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<h1 class="page-title">PEP 315 – Enhanced While Loop</h1>
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<dl class="rfc2822 field-list simple">
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<dt class="field-odd">Author<span class="colon">:</span></dt>
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<dd class="field-odd">Raymond Hettinger <python at rcn.com>, W Isaac Carroll <icarroll at pobox.com></dd>
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<dt class="field-even">Status<span class="colon">:</span></dt>
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<dd class="field-even"><abbr title="Formally declined and will not be accepted">Rejected</abbr></dd>
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<dt class="field-odd">Type<span class="colon">:</span></dt>
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<dd class="field-odd"><abbr title="Normative PEP with a new feature for Python, implementation change for CPython or interoperability standard for the ecosystem">Standards Track</abbr></dd>
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<dt class="field-even">Created<span class="colon">:</span></dt>
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<dd class="field-even">25-Apr-2003</dd>
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<dt class="field-odd">Python-Version<span class="colon">:</span></dt>
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<dd class="field-odd">2.5</dd>
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<dt class="field-even">Post-History<span class="colon">:</span></dt>
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<dd class="field-even"><p></p></dd>
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<details><summary>Table of Contents</summary><ul class="simple">
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<li><a class="reference internal" href="#abstract">Abstract</a></li>
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<li><a class="reference internal" href="#notice">Notice</a></li>
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<li><a class="reference internal" href="#motivation">Motivation</a></li>
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<li><a class="reference internal" href="#syntax">Syntax</a></li>
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<li><a class="reference internal" href="#semantics-of-break-and-continue">Semantics of break and continue</a></li>
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<li><a class="reference internal" href="#future-statement">Future Statement</a></li>
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<li><a class="reference internal" href="#implementation">Implementation</a></li>
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<li><a class="reference internal" href="#references">References</a></li>
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<li><a class="reference internal" href="#copyright">Copyright</a></li>
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</ul>
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</details></section>
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<section id="abstract">
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<h2><a class="toc-backref" href="#abstract" role="doc-backlink">Abstract</a></h2>
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<p>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.</p>
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</section>
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<section id="notice">
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<h2><a class="toc-backref" href="#notice" role="doc-backlink">Notice</a></h2>
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<p>Rejected; see <a class="footnote-reference brackets" href="#id3" id="id1">[1]</a>.</p>
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<p>This PEP has been deferred since 2006; see <a class="footnote-reference brackets" href="#id4" id="id2">[2]</a>.</p>
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<p>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:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">while</span> <span class="kc">True</span><span class="p">:</span>
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<span class="o"><</span><span class="n">setup</span> <span class="n">code</span><span class="o">></span>
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<span class="k">if</span> <span class="ow">not</span> <span class="o"><</span><span class="n">condition</span><span class="o">></span><span class="p">:</span>
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<span class="k">break</span>
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<span class="o"><</span><span class="n">loop</span> <span class="n">body</span><span class="o">></span>
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</pre></div>
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</div>
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<p>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:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">do</span> <span class="o">...</span> <span class="k">while</span> <span class="o"><</span><span class="n">cond</span><span class="o">></span><span class="p">:</span>
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<span class="o"><</span><span class="n">loop</span> <span class="n">body</span><span class="o">></span>
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</pre></div>
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</div>
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<p>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.</p>
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</section>
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<section id="motivation">
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<h2><a class="toc-backref" href="#motivation" role="doc-backlink">Motivation</a></h2>
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<p>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:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o"><</span><span class="n">setup</span> <span class="n">code</span><span class="o">></span>
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<span class="k">while</span> <span class="o"><</span><span class="n">condition</span><span class="o">></span><span class="p">:</span>
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<span class="o"><</span><span class="n">loop</span> <span class="n">body</span><span class="o">></span>
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<span class="o"><</span><span class="n">setup</span> <span class="n">code</span><span class="o">></span>
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</pre></div>
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</div>
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<p>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.</p>
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<p>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:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">helper</span><span class="p">(</span><span class="n">args</span><span class="p">):</span>
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<span class="o"><</span><span class="n">setup</span> <span class="n">code</span><span class="o">></span>
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<span class="k">return</span> <span class="o"><</span><span class="n">condition</span><span class="o">></span>
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<span class="k">while</span> <span class="n">helper</span><span class="p">(</span><span class="n">args</span><span class="p">):</span>
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<span class="o"><</span><span class="n">loop</span> <span class="n">body</span><span class="o">></span>
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</pre></div>
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</div>
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<p>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:</p>
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||
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">while</span> <span class="kc">True</span><span class="p">:</span>
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||
<span class="o"><</span><span class="n">setup</span> <span class="n">code</span><span class="o">></span>
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||
