User Input

Programs become far more useful when they can respond to the person using them. Instead of working only with values written into the code, an interactive program can ask for a name, a quantity, or a choice, and act on the answer.

Python reads input typed by the user at runtime with the built-in input() function.

Basic User Input

When Python reaches an input() call, the program pauses and waits. The user types something and presses Enter, and input() returns what was typed.

Example: Reading and Displaying Input

username = input()
print(f"Welcome, {username}!")

Example run (the user types Asha):

Asha
Welcome, Asha!

What happens:

1. input() pauses the program, showing a blinking cursor.
2. The user types Asha and presses Enter.
3. input() returns the string "Asha", which is stored in username.
4. The program continues and prints the welcome message.

Without any message, however, the user does not know what to type. That is what a prompt is for.

Using a Prompt Message

Pass a prompt string to input() to display a message before waiting for input. The user types on the same line as the prompt.

Example: Input With a Prompt

username = input("Please enter your username: ")
print(f"Hello, {username}")

Example run:

Please enter your username: asha_k
Hello, asha_k

Tip: End the prompt with a colon and a space (": ") so the user's typing does not run into the message.

Taking Multiple Inputs

A program can ask for input as many times as needed. Python pauses at each input() call until the user responds.

Example: Multiple Inputs

city = input("Which city do you live in? ")
food = input("What is your favorite food? ")
year = input("Which year were you born? ")

print(f"You live in {city}, love {food}, and were born in {year}.")

Example run:

Which city do you live in? Chennai
What is your favorite food? Dosa
Which year were you born? 2001
You live in Chennai, love Dosa, and were born in 2001.

Reading Several Values on One Line

To let the user type several values at once, combine input() with split():

first, last = input("Enter your first and last name: ").split()
print(f"First: {first}, Last: {last}")

Example run:

Enter your first and last name: Meena Raman
First: Meena, Last: Raman

split() breaks the text into a list at each space, and the two values are unpacked into first and last. If the user types a different number of words, Python raises a ValueError.

Input Is Always a String

input() always returns a string — even if the user types a number.

age = input("Enter your age: ")
print(type(age))

Example run:

Enter your age: 25
<class 'str'>

This matters as soon as you try to calculate:

age = input("Enter your age: ")
print(age + 1)

Example run:

Enter your age: 25
TypeError: can only concatenate str (not "int") to str

age is the text "25", not the number 25, so it cannot be added to 1.

Converting Input to a Number

To perform calculations, convert the input with int() (whole numbers) or float() (decimal numbers).

radius = input("Enter the radius of the circle: ")
area = 3.14 * float(radius) ** 2

print(f"Circle area: {area:.2f}")

Example run:

Enter the radius of the circle: 5
Circle area: 78.50

What happens:

1. The user types 5, and input() returns the string "5".
2. float(radius) converts it to the number 5.0.
3. ** 2 squares it (exponentiation runs before multiplication), giving 25.0.
4. 3.14 * 25.0 is 78.5, shown as 78.50 by the :.2f format.

You can also convert immediately when reading:

quantity = int(input("How many tickets? "))
print("Total:", quantity * 150)

Example run:

How many tickets? 3
Total: 450

Note: For more accurate circle calculations, use math.pi instead of 3.14.

Validating User Input

Users make mistakes: they type letters where numbers are expected, leave fields empty, or enter values out of range. If the program trusts input blindly, it can crash:

number = float(input("Enter a number: "))

Example run:

Enter a number: ten
ValueError: could not convert string to float: 'ten'

Input validation checks the data and handles problems gracefully instead of crashing.

Example: Keep Asking Until the Input Is Valid

while True:
    value = input("Enter a valid number: ")
    try:
        number = float(value)
        break
    except ValueError:
        print("Invalid input. Please enter a numeric value.")

print("Input accepted. Thank you!")

Example run:

Enter a valid number: abc
Invalid input. Please enter a numeric value.
Enter a valid number: 12.5
Input accepted. Thank you!

How it works:

1. while True: repeats until something explicitly stops the loop.
2. float(value) tries to convert the text.
3. If the conversion succeeds, break exits the loop.
4. If it fails, Python raises a ValueError, the except block prints a message, and the loop asks again.

Example: Validating a Range

Checking the type is not always enough — the value must also make sense:

while True:
    try:
        age = int(input("Enter your age: "))
    except ValueError:
        print("Please enter a whole number.")
        continue

    if 1 <= age <= 120:
        break
    print("Please enter an age between 1 and 120.")

print("Age recorded:", age)

Example run:

Enter your age: -4
Please enter an age between 1 and 120.
Enter your age: 30
Age recorded: 30

The first check catches text that is not a number; the second rejects numbers that are outside the allowed range.

Cleaning Text Input

For text, strip() removes accidental spaces, and lower() makes comparisons case-insensitive:

answer = input("Continue? (yes/no): ").strip().lower()

if answer == "yes":
    print("Continuing...")
else:
    print("Stopped.")

Example run:

Continue? (yes/no):   YES
Continuing...

Why Input Validation Matters

• Prevents runtime errors — invalid data does not crash the program.
• Improves the user experience — users get clear guidance instead of a traceback.
• Makes programs more reliable — calculations only run on sensible values.
• Handles unexpected behavior — real users type all kinds of things.

Common Mistakes

Related Concepts

• Python Type Casting — converting strings to numbers
• Python Exception Handling — try and except for invalid input
• Python while Loops — repeating a prompt until the input is valid
• Python String Methods — cleaning text input with strip() and lower()