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<h1 class="page-title">PEP 341 – Unifying try-except and try-finally</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">Georg Brandl <georg at python.org></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abbr title="Accepted and implementation complete, or no longer active">Final</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">04-May-2005</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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<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="#rationale-proposal">Rationale/Proposal</a></li>
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<li><a class="reference internal" href="#changes-to-the-grammar">Changes to the grammar</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 a change in the syntax and semantics of try
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statements to allow combined try-except-finally blocks. This
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means in short that it would be valid to write:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">try</span><span class="p">:</span>
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<span class="o"><</span><span class="n">do</span> <span class="n">something</span><span class="o">></span>
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<span class="k">except</span> <span class="ne">Exception</span><span class="p">:</span>
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<span class="o"><</span><span class="n">handle</span> <span class="n">the</span> <span class="n">error</span><span class="o">></span>
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<span class="k">finally</span><span class="p">:</span>
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<span class="o"><</span><span class="n">cleanup</span><span class="o">></span>
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</pre></div>
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</div>
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</section>
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<section id="rationale-proposal">
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<h2><a class="toc-backref" href="#rationale-proposal" role="doc-backlink">Rationale/Proposal</a></h2>
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<p>There are many use cases for the try-except statement and
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for the try-finally statement per se; however, often one needs
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to catch exceptions and execute some cleanup code afterwards.
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It is slightly annoying and not very intelligible that
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one has to write:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">f</span> <span class="o">=</span> <span class="kc">None</span>
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<span class="k">try</span><span class="p">:</span>
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<span class="k">try</span><span class="p">:</span>
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<span class="n">f</span> <span class="o">=</span> <span class="nb">open</span><span class="p">(</span><span class="n">filename</span><span class="p">)</span>
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<span class="n">text</span> <span class="o">=</span> <span class="n">f</span><span class="o">.</span><span class="n">read</span><span class="p">()</span>
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<span class="k">except</span> <span class="ne">IOError</span><span class="p">:</span>
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<span class="nb">print</span> <span class="s1">'An error occurred'</span>
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<span class="k">finally</span><span class="p">:</span>
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<span class="k">if</span> <span class="n">f</span><span class="p">:</span>
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<span class="n">f</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
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</pre></div>
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</div>
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<p>So it is proposed that a construction like this:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">try</span><span class="p">:</span>
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<span class="o"><</span><span class="n">suite</span> <span class="mi">1</span><span class="o">></span>
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<span class="k">except</span> <span class="n">Ex1</span><span class="p">:</span>
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<span class="o"><</span><span class="n">suite</span> <span class="mi">2</span><span class="o">></span>
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<span class="o"><</span><span class="n">more</span> <span class="k">except</span><span class="p">:</span> <span class="n">clauses</span><span class="o">></span>
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<span class="k">else</span><span class="p">:</span>
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<span class="o"><</span><span class="n">suite</span> <span class="mi">3</span><span class="o">></span>
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<span class="k">finally</span><span class="p">:</span>
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<span class="o"><</span><span class="n">suite</span> <span class="mi">4</span><span class="o">></span>
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</pre></div>
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</div>
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<p>be exactly the same as the legacy:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">try</span><span class="p">:</span>
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<span class="k">try</span><span class="p">:</span>
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<span class="o"><</span><span class="n">suite</span> <span class="mi">1</span><span class="o">></span>
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<span class="k">except</span> <span class="n">Ex1</span><span class="p">:</span>
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<span class="o"><</span><span class="n">suite</span> <span class="mi">2</span><span class="o">></span>
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<span class="o"><</span><span class="n">more</span> <span class="k">except</span><span class="p">:</span> <span class="n">clauses</span><span class="o">></span>
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<span class="k">else</span><span class="p">:</span>
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<span class="o"><</span><span class="n">suite</span> <span class="mi">3</span><span class="o">></span>