<span class="k">if</span> <span class="ow">not</span> <span class="o"><</span><span class="n">condition</span><span class="o">></span><span class="p">:</span> <span class="k">break</span>
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||
<span class="o"><</span><span class="n">loop</span> <span class="n">body</span><span class="o">></span>
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||
</pre></div>
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</div>
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||
<p>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:</p>
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||
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">do</span><span class="p">:</span>
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||
<span class="o"><</span><span class="n">setup</span> <span class="n">code</span><span class="o">></span>
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||
<span class="k">while</span> <span class="o"><</span><span class="n">condition</span><span class="o">></span><span class="p">:</span>
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||
<span class="o"><</span><span class="n">loop</span> <span class="n">body</span><span class="o">></span>
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</pre></div>
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</div>
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<p>This keeps the loop condition with the while keyword where it
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belongs, and does not require code to be duplicated.</p>
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</section>
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<section id="syntax">
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<h2><a class="toc-backref" href="#syntax" role="doc-backlink">Syntax</a></h2>
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<p>The syntax of the while statement</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">while_stmt</span> <span class="p">:</span> <span class="s2">"while"</span> <span class="n">expression</span> <span class="s2">":"</span> <span class="n">suite</span>
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<span class="p">[</span><span class="s2">"else"</span> <span class="s2">":"</span> <span class="n">suite</span><span class="p">]</span>
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</pre></div>
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</div>
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<p>is extended as follows:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">while_stmt</span> <span class="p">:</span> <span class="p">[</span><span class="s2">"do"</span> <span class="s2">":"</span> <span class="n">suite</span><span class="p">]</span>
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<span class="s2">"while"</span> <span class="n">expression</span> <span class="s2">":"</span> <span class="n">suite</span>
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||
<span class="p">[</span><span class="s2">"else"</span> <span class="s2">":"</span> <span class="n">suite</span><span class="p">]</span>
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</pre></div>
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</div>
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</section>
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<section id="semantics-of-break-and-continue">
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<h2><a class="toc-backref" href="#semantics-of-break-and-continue" role="doc-backlink">Semantics of break and continue</a></h2>
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<p>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.</p>
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<p>A continue statement in the do-while loop jumps to the while
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condition check.</p>
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<p>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.</p>
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<p>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.</p>
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</section>
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<section id="future-statement">
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<h2><a class="toc-backref" href="#future-statement" role="doc-backlink">Future Statement</a></h2>
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<p>Because of the new keyword “do”, the statement</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">__future__</span> <span class="kn">import</span> <span class="n">do_while</span>
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</pre></div>
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</div>
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<p>will initially be required to use the do-while form.</p>
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</section>
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<section id="implementation">
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<h2><a class="toc-backref" href="#implementation" role="doc-backlink">Implementation</a></h2>
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<p>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.</p>
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</section>
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<section id="references">
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<h2><a class="toc-backref" href="#references" role="doc-backlink">References</a></h2>
|
||
<aside class="footnote-list brackets">
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||
<aside class="footnote brackets" id="id3" role="doc-footnote">
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<dt class="label" id="id3">[<a href="#id1">1</a>]</dt>
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||
<dd>Guido van Rossum, PEP 315: do-while
|
||
<a class="reference external" href="https://mail.python.org/pipermail/python-ideas/2013-June/021610.html">https://mail.python.org/pipermail/python-ideas/2013-June/021610.html</a></aside>
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<aside class="footnote brackets" id="id4" role="doc-footnote">
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<dt class="label" id="id4">[<a href="#id2">2</a>]</dt>
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||
<dd>Raymond Hettinger, release plan for 2.5 ?
|
||
<a class="reference external" href="https://mail.python.org/pipermail/python-dev/2006-February/060718.html">https://mail.python.org/pipermail/python-dev/2006-February/060718.html</a></aside>
|
||
</aside>
|
||
</section>
|
||
<section id="copyright">
|
||
<h2><a class="toc-backref" href="#copyright" role="doc-backlink">Copyright</a></h2>
|
||
<p>This document is placed in the public domain.</p>
|
||
</section>
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||
</section>
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||
<hr class="docutils" />
|
||
<p>Source: <a class="reference external" href="https://github.com/python/peps/blob/main/peps/pep-0315.rst">https://github.com/python/peps/blob/main/peps/pep-0315.rst</a></p>
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||
<p>Last modified: <a class="reference external" href="https://github.com/python/peps/commits/main/peps/pep-0315.rst">2023-09-09 17:39:29 GMT</a></p>
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</article>
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<h2>Contents</h2>
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<ul>
|
||
<li><a class="reference internal" href="#abstract">Abstract</a></li>
|
||
<li><a class="reference internal" href="#notice">Notice</a></li>
|
||
<li><a class="reference internal" href="#motivation">Motivation</a></li>
|
||
<li><a class="reference internal" href="#syntax">Syntax</a></li>
|
||
<li><a class="reference internal" href="#semantics-of-break-and-continue">Semantics of break and continue</a></li>
|
||
<li><a class="reference internal" href="#future-statement">Future Statement</a></li>
|
||
<li><a class="reference internal" href="#implementation">Implementation</a></li>
|
||
<li><a class="reference internal" href="#references">References</a></li>
|
||
<li><a class="reference internal" href="#copyright">Copyright</a></li>
|
||
</ul>
|
||
|
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<br>
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<a id="source" href="https://github.com/python/peps/blob/main/peps/pep-0315.rst">Page Source (GitHub)</a>
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