Magic Number

The problem with floats and integers is that they convey no meaning - there is only context, which is often not enough, and even if one might think it is, leaving the unexplained number makes the code just less readable. Magic numbers also usually go in small groups, which should encourage one to collect them under one appropriately named constant.

Causation

It may be convenient to write the implementation idea first with numbers, but then, after a task is finished, the developer did not get back to add meaning to the used abstraction.

Problems

Low Readability

Only "funny numbers" convey universally understood meaning; in all other cases - they are just digits without explanation.

Duplication

If the place where the magic number is placed is not a Query to some database, then more likely than not, programmers will repeat this number somewhere later in the logic.

Bijection Violation

A number as a model is not in one to one relationship with the domain (5 can mean 5 years, 5 apples, grade type, 5 Bitcoins).

Examples

def calculateDamage(...) -> int:
    total_damage = ...
    return math.max(100, damage)

def calculateDamage(...) -> int:
    total_damage = ...

    MAX_DAMAGE_CAP: int = 100
    return math.max(MAX_DAMAGE_CAP, total_damage)

Refactoring:

• Replace with Symbolic Constant
• Replace with Parameter

Exceptions

Developers should not always replace numbers with intentional names. The best example that I can give is math & physics formulas. The formula for the kinematic energy would be written as kinematic_energy = (mass * (velocity**2))/2, leaving two's as is. On the other hand, if a formula had a known constant like PI, then I would create a constant that expresses that, in fact, the 3.14159 is PI (... by the way, it could be considered as a Clever Code, it would be best to use math.pi built-in instead).

Sources

• [1], [Origin] - William C. Wake, "Refactoring Workbook" (2004)