Engineering Craft › Clean Code & Principles

YAGNI (You Aren't Gonna Need It)

Don't build things before you need them.

Also known as: You Aren't Gonna Need It, You Ain't Gonna Need It

YAGNI (“You Aren’t Gonna Need It”) says: don’t build something because you might need it later. Build it when you actually do. It comes from Extreme Programming.

# Needed today
def send_welcome_email(user):
    ...

# Built "just in case", used by nothing
class NotificationManager:
    def __init__(self, channels=("email", "sms", "push", "slack"), retry_policy=None,
                 template_engine=None, plugins=None): ...

The second one has four channels, a plugin system and a template engine, and the product only sends one email.

Why guessing is expensive

• You’re probably wrong about what the future needs, so the extra code doesn’t fit when the real requirement arrives.
• Extra code has a cost now. It has to be written, tested, read, maintained and debugged, even if it’s never used.
• It makes the real work harder, by adding more things to understand.
• Waiting is cheap if your code is easy to change.

Typical offenders

• Configuration options nobody asked for.
• Generic frameworks built for one use case.
• Database columns or API fields “for later”.
• Abstractions with only one implementation.

What YAGNI doesn’t mean

It is not “skip tests”, “write messy code” or “ignore the design”. Good structure, clear names and tests are what make it cheap to add features later. And some things are expensive to retrofit (security, data models you can’t easily migrate, public API shapes), so think about those up front.

Ask: “Is there a requirement today that needs this?” If not, leave it out. Related ideas: KISS and premature abstraction.