📘 Lesson 07 · Beginner

Python Input & Output

Learn input() to read user data and print() with sep, end, and f-string formatting.

Getting Input from the User

input() pauses your program, shows a prompt, waits for the user to type something and press Enter, then returns what they typed as a string. This last point is crucial: even if the user types 42, you get the string "42", not the integer 42. Always convert with int() or float() before doing arithmetic.

input.py

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

# Must convert to int for maths
age = int(input("Your age? "))
print(f"In 10 years you will be {age + 10}")

height = float(input("Height in metres? "))
print(f"Height: {height}m")

▶ Output

What is your name? Alice
Hello, Alice!
Your age? 25
In 10 years you will be 35
Height in metres? 1.72
Height: 1.72m

Customising print()

print() has two useful keyword arguments. sep controls what goes between values (default: a space). end controls what comes after the last value (default: a newline \n). Setting end="" keeps the cursor on the same line so the next print continues right after.

print_params.py

print("a", "b", "c")                  # default: space between
print("2024", "01", "15", sep="-")   # custom separator
print("Loading", end="...")           # no newline
print("Done!")                          # continues on same line

▶ Output

a b c
2024-01-15
Loading...Done!

Building an Interactive Program

Combining input(), type conversion, and f-strings, you can build useful programs right away. Here is a simple BMI calculator — it takes two inputs, computes the result, and displays a formatted message:

bmi.py

print("=== BMI Calculator ===")
weight = float(input("Weight (kg): "))
height = float(input("Height (m):  "))

bmi = weight / (height ** 2)
print(f"\nBMI: {bmi:.1f}")

if bmi < 18.5:
    print("Underweight")
elif bmi < 25:
    print("Normal weight")
else:
    print("Overweight")

▶ Output

=== BMI Calculator ===
Weight (kg): 70
Height (m):  1.75

BMI: 22.9
Normal weight

⚠️

If the user types text when you expect a number, Python raises a ValueError and crashes. Use try/except (Lesson 18) to handle invalid input gracefully.

🧠 Quick Check

What type does input() always return?

int

float

str

bool