PEP 315: Enhanced While Loop

[W Isaac Carroll]

PEP: 315
Title: Enhanced While Loop
Version: $Revision: 1.1 $
Last-Modified: $Date: 2003/05/02 22:53:32 $
Author: W Isaac Carroll <icarroll at pobox.com>
Status: Draft
Type: Standards Track
Content-Type: text/plain
Created: 25-Apr-2003
Python-Version: 2.4
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.


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 will behave somewhat
     differently than in the standard while loop.  Instead of jumping
     back to the loop condition, it will jump to the beginning of the
     first suite of the loop.  This is to ensure that the setup code
     has a chance to do its job before the condition is evaluated.


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.


Copyright

     This document is placed in the public domain.