Rule of three (''"Three strikes and you refactor"'') is a
code refactoring
In computer programming and software design, code refactoring is the process of restructuring existing computer code—changing the '' factoring''—without changing its external behavior. Refactoring is intended to improve the design, structur ...
rule of thumb
In English, the phrase ''rule of thumb'' refers to an approximate method for doing something, based on practical experience rather than theory. This usage of the phrase can be traced back to the 17th century and has been associated with various t ...
to decide when similar pieces of code should be refactored to avoid duplication. It states that two instances of similar code do not require refactoring, but when similar code is used three times, it should be extracted into a new procedure. The rule was popularised by
Martin Fowler in ''Refactoring'' and attributed to Don Roberts.
Duplication
Duplication, duplicate, and duplicator may refer to:
Biology and genetics
* Gene duplication, a process which can result in free mutation
* Chromosomal duplication, which can cause Bloom and Rett syndrome
* Polyploidy, a phenomenon also known ...
is considered a bad practice in programming because it makes the code harder to
maintain. When the rule encoded in a replicated piece of code changes, whoever maintains the code will have to change it in all places correctly.
However, choosing an appropriate design to avoid duplication might benefit from more examples to see patterns in. Attempting premature refactoring risks selecting a wrong abstraction, which can result in worse code as new requirements emerge
Sandi Metz
Sandi Metz is an American software engineer and author. She is the author of ''Practical Object-Oriented Design in Ruby''. Metz teaches workshops around the country for new and experienced developers, emphasizing good programming habits and pra ...
''"The Wrong Abstraction"'' (2016)
/ref> and will eventually need to be refactored again.
The rule implies that the cost of maintenance certainly outweighs the cost of refactoring and potential bad design when there are three copies, and may or may not if there are only two copies.
See also
* Copy-and-paste programming
In human–computer interaction and user interface design, cut, copy, and paste are related commands that offer an interprocess communication technique for transferring data through a computer's user interface. The ''cut'' command removes the ...
* Don't repeat yourself
"Don't repeat yourself" (DRY) is a principle of software development aimed at reducing repetition of software patterns, replacing it with abstractions or using data normalization to avoid redundancy.
The DRY principle is stated as "Every piece o ...
(DRY)
References
{{reflist
Computer programming folklore
Programming principles