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<span class="k">finally</span><span class="p">:</span>
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<span class="o"><</span><span class="n">suite</span> <span class="mi">4</span><span class="o">></span>
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</pre></div>
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</div>
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<p>This is backwards compatible, and every try statement that is
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legal today would continue to work.</p>
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</section>
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<section id="changes-to-the-grammar">
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<h2><a class="toc-backref" href="#changes-to-the-grammar" role="doc-backlink">Changes to the grammar</a></h2>
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||
<p>The grammar for the try statement, which is currently:</p>
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||
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">try_stmt</span><span class="p">:</span> <span class="p">(</span><span class="s1">'try'</span> <span class="s1">':'</span> <span class="n">suite</span> <span class="p">(</span><span class="n">except_clause</span> <span class="s1">':'</span> <span class="n">suite</span><span class="p">)</span><span class="o">+</span>
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||
<span class="p">[</span><span class="s1">'else'</span> <span class="s1">':'</span> <span class="n">suite</span><span class="p">]</span> <span class="o">|</span> <span class="s1">'try'</span> <span class="s1">':'</span> <span class="n">suite</span> <span class="s1">'finally'</span> <span class="s1">':'</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>would have to become:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">try_stmt</span><span class="p">:</span> <span class="s1">'try'</span> <span class="s1">':'</span> <span class="n">suite</span>
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<span class="p">(</span>
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<span class="p">(</span><span class="n">except_clause</span> <span class="s1">':'</span> <span class="n">suite</span><span class="p">)</span><span class="o">+</span>
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<span class="p">[</span><span class="s1">'else'</span> <span class="s1">':'</span> <span class="n">suite</span><span class="p">]</span>
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<span class="p">[</span><span class="s1">'finally'</span> <span class="s1">':'</span> <span class="n">suite</span><span class="p">]</span>
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<span class="o">|</span>
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<span class="s1">'finally'</span> <span class="s1">':'</span> <span class="n">suite</span>
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<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="implementation">
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<h2><a class="toc-backref" href="#implementation" role="doc-backlink">Implementation</a></h2>
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<p>As the PEP author currently does not have sufficient knowledge
|
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of the CPython implementation, he is unfortunately not able
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to deliver one. Thomas Lee has submitted a patch <a class="footnote-reference brackets" href="#id5" id="id1">[2]</a>.</p>
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<p>However, according to Guido, it should be a piece of cake to
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implement <a class="footnote-reference brackets" href="#id4" id="id2">[1]</a> – at least for a core hacker.</p>
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<p>This patch was committed 17 December 2005, SVN revision 41740 <a class="footnote-reference brackets" href="#id6" id="id3">[3]</a>.</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>
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<aside class="footnote-list brackets">
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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">1</a>]</dt>
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<dd><a class="reference external" href="https://mail.python.org/pipermail/python-dev/2005-May/053319.html">https://mail.python.org/pipermail/python-dev/2005-May/053319.html</a></aside>
|
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<aside class="footnote brackets" id="id5" role="doc-footnote">
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<dt class="label" id="id5">[<a href="#id1">2</a>]</dt>
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<dd><a class="reference external" href="https://bugs.python.org/issue1355913">https://bugs.python.org/issue1355913</a></aside>
|
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<aside class="footnote brackets" id="id6" role="doc-footnote">
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<dt class="label" id="id6">[<a href="#id3">3</a>]</dt>
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<dd><a class="reference external" href="https://mail.python.org/pipermail/python-checkins/2005-December/048457.html">https://mail.python.org/pipermail/python-checkins/2005-December/048457.html</a></aside>
|
||
</aside>
|
||
</section>
|
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<section id="copyright">
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||
<h2><a class="toc-backref" href="#copyright" role="doc-backlink">Copyright</a></h2>
|
||
<p>This document has been placed in the public domain.</p>
|
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</section>
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</section>
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<hr class="docutils" />
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<p>Source: <a class="reference external" href="https://github.com/python/peps/blob/main/peps/pep-0341.rst">https://github.com/python/peps/blob/main/peps/pep-0341.rst</a></p>
|
||
<p>Last modified: <a class="reference external" href="https://github.com/python/peps/commits/main/peps/pep-0341.rst">2023-09-09 17:39:29 GMT</a></p>
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<h2>Contents</h2>
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<ul>
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<li><a class="reference internal" href="#abstract">Abstract</a></li>
|
||
<li><a class="reference internal" href="#rationale-proposal">Rationale/Proposal</a></li>
|
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<li><a class="reference internal" href="#changes-to-the-grammar">Changes to the grammar</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>
|
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</ul>
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<br>
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