You aren't gonna need it

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"You aren't gonna need it"[1][2] (acronym: YAGNI)[3] is a principle of extreme programming (XP) that states a programmer should not add functionality until deemed necessary.[4] XP co-founder Ron Jeffries has written: "Always implement things when you actually need them, never when you just foresee that you need them."[5] Other forms of the phrase include "You aren't going to need it"[6][7] and "You ain't gonna need it".[8]


YAGNI is a principle behind the XP practice of "do the simplest thing that could possibly work" (DTSTTCPW).[2][3] It is meant to be used in combination with several other practices, such as continuous refactoring, continuous automated unit testing, and continuous integration. Used without continuous refactoring, it could lead to disorganized code and massive rework. Continuous refactoring in turn relies on automated unit tests or static analysis tools as a safety net and continuous integration to prevent wider integration problems.[citation needed] The efficacy of YAGNI, even when considered in combination with the supporting practices, is controversial.[citation needed] YAGNI's dependency on supporting practices is part of the original definition of XP.


According to those[who?] who advocate the YAGNI approach, the temptation to write code that is not necessary at the moment, but might be in the future, has the following disadvantages:[citation needed]

  • The time spent is taken from adding, testing or improving the necessary functionality.
  • The new features must be debugged, documented, and supported.
  • Any new feature imposes constraints on what can be done in the future, so an unnecessary feature may preclude needed features from being added in the future.
  • Until the feature is actually needed, it is difficult to fully define what it should do and to test it. If the new feature is not properly defined and tested, it may not work correctly, even if it eventually is needed.
  • It leads to code bloat; the software becomes larger and more complicated.
  • Unless there are specifications and some kind of revision control, the feature may not be known to programmers who could make use of it.
  • Adding the new feature may suggest other new features. If these new features are implemented as well, this could result in a snowball effect towards feature creep.

See also[edit]


  1. ^ Extreme Programming Installed, Ronald E. Jeffries, Ann Anderson, Chet Hendrickson, 2001, 265 pages, p. 190, webpage: Books-Google-dIsC, quote: "YAGNI: 'You Aren't Gonna Need It.' This slogan, one of XP's most famous..., reminds us always to work on the story we have, not something we think we're going to need."
  2. ^ a b Extreme Programming examined, Giancarlo Succi, Michele Marchesi, 2001, 569 pages, p., webpage: Books-Google-VSCh, quote: "XP says 'do the simplest thing that could possibly work ' because 'you aren't gonna need it'."
  3. ^ a b Object-oriented & classical software engineering, Stephen R. Schach, 2007, 618 pages, p., webpage: Book-Google-hWwh, quote: "Two acronyms now associated with extreme programming are YAGNI (you aren't gonna need it) and DTSTTCPW (do the simplest thing that could possibly work)."
  4. ^ Lowell Lindstrom; Carmen Zannier; Erdogmus, Hakan (2004). "Extreme Programming and Agile Methods". XP/Agile Universe 2004: 4th Conference on Extreme Programming and Agile Methods (Calgary, Canada, August 15–18) (Lecture Notes in Computer Science). Berlin: Springer. p. 121. ISBN 3-540-22839-X. 
  5. ^ Ron Jeffries. "Practices". X Programming. Retrieved 2007-11-07.  |contribution= ignored (help)
  6. ^ Martin Fowler; Kent Beck (8 July 1999). Refactoring: Improving the Design of Existing Code. Addison-Wesley Professional, 431 pages, p. 68, webpage: BGoogle-1M. ISBN 0201485672, ISBN 978-0201485677. Quote: "you aren't going to need it".
  7. ^ Mary Poppendieck; Tom Poppendieck (2003). Lean Software Development: An Agile Toolkit, p.59, webpage: BGoogle-hQ. Quote: "Kent Beck, Extreme Programming Explained, Chapter 17, uses the acronym YAGNI (You Aren't Going to Need It) for this practice and explains its rationale."
  8. ^ Russ Olsen (2007). Design Patterns in Ruby, p.13, webpage: [1]. ISBN 0321490452, ISBN 9780321490452. Quote: "This design principle comes out of the Extreme Programming world and is elegantly summed up by the phrase You Ain’t Gonna Need It (YAGNI for short)."