Skip to content

lesson.md

Latest commit

 

History

History
291 lines (209 loc) · 6.87 KB

lesson.md

File metadata and controls

291 lines (209 loc) · 6.87 KB
Raw
layout default
title Input and Output
parent Lessons
nav_order 5
permalink /lessons/input-and-output/
course_lesson true
course_index 05
previous_page /lessons/operators/
previous_title Operators
next_page /lessons/control-flow/
next_title Control Flow

05 - Input and Output

Programs become useful when they receive information and communicate results clearly.

A simple picture

Input is the program's ear: it listens to what a person types. Output is the program's voice: it displays a clear answer. A friendly program asks one clear question at a time.

Reading input

input() displays a prompt, waits for the user, and returns the entered text without the final Enter key:

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

.strip() removes accidental spaces at the beginning and end. It does not remove spaces between words.

Important fact: input() always returns a string (str). This is true even when the person types digits.

Try this program and enter 25:

age_text = input("Enter your age: ")

print(age_text)
print(type(age_text))

Output:

25
<class 'str'>

The value looks like a number to us, but Python received keyboard characters. Convert it only when the program needs number operations:

age_text = input("Enter your age: ")
age = int(age_text)

print(age)
print(type(age))
keyboard -> input() -> str -> int() or float() when needed

Convert numeric input explicitly:

quantity = int(input("Quantity: "))
price = float(input("Price: "))

The inner input() runs first. Its string result is passed to int() or float().

Displaying output

print() displays values and ends with a newline by default:

print("Python")
print("is enjoyable")

It accepts multiple values and separates them with a space:

print("Total", 125)  # Total 125

Change the separator with sep:

print(2026, 8, 5, sep="-")  # 2026-8-5

Change the ending with end:

print("Loading", end="...")
print("done")             # Loading...done

Escape characters

Escape characters represent special formatting inside a string:

Sequence Meaning
\n new line
\t tab
\\ backslash
\" double quote
\' single quote 
print("Name:\tRavi\nCity:\tPune")

Use triple quotes for a multi-line string when appropriate.

f-strings

An f-string inserts expressions inside {}:

name = "Ravi"
total = 125.5
print(f"{name} paid Rs. {total:.2f}")

Useful format specifiers include .2f for two decimal places and >10 for right alignment:

print(f"{'Item':<12}{'Price':>8}")
print(f"{'Pen':<12}{12.5:>8.2f}")

Designing a good interaction

Write the conversation before writing code:

Product name: Notebook
Quantity: 2
Unit price: 45.50
Total: 91.00

Then identify the type of every input and the formula for the result.

Real-world example

item = input("Item: ").strip()
quantity = int(input("Quantity: "))
unit_price = float(input("Unit price: "))
total = quantity * unit_price

print("\n--- Receipt ---")
print(f"Item:     {item}")
print(f"Quantity: {quantity}")
print(f"Total:    Rs. {total:.2f}")

This is clear because each value has a label and the money value has consistent formatting.

Common mistakes

  • Forgetting that input is text.
  • Leaving user spaces uncleaned.
  • Printing a calculation expression as text instead of evaluating it.
  • Using many commas when one formatted f-string would be clearer.
  • Mixing prompts, calculations, and output so the program is difficult to change.

Bug Hunter

Bug 1 — the number is still text

age = input("Age: ")
next_age = age + 1
print(next_age)

Clue: convert the input before adding a number.

Bug 2 — conversion happens too late

first = input("First number: ")
second = input("Second number: ")
total = first + second
print(int(total))

If the inputs are 2 and 3, this prints 23. Convert each input before addition.

Bug 3 — forgotten f-string marker

name = input("Name: ")
print("Hello, {name}!")

Clue: an f-string begins with f before the opening quote.

Show Bug Hunter fixes 
# Bug 1
age = int(input("Age: "))
next_age = age + 1
print(next_age)

# Bug 2
first = int(input("First number: "))
second = int(input("Second number: "))
total = first + second
print(total)

# Bug 3
name = input("Name: ")
print(f"Hello, {name}!")
Optional deeper look: where do input and output go?

The terminal provides a text input stream called standard input. input() reads one line from it. print() writes text to standard output. A stream is simply a flow of data that a program can read or write. Keeping conversion separate makes the program easier to test later.

Practice

Try these problems on this page. First write the sample conversation, then write the program.

Problems

  1. Read a name and greet the user.
  2. Read two numbers and print their sum in a sentence.
  3. Print a date using sep.
  4. Create a formatted product receipt.
  5. Read hours and hourly rate and print salary.
  6. Predict the output of small programs using sep and end.
  7. Build a temperature conversion prompt and result.
  8. Print a three-column table using f-strings.
  9. Create an invoice with aligned labels.
  10. Design the input and output for a menu item order before writing code.
Show hints
  1. Store input() in a variable and use an f-string.
  2. Convert both inputs to numbers.
  3. Pass the date pieces to print() with sep="-".
  4. Give every value a label.
  5. Multiply hours by the hourly rate.
  6. sep goes between values; end replaces the final newline.
  7. Convert the temperature to float.
  8. Use alignment such as :<12 and :>8.
  9. Print one label and value per line.
  10. Write example input and output before coding.
Show solution ideas
  1. name = input("Name: ") followed by print(f"Hello, {name}!").
  2. Convert with int() and calculate first + second.
  3. print(year, month, day, sep="-").
  4. Use quantity times unit price and format money with .2f.
  5. salary = float(hours) * float(rate).
  6. Read the code left to right and mark each separator and ending character.
  7. Use fahrenheit = celsius * 9 / 5 + 32.
  8. Use f-string width specifiers.
  9. Separate input, calculation, and display with clear labels.
  10. Decide the prompt, type, and output for every field.

Homework

Create a formatted travel booking confirmation that accepts a passenger name, destination, ticket count, and total price.

Checkpoint

Build a program that reads a product name, quantity, and price and prints a formatted receipt line.