08-input-output

08 · Input & Output

Part 1 — Fundamentals · Estimated time: 20–30 min · Prerequisite: 02 · Data Types & Numbers

So far your programs have only talked to you with print(). Now they'll listen back with input(). This is what makes a program interactive — it can ask a question, take an answer, and respond. We'll also polish how print() displays things.

What you'll learn

• print() with several values, plus the sep= and end= options
• input() — and the crucial fact that it always returns a string
• Converting input to numbers with int() / float()
• Formatting numbers neatly in f-strings (decimals, percentages, alignment)

ℹ️ These programs are interactive — when you run them, type your answer and press Enter. Each interactive example's docstring also shows how to feed input automatically with printf '...' | python file.py if you'd like.

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1. Printing, with options

print("a", "b", "c")            # a b c   (print puts spaces between values)
print("a", "b", "c", sep="-")   # a-b-c   (sep changes the separator)
print("Loading", end="...")     # no line break yet
print("done")                   # Loading...done

By default print() adds a space between values and a newline at the end. sep= and end= let you change both.

▶️ Run it: python examples/01_print_options.py

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2. Reading input

name = input("What is your name? ")   # shows the prompt, waits for Enter
print(f"Hello, {name}!")

input() shows your prompt, waits for the user to type a line, and hands it back as a string — always, even if they typed digits.

▶️ Run it (then type your answer): python examples/02_getting_input.py

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3. Input is text — convert it for maths

Because input() gives you a string, "5" + "3" would be "53", not 8. To do maths, convert first with int() or float().

age_text = input("How old are you? ")   # e.g. "20" (a string)
age = int(age_text)                     # 20 (a number)
print(f"Next year you'll be {age + 1}.")

▶️ Run it (then type a number): python examples/03_input_numbers.py

💡 Try it yourself: ask for a price as a float, then print it doubled.

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4. Formatting numbers in f-strings

Put a : after the value inside the braces to control how it looks.

price = 3.14159
print(f"{price:.2f}")    # 3.14   two decimal places
ratio = 0.25
print(f"{ratio:.0%}")    # 25%    as a percentage
name = "Sam"
print(f"|{name:<8}|")    # |Sam     |   left-aligned in 8 spaces
print(f"|{name:>8}|")    # |     Sam|   right-aligned

▶️ Run it: python examples/04_formatting.py

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Common mistakes

Mistake	What happens	Fix
input() + 1	TypeError (string + int)	Convert first: int(input(...)) + 1.
"5" + "3" expecting 8	gives "53" (text joined)	Convert both to numbers before adding.
int(input("Age: ")) and user types "abc"	ValueError	Ask for a number; (handling bad input nicely comes in the Exceptions lesson).
Forgetting the prompt text	user sees a blank cursor	Pass a prompt string: input("Name: ").

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Recap / cheat-sheet

print(a, b, c)               # a b c
print(a, b, sep=", ")        # a, b
print("x", end="")           # no trailing newline

text = input("Prompt: ")     # ALWAYS a string
number = int(input("n: "))   # convert for maths
price = float(input("$: "))

f"{value:.2f}"               # 2 decimals
f"{value:.0%}"               # percentage
f"{text:<10}" / f"{text:>10}"  # left / right align

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Exercises

These exercises are interactive. Run with python exercises/<file>.py and type your answers, then compare with the matching file in solutions/. Each docstring shows a Sample run (the parts you type are after each prompt).

1. exercises/01_greeting.py — ask for a name and greet the user.
2. exercises/02_add_two_numbers.py — add two numbers the user enters.
3. exercises/03_rectangle_area.py — ask for width and height, print the area.

Try each yourself before opening the solution.

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