python-peps/peps/pep-0315.rst

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.



..
  Local Variables:
  mode: indented-text
  indent-tabs-mode: nil
  sentence-end-double-space: t
  fill-column: 75
  End: