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