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Module 005: Basic Input and Output

Phase: 1. Fundamentals | Estimated time: 1.5 hours | Milestone Project: No

Prerequisites

Learning Objectives

By the end of this module, you will be able to:

  • Get text input from the user using input()
  • Print dynamic output that includes user-provided values
  • Convert string input to numeric types for calculation
  • Build a simple interactive program

Why This Matters

Programs that only display fixed output are boring. The input() function is your gateway to building interactive applications — from simple Q&A scripts to games, calculators, and data-entry tools.

Concept Explanation

The input() Function

input() pauses the program and waits for the user to type something and press Enter. Whatever the user types is returned as a string:

name = input()
print("Hello,", name)

You can pass a prompt string to input() that is displayed to the user:

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

What input() Returns

input() always returns a string. Even if the user types a number:

age = input("How old are you? ")
print(age, type(age))  # e.g., 25 <class 'str'>

If you need a number, you must convert it yourself using int() or float():

age = int(input("How old are you? "))
print("Next year you'll be", age + 1)

Combining Input with Print

You can build sentences using f-string style (we'll cover f-strings properly in Module 008) or by passing multiple arguments to print():

name = input("Enter your name: ")
color = input("Enter your favorite color: ")
print(name, "likes", color)

Simple Interactive Programs

A simple calculator using input:

num1 = float(input("Enter first number: "))
num2 = float(input("Enter second number: "))
print("Sum:", num1 + num2)
print("Difference:", num1 - num2)

The End of Input

When running in a terminal, input() waits forever until the user types something. In a Jupyter notebook, you may need to run code cells that simulate input differently (see the notebook).

Common Pitfalls

  1. Forgetting that input() returns a string: input() + 5 causes TypeError. Convert to int or float first.
  2. Not stripping whitespace: input() preserves leading/trailing spaces. Use .strip() to clean input.
  3. Using input() in an online/notebook environment: Some environments don't support interactive input. Use a simulated approach or pass the prompt carefully.
  4. Assuming the prompt needs a newline: input() does not add a newline after the prompt. Add a space at the end of your prompt string for readability.
  5. Chaining conversions unsafely: int(input()) raises ValueError if the user types non-numeric text. We'll learn error handling for this in a later module.

Hands-On Walkthrough

Let's build a simple "Personal Greeting" program:

# Ask for user's details
name = input("What is your name? ")
age = int(input("How old are you? "))
city = input("Which city do you live in? ")

# Generate a personalized message
print()
print("Hello,", name + "!")
print("You are", age, "years old and live in", city + ".")
print("In 5 years, you will be", age + 5, "years old.")

Example run:

What is your name? Alice
How old are you? 30
Which city do you live in? London

Hello, Alice!
You are 30 years old and live in London.
In 5 years, you will be 35 years old.

Now let's build an interest calculator:

principal = float(input("Enter the principal amount: "))
rate = float(input("Enter the interest rate (as %, e.g. 5 for 5%): "))
years = int(input("Number of years: "))

interest = principal * (rate / 100) * years
total = principal + interest

print("Interest earned:", interest)
print("Total after", years, "years:", total)

Key Takeaways

  • input() reads a line of text from the user as a string.
  • Always convert numeric input with int() or float() before doing math.
  • input() accepts an optional prompt string.
  • Combine input() and print() for interactive programs.
  • Use .strip() to clean whitespace from input.
  • input() returns a string regardless of what the user types.
  • Prompt strings should end with a space for readability.
  • Simple interactive programs follow: input → process → output.

Further Reading

Next Module

Learn the building blocks of computation in Module 006: Operators.