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PEP 315 -- Enhanced While Loop

PEP:315
Title:Enhanced While Loop
Author:Raymond Hettinger <python at rcn.com>, W Isaac Carroll <icarroll at pobox.com>
Status:Rejected
Type:Standards Track
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.

Source: https://github.com/python/peps/blob/master/pep-0315.txt