Lesson 03 · Input and Output¶

The input() function¶

input() pauses the program and waits for the user to type something and press Enter:

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

Run:

What's your name? Andrew
Hello, Andrew

The text inside the parentheses is the prompt — the message shown to the user.

input() ALWAYS returns str¶

This is the most common source of errors for beginners:

age = input("Your age: ")
print(type(age))   # <class 'str'>

Even if the user types 14, Python receives the string "14", not the number 14.

Converting input¶

age = int(input("Your age: "))
print(type(age))   # <class 'int'>
print(age + 1)     # works!

height = float(input("Your height (m): "))
print(f"Height in cm: {height * 100}")

The print() function — details¶

The sep separator¶

print("Mon", "Tue", "Wed")              # Mon Tue Wed
print("Mon", "Tue", "Wed", sep=", ")    # Mon, Tue, Wed
print("Mon", "Tue", "Wed", sep="\n")    # each on a new line

The end terminator¶

print("One", end=" ")
print("Two", end=" ")
print("Three")
# One Two Three — all on the same line

Empty line¶

print()   # prints an empty line

Combined examples¶

Simple greeting program¶

first_name = input("Your first name: ")
age = int(input("Your age: "))

print()
print(f"Hello, {first_name}!")
print(f"Next year you will be {age + 1} years old.")

Reading multiple values¶

a = float(input("First number: "))
b = float(input("Second number: "))

print(f"{a} + {b} = {a + b}")
print(f"{a} × {b} = {a * b}")

Exercises¶

Exercise 1 — Personalized greeting¶

Write a program that asks for name and age, then displays: "Hi, [name]! You are [age] years old."

name = input("Your name: ")
age = int(input("Your age: "))
print(f"Hi, {name}! You are {age} years old.")

Exercise 2 — Rectangle area¶

Ask for the length and width of a rectangle, calculate the area and the perimeter.

length = float(input("Length (m): "))
width = float(input("Width (m): "))
area = length * width
perimeter = 2 * (length + width)
print(f"Area: {area} m²")
print(f"Perimeter: {perimeter} m")

Exercise 3 — How many seconds?¶

Ask for hours and minutes, display the total in seconds.

hours = int(input("Hours: "))
minutes = int(input("Minutes: "))
seconds = hours * 3600 + minutes * 60
print(f"{hours}h {minutes}min = {seconds} seconds")

Mini-project: Interactive calculator¶

Write a program that asks for two numbers and displays all the basic operations.

Example run:

First number: 10
Second number: 3

10.0 + 3.0 = 13.0
10.0 - 3.0 = 7.0
10.0 × 3.0 = 30.0
10.0 / 3.0 = 3.3333333333333335
10.0 // 3.0 = 3.0
10.0 % 3.0 = 1.0
10.0 ^ 3.0 = 1000.0

a = float(input("First number: "))
b = float(input("Second number: "))

print()
print(f"{a} + {b} = {a + b}")
print(f"{a} - {b} = {a - b}")
print(f"{a} × {b} = {a * b}")
print(f"{a} / {b} = {a / b}")
print(f"{a} // {b} = {a // b}")
print(f"{a} % {b} = {a % b}")
print(f"{a} ^ {b} = {a ** b}")

Summary¶

• input("message") reads text from the keyboard — ALWAYS returns str
• Convert with int(input(...)) or float(input(...))
• print(a, b, sep=", ") controls the separator
• print("text", end="") controls what comes after the line

Next step: → Lesson 04: Conditions