Python Basics — input() Function

input() Function

The input() function reads text from the user via the keyboard. It always returns a string, even if the user types a number.

Basic Usage

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

Interaction:

What is your name? Alice
Hello, Alice!

input() Always Returns a String

age = input("Enter your age: ")
print(type(age))  # <class 'str'> — always a string!
print(age)        # "25" (a string, not a number)

This means you cannot do math with input() directly:

# ❌ This won't work as expected
num = input("Enter a number: ")  # User types: 5
# print(num + 10)  # TypeError: can only concatenate str to str

Type Casting User Input

Convert the input string to the type you need:

# Integer input
age = int(input("Enter your age: "))
print(f"Next year you'll be {age + 1}")

# Float input
price = float(input("Enter the price: "))
tax = price * 0.08
print(f"Tax: ${tax:.2f}")

# Boolean-like input
answer = input("Continue? (yes/no): ")
should_continue = answer.lower() == "yes"
print(should_continue)  # True or False

Handling Invalid Input

Users sometimes type unexpected things. Use try/except to handle errors:

try:
    age = int(input("Enter your age: "))
    print(f"You are {age} years old.")
except ValueError:
    print("That's not a valid number!")

Input Validation Loop

Keep asking until the user enters valid input:

while True:
    user_input = input("Enter a number (1-100): ")
    try:
        number = int(user_input)
        if 1 <= number <= 100:
            break
        else:
            print("Please enter a number between 1 and 100.")
    except ValueError:
        print("Invalid input. Please enter a number.")

print(f"You entered: {number}")

Multi-Line Input

Read multiple values on separate lines:

print("Enter three numbers (one per line):")
a = int(input())
b = int(input())
c = int(input())
print(f"Sum: {a + b + c}")

Reading Multiple Values on One Line

# User types: 10 20 30
values = input("Enter three numbers separated by spaces: ").split()
a, b, c = int(values[0]), int(values[1]), int(values[2])
print(f"Sum: {a + b + c}")

# More concise with map()
x, y, z = map(int, input("Enter three numbers: ").split())
print(f"Sum: {x + y + z}")

Command-Line Arguments (sys.argv)

For scripts, you can also accept input via command-line arguments:

import sys

# Run: python script.py Alice 25
if len(sys.argv) >= 3:
    name = sys.argv[1]
    age = sys.argv[2]
    print(f"Name: {name}, Age: {age}")
else:
    print("Usage: python script.py <name> <age>")

python script.py Alice 25
# Name: Alice, Age: 25

Practical Example: Simple Calculator

print("=== Simple Calculator ===")
num1 = float(input("Enter first number: "))
operator = input("Enter operator (+, -, *, /): ")
num2 = float(input("Enter second number: "))

if operator == "+":
    result = num1 + num2
elif operator == "-":
    result = num1 - num2
elif operator == "*":
    result = num1 * num2
elif operator == "/":
    if num2 != 0:
        result = num1 / num2
    else:
        print("Error: Division by zero!")
        result = None
else:
    print(f"Unknown operator: {operator}")
    result = None

if result is not None:
    print(f"{num1} {operator} {num2} = {result}")

Common Mistakes

Mistake	Example	Fix
Forgetting to convert	int(input()) + 10 not input() + 10	Always cast for math
No error handling	int(input(...)) crashes on “abc”	Wrap in try/except
Trailing whitespace	User adds spaces	Use .strip(): input().strip()

Summary

• input() always returns a string — cast with int(), float() as needed
• Use try/except to handle invalid input gracefully
• Use a while loop for input validation
• Use .split() and map() to read multiple values on one line
• Use sys.argv for command-line arguments
• Always .strip() user input to remove accidental whitespace

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Next up: String Formatting →