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| PEP: 315 | |
| Title: Enhanced While Loop | |
| Version: $Revision$ | |
| Last-Modified: $Date$ | |
| Author: Raymond Hettinger <python@rcn.com>, W Isaac Carroll <icarroll@pobox.com> | |
| Status: Rejected | |
| Type: Standards Track | |
| Content-Type: text/x-rst | |
| Created: 25-Apr-2003 | |
| Python-Version: 2.5 | |
| Post-History: | |
| Abstract | |
| ======== | |
| This PEP proposes adding an optional "do" clause to the beginning | |
| of the while loop to make loop code clearer and reduce errors | |
| caused by code duplication. | |
| Notice | |
| ====== | |
| Rejected; see [1]_. | |
| This PEP has been deferred since 2006; see [2]_. | |
| Subsequent efforts to revive the PEP in April 2009 did not | |
| meet with success because no syntax emerged that could | |
| compete with the following form:: | |
| while True: | |
| <setup code> | |
| if not <condition>: | |
| break | |
| <loop body> | |
| A syntax alternative to the one proposed in the PEP was found for | |
| a basic do-while loop but it gained little support because the | |
| condition was at the top:: | |
| do ... while <cond>: | |
| <loop body> | |
| Users of the language are advised to use the while-True form with | |
| an inner if-break when a do-while loop would have been appropriate. | |
| Motivation | |
| ========== | |
| It is often necessary for some code to be executed before each | |
| evaluation of the while loop condition. This code is often | |
| duplicated outside the loop, as setup code that executes once | |
| before entering the loop:: | |
| <setup code> | |
| while <condition>: | |
| <loop body> | |
| <setup code> | |
| The problem is that duplicated code can be a source of errors if | |
| one instance is changed but the other is not. Also, the purpose | |
| of the second instance of the setup code is not clear because it | |
| comes at the end of the loop. | |
| It is possible to prevent code duplication by moving the loop | |
| condition into a helper function, or an if statement in the loop | |
| body. However, separating the loop condition from the while | |
| keyword makes the behavior of the loop less clear:: | |
| def helper(args): | |
| <setup code> | |
| return <condition> | |
| while helper(args): | |
| <loop body> | |
| This last form has the additional drawback of requiring the loop's | |
| else clause to be added to the body of the if statement, further | |
| obscuring the loop's behavior:: | |
| while True: | |
| <setup code> | |
| if not <condition>: break | |
| <loop body> | |
| This PEP proposes to solve these problems by adding an optional | |
| clause to the while loop, which allows the setup code to be | |
| expressed in a natural way:: | |
| do: | |
| <setup code> | |
| while <condition>: | |
| <loop body> | |
| This keeps the loop condition with the while keyword where it | |
| belongs, and does not require code to be duplicated. | |
| Syntax | |
| ====== | |
| The syntax of the while statement :: | |
| while_stmt : "while" expression ":" suite | |
| ["else" ":" suite] | |
| is extended as follows:: | |
| while_stmt : ["do" ":" suite] | |
| "while" expression ":" suite | |
| ["else" ":" suite] | |
| Semantics of break and continue | |
| =============================== | |
| In the do-while loop the break statement will behave the same as | |
| in the standard while loop: It will immediately terminate the loop | |
| without evaluating the loop condition or executing the else | |
| clause. | |
| A continue statement in the do-while loop jumps to the while | |
| condition check. | |
| In general, when the while suite is empty (a pass statement), | |
| the do-while loop and break and continue statements should match | |
| the semantics of do-while in other languages. | |
| Likewise, when the do suite is empty, the do-while loop and | |
| break and continue statements should match behavior found | |
| in regular while loops. | |
| Future Statement | |
| ================ | |
| Because of the new keyword "do", the statement :: | |
| from __future__ import do_while | |
| will initially be required to use the do-while form. | |
| Implementation | |
| ============== | |
| The first implementation of this PEP can compile the do-while loop | |
| as an infinite loop with a test that exits the loop. | |
| References | |
| ========== | |
| .. [1] Guido van Rossum, PEP 315: do-while | |
| https://mail.python.org/pipermail/python-ideas/2013-June/021610.html | |
| .. [2] Raymond Hettinger, release plan for 2.5 ? | |
| https://mail.python.org/pipermail/python-dev/2006-February/060718.html | |
| Copyright | |
| ========= | |
| This document is placed in the public domain. | |
| .. | |
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