Engineering Craft › Clean Code & Principles
Code Smell
A surface sign that the design may have a deeper problem.
Also known as: code smells, smell, bad smell
A code smell is a surface sign that the design might have a deeper problem. It isn’t a bug (the code may work fine), but it’s a hint to look closer, like noticing a smell in the kitchen.
Common smells
| Smell | What it looks like |
|---|---|
| Long method | A function you have to scroll through |
| God object | One class that knows and does everything |
| Duplicated code | The same logic copied in several places |
| Feature envy | A method that mostly uses another class’s data |
| Primitive obsession | Using raw strings and ints for concepts like money or email |
| Long parameter list | Functions taking six or seven arguments |
| Shotgun surgery | One change requires edits across many files |
| Dead code | Unused functions and branches |
| Magic numbers | Unexplained constants like if status == 3 |
def process(order, user, address, coupon, gift, express, notify, retries): # smell
...What to do about one
A smell is a prompt, not a verdict. Ask whether it actually hurts: is this code hard to change, hard to test, a source of bugs? Then:
- If it’s in code you’re about to change, refactor it as part of that work.
- If it’s stable code nobody touches, leave it alone.
- If it needs more than a quick fix, note it as technical debt.
Don’t hunt for smells everywhere, or rewrite working code just because it looks unfashionable. The skill is noticing them in code you work with, and choosing a fix that is proportionate. Make small, safe steps with tests passing.