int(input("Your age: "))
print(f"Next year you will be {age + 1}")

Your age: 21
Next year you will be 22

Use float() for decimals:

qty = int(input("Quantity: "))
price = float(input("Price: "))
print(f"Total: {qty * price}")

Quantity: 4
Price: 2.5
Total: 10.0

Cleaning up text#

Users type extra spaces and random capitalisation. strip() removes spaces at both ends, and title() capitalises each word:

name = input("Name: ").strip().title()
print(f"[{name}]")

Name:   aSHA  
[Asha]

Other useful cleaners: .lower() (compare answers without worrying about case) and .upper().

Several values on one line#

split() breaks a string at spaces into a list. Combine it with map(int, ...) to convert every piece:

a, b, c = map(int, input("Three numbers: ").split())
print(a + b + c)

Three numbers: 3 4 5
12

When the input is invalid#

If the user types something that cannot be converted, the program crashes:

age = int(input("Your age: "))

Your age: abc
ValueError: invalid literal for int() with base 10: 'abc'

Never trust user input. A while True loop can keep asking until the answer is valid:

while True:
    text = input("Enter your age: ")
    if text.isdigit() and 0 < int(text) < 130:
        age = int(text)
        break
    print("Please enter a whole number between 1 and 129.")

print(f"Thanks! Age recorded: {age}")

Enter your age: abc
Please enter a whole number between 1 and 129.
Enter your age: -3
Please enter a whole number between 1 and 129.
Enter your age: 30
Thanks! Age recorded: 30

isdigit() is True only when the string contains nothing but digits, so "abc" and "-3" are rejected before int() is called. For decimals and negative numbers, a try / except block is the more general approach; see Exceptions.

Common mistakes#

Practice#

Temperature in Celsius: 72
72.0°C is 161.6°F

celsius = float(input("Temperature in Celsius: "))
fahrenheit = celsius * 9 / 5 + 32
print(f"{celsius}°C is {fahrenheit:.1f}°F")

Temperature in Celsius: 72
72.0°C is 161.6°F

Key takeaway#