Rule of three (computer programming)
Rule of three is a code refactoring rule of thumb to decide when a replicated piece of code should be replaced by a new procedure. It states that the code can be copied once, but that when the same code is used three times, it should be extracted into a new procedure. The rule was introduced by Martin Fowler in Refactoring and attributed to Don Roberts.
Duplication in programming is a bad practice because it makes the code harder to maintain. Triplication is worse because it makes it harder yet. When the rule encoded in a replicated piece of code changes, whoever maintains the code will have to change it in all places correctly. This process is error-prone and often leads to problems. If the code exists in only one place, then it can be easily changed there. This rule is typically only applied to a small number of lines of code, or even single lines of code. For example, if a program calls a function, and then calls it again when it fails, it is acceptable to have two call sites; however, if it is to be called five times before giving up, there should only be one call site inside a loop.
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