Code Smell 02 - Constants and Magic Numbers

Code Smell 02 - Constants and Magic Numbers

A method makes calculations with lots of numbers without describing their semantics

Maxi Contieri
·Oct 20, 2020·

1 min read

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Table of contents

  • Problems
  • Solutions
  • Examples
  • Sample Code
  • Detection
  • Tags
  • More info
  • Credits

TL;DR: Avoid Magic numbers without explanation. We don't know their source and are afraid to change them.


  • Coupling
  • Low testability
  • Low readability


1) Rename the constant with a semantic and name (meaningful and intention revealing).

2) Replace constants with parameters, so you can mock them from the outside.

3) The constant definition is often a different object than the constant (ab)user.


  • Algorithms Hyper Parameters

Sample Code


def energy(mass)
    mass * 299792458 ** 2


# Storing magnitudes without units is another smell
class PhysicsConstants
   LIGHT_SPEED = 299792458.freeze

def energy(mass)
    mass * PhysicsConstants::LIGHT_SPEED ** 2


Many linters can detect number literals in attributes and methods.


  • Hard coded
  • Constants

More info


Photo by Kristopher Roller on Unsplash

In a purely functional program, the value of a [constant] never changes, and yet, it changes all the time! A paradox!

Joel Spolsky

This article is part of the CodeSmell Series.

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Technical Opinions are my own. I don't have the revealed truth.

Software Design is a creative activity. These are hints and not rigid rules.

I write on BackEnd Business Systems and OOP Design. My advice/experience might not suit other systems.

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