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<!DOCTYPE html> <html lang="en"> <head> <meta charset="utf-8"> <meta name="viewport" content="width=device-width, initial-scale=1.0"> <meta name="color-scheme" content="light dark"> <title>PEP 341 – Unifying try-except and try-finally | peps.python.org</title> <link rel="shortcut icon" href="../_static/py.png"> <link rel="canonical" href="https://peps.python.org/pep-0341/"> <link rel="stylesheet" href="../_static/style.css" type="text/css"> <link rel="stylesheet" href="../_static/mq.css" type="text/css"> <link rel="stylesheet" href="../_static/pygments.css" type="text/css" media="(prefers-color-scheme: light)" id="pyg-light"> <link rel="stylesheet" href="../_static/pygments_dark.css" type="text/css" media="(prefers-color-scheme: dark)" id="pyg-dark"> <link rel="alternate" type="application/rss+xml" title="Latest PEPs" href="https://peps.python.org/peps.rss"> <meta property="og:title" content='PEP 341 – Unifying try-except and try-finally | peps.python.org'> <meta property="og:description" content="This PEP proposes a change in the syntax and semantics of try statements to allow combined try-except-finally blocks. 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This means in short that it would be valid to write:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">try</span><span class="p">:</span> <span class="o"><</span><span class="n">do</span> <span class="n">something</span><span class="o">></span> <span class="k">except</span> <span class="ne">Exception</span><span class="p">:</span> <span class="o"><</span><span class="n">handle</span> <span class="n">the</span> <span class="n">error</span><span class="o">></span> <span class="k">finally</span><span class="p">:</span> <span class="o"><</span><span class="n">cleanup</span><span class="o">></span> </pre></div> </div> </section> <section id="rationale-proposal"> <h2><a class="toc-backref" href="#rationale-proposal" role="doc-backlink">Rationale/Proposal</a></h2> <p>There are many use cases for the try-except statement and for the try-finally statement per se; however, often one needs to catch exceptions and execute some cleanup code afterwards. It is slightly annoying and not very intelligible that one has to write:</p> <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> <span class="k">try</span><span class="p">:</span> <span class="k">try</span><span class="p">:</span> <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> <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> <span class="k">except</span> <span class="ne">IOError</span><span class="p">:</span> <span class="nb">print</span> <span class="s1">'An error occurred'</span> <span class="k">finally</span><span class="p">:</span> <span class="k">if</span> <span class="n">f</span><span class="p">:</span> <span class="n">f</span><span class="o">.</span><span class="n">close</span><span class="p">()</span> </pre></div> </div> <p>So it is proposed that a construction like this:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">try</span><span class="p">:</span> <span class="o"><</span><span class="n">suite</span> <span class="mi">1</span><span class="o">></span> <span class="k">except</span> <span class="n">Ex1</span><span class="p">:</span> <span class="o"><</span><span class="n">suite</span> <span class="mi">2</span><span class="o">></span> <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> <span class="k">else</span><span class="p">:</span> <span class="o"><</span><span class="n">suite</span> <span class="mi">3</span><span class="o">></span> <span class="k">finally</span><span class="p">:</span> <span class="o"><</span><span class="n">suite</span> <span class="mi">4</span><span class="o">></span> </pre></div> </div> <p>be exactly the same as the legacy:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">try</span><span class="p">:</span> <span class="k">try</span><span class="p">:</span> <span class="o"><</span><span class="n">suite</span> <span class="mi">1</span><span class="o">></span> <span class="k">except</span> <span class="n">Ex1</span><span class="p">:</span> <span class="o"><</span><span class="n">suite</span> <span class="mi">2</span><span class="o">></span> <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> <span class="k">else</span><span class="p">:</span> <span class="o"><</span><span class="n">suite</span> <span class="mi">3</span><span class="o">></span> <span class="k">finally</span><span class="p">:</span> <span class="o"><</span><span class="n">suite</span> <span class="mi">4</span><span class="o">></span> </pre></div> </div> <p>This is backwards compatible, and every try statement that is legal today would continue to work.</p> </section> <section id="changes-to-the-grammar"> <h2><a class="toc-backref" href="#changes-to-the-grammar" role="doc-backlink">Changes to the grammar</a></h2> <p>The grammar for the try statement, which is currently:</p> <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> <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> </pre></div> </div> <p>would have to become:</p> <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> <span class="p">(</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> <span class="p">[</span><span class="s1">'else'</span> <span class="s1">':'</span> <span class="n">suite</span><span class="p">]</span> <span class="p">[</span><span class="s1">'finally'</span> <span class="s1">':'</span> <span class="n">suite</span><span class="p">]</span> <span class="o">|</span> <span class="s1">'finally'</span> <span class="s1">':'</span> <span class="n">suite</span> <span class="p">)</span> </pre></div> </div> </section> <section id="implementation"> <h2><a class="toc-backref" href="#implementation" role="doc-backlink">Implementation</a></h2> <p>As the PEP author currently does not have sufficient knowledge of the CPython implementation, he is unfortunately not able to deliver one. Thomas Lee has submitted a patch <a class="footnote-reference brackets" href="#id5" id="id1">[2]</a>.</p> <p>However, according to Guido, it should be a piece of cake to implement <a class="footnote-reference brackets" href="#id4" id="id2">[1]</a> – at least for a core hacker.</p> <p>This patch was committed 17 December 2005, SVN revision 41740 <a class="footnote-reference brackets" href="#id6" id="id3">[3]</a>.</p> </section> <section id="references"> <h2><a class="toc-backref" href="#references" role="doc-backlink">References</a></h2> <aside class="footnote-list brackets"> <aside class="footnote brackets" id="id4" role="doc-footnote"> <dt class="label" id="id4">[<a href="#id2">1</a>]</dt> <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> <aside class="footnote brackets" id="id5" role="doc-footnote"> <dt class="label" id="id5">[<a href="#id1">2</a>]</dt> <dd><a class="reference external" href="https://bugs.python.org/issue1355913">https://bugs.python.org/issue1355913</a></aside> <aside class="footnote brackets" id="id6" role="doc-footnote"> <dt class="label" id="id6">[<a href="#id3">3</a>]</dt> <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> <section id="copyright"> <h2><a class="toc-backref" href="#copyright" role="doc-backlink">Copyright</a></h2> <p>This document has been placed in the public domain.</p> </section> </section> <hr class="docutils" /> <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">2025-02-01 08:59:27 GMT</a></p> </article> <nav id="pep-sidebar"> <h2>Contents</h2> <ul> <li><a class="reference internal" href="#abstract">Abstract</a></li> <li><a class="reference internal" href="#rationale-proposal">Rationale/Proposal</a></li> <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> </ul> <br> <a id="source" href="https://github.com/python/peps/blob/main/peps/pep-0341.rst">Page Source (GitHub)</a> </nav> </section> <script src="../_static/colour_scheme.js"></script> <script src="../_static/wrap_tables.js"></script> <script src="../_static/sticky_banner.js"></script> </body> </html>