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Version: 1.0

YAGNI

What Is YAGNI?​

YAGNI stands for You Aren’t Gonna Need It — a key principle from Agile and Extreme Programming (XP) that advises:

Don’t implement something until it’s actually needed.

Core Idea​

  • Don’t build features, methods, or abstractions just in case.
  • Only write code when there’s a real, immediate need.

Example​

  • Violating YAGNI
def send_notification(user: User, method: int="email", retries: int=3) -> None:
    if method == "email":
        print(f"Sending email to {user}")
    elif method == "sms":
        print(f"Sending SMS to {user}")
    # In reality, only 'email' is ever used in the app

You spent time building support for sms and retries, but you don’t use them. This is extra logic to maintain, test, and potentially fix in the future — wasted effort.

  • YAGNI Applied
def send_email(user: User):
    print(f"Sending email to {user}")

You can always refactor later if you do need SMS or retries. But until then, this is simpler, easier to test, and easier to maintain.

Common Violations​

  • Writing code for features you might need later
  • Adding configuration or flags for future use
  • Building complex inheritance or plugin systems early

Summary​

YAGNI = Don’t do extra work for hypothetical needs. Build what you need now, not what you think you might need.