3.3 · Input & Output

Goal: read user input correctly and print useful output.

Output with print

print("Hello")
print("x =", 5)                    # Hello x = 5
print("Score:", 75, "Grade:", "A") # Score: 75 Grade: A

Optional arguments:

print("a", "b", "c", sep=" - ")    # a - b - c
print("loading", end="...")        # no newline

Format strings

The clean way to embed values:

name = "Alice"
score = 86
print(f"{name} scored {score}")    # Alice scored 86
print(f"{score / 100:.2%}")        # 86.00%
print(f"Pi is approximately {3.14159:.2f}")  # Pi is approximately 3.14

Inside {} you can put any expression. The colon syntax controls formatting (.2f = 2 decimal places, % = percentage).

Input with input

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

⚠️ input always returns a string. Convert with int() or float() if you need a number.

age = int(input("Age? "))
height = float(input("Height in m? "))

Validating numeric input

raw = input("Age? ")
if raw.isdigit():
    age = int(raw)
    print(f"You are {age}.")
else:
    print("That's not a valid age.")

Or with try / except for floats:

try:
    height = float(input("Height (m)? "))
    print(height * height)
except ValueError:
    print("Invalid number.")

File input / output (optional preview)

In Module D this is informally introduced; file handling is explicitly Elective 2C material. Quick taste:

with open("scores.txt", "r") as f:
    for line in f:
        print(line.strip())

Worked example · Simple calculator

a = float(input("First number? "))
op = input("Operator (+ - * /)? ")
b = float(input("Second number? "))

if op == "+":
    print(a + b)
elif op == "-":
    print(a - b)
elif op == "*":
    print(a * b)
elif op == "/":
    if b == 0:
        print("Division by zero!")
    else:
        print(a / b)
else:
    print("Unknown operator")

Common student mistakes

• Forgetting that input() returns a string.
• Comparing input() to a number: if input("?") == 5: is always False (string vs int).
• Not handling ValueError from int() / float().

Exam-style question

Q (4 marks): Write a Python program that asks the user to enter the temperature in Celsius and outputs the equivalent in Fahrenheit (formula: F = C × 9/5 + 32), formatted to 1 decimal place.

Sample answer:

c = float(input("Temperature in °C? "))
f = c * 9 / 5 + 32
print(f"{c}°C is {f:.1f}°F")

Key takeaways

• print outputs; input reads strings.
• Format strings (f"…") make output readable.
• Convert string input to numbers as needed.

➡️ Next: 3.4 Selection