127 lines
3.4 KiB
Plaintext
127 lines
3.4 KiB
Plaintext
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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: W Isaac Carroll <icarroll@pobox.com>
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Status: Draft
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Type: Standards Track
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Content-Type: text/plain
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Created: 25-Apr-2003
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Python-Version: 2.4
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Post-History:
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Abstract
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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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Motivation
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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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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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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 will behave somewhat
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differently than in the standard while loop. Instead of jumping
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back to the loop condition, it will jump to the beginning of the
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first suite of the loop. This is to ensure that the setup code
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has a chance to do its job before the condition is evaluated.
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Future Statement
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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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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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Copyright
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This document is placed in the public domain.
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Local Variables:
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mode: indented-text
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indent-tabs-mode: nil
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sentence-end-double-space: t
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fill-column: 75
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End:
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