Programming Fundamentals › Programming Basics

Magic Number

An unexplained literal in code that should be a named constant.

Also known as: magic string, hard-coded value, hardcoded constant

A magic number is an unexplained literal value in code. The reader can’t tell what it means or why it has that value.

if user.age >= 18 and attempts < 5:
    price = base * 1.2 + 4.99

What’s 1.2? What’s 4.99? Why 5? Give them names:

LEGAL_AGE = 18
MAX_LOGIN_ATTEMPTS = 5
VAT_MULTIPLIER = 1.2
SHIPPING_FEE = 4.99

if user.age >= LEGAL_AGE and attempts < MAX_LOGIN_ATTEMPTS:
    price = base * VAT_MULTIPLIER + SHIPPING_FEE

Why it matters

• Meaning: the name documents the intent.
• One place to change: when the VAT rate moves, you edit one line instead of hunting for 1.2.
• Fewer bugs: the same 86400 typed in five places will eventually be typed wrongly in one.

Not everything is magic

0, 1, -1 and 2 in obvious contexts (items[0], n % 2 == 0) don’t need names. Name a value when its meaning isn’t obvious from the code, or when it might change.

Magic strings count too: if status == "A3" should be a named constant or an enum.

Where to put them

A constant near the code that uses it. Values that differ between environments, or that operations staff need to change, belong in configuration instead.