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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 415 – Implement context suppression with exception attributes | peps.python.org</title> <link rel="shortcut icon" href="../_static/py.png"> <link rel="canonical" href="https://peps.python.org/pep-0415/"> <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 415 – Implement context suppression with exception attributes | peps.python.org'> <meta property="og:description" content="PEP 409 introduced support for the raise exc from None construct to allow the display of the exception context to be explicitly suppressed. 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Proposals</h1> <ul class="breadcrumbs"> <li><a href="https://www.python.org/" title="The Python Programming Language">Python</a> » </li> <li><a href="../pep-0000/">PEP Index</a> » </li> <li>PEP 415</li> </ul> <button id="colour-scheme-cycler" onClick="setColourScheme(nextColourScheme())"> <svg aria-hidden="true" class="colour-scheme-icon-when-auto"><use href="#svg-sun-half"></use></svg> <svg aria-hidden="true" class="colour-scheme-icon-when-dark"><use href="#svg-moon"></use></svg> <svg aria-hidden="true" class="colour-scheme-icon-when-light"><use href="#svg-sun"></use></svg> <span class="visually-hidden">Toggle light / dark / auto colour theme</span> </button> </header> <article> <section id="pep-content"> <h1 class="page-title">PEP 415 – Implement context suppression with exception attributes</h1> <dl class="rfc2822 field-list simple"> <dt class="field-odd">Author<span class="colon">:</span></dt> <dd class="field-odd">Benjamin Peterson <benjamin at python.org></dd> <dt class="field-even">BDFL-Delegate<span class="colon">:</span></dt> <dd class="field-even">Alyssa Coghlan</dd> <dt class="field-odd">Status<span class="colon">:</span></dt> <dd class="field-odd"><abbr title="Accepted and implementation complete, or no longer active">Final</abbr></dd> <dt class="field-even">Type<span class="colon">:</span></dt> <dd class="field-even"><abbr title="Normative PEP with a new feature for Python, implementation change for CPython or interoperability standard for the ecosystem">Standards Track</abbr></dd> <dt class="field-odd">Created<span class="colon">:</span></dt> <dd class="field-odd">26-Feb-2012</dd> <dt class="field-even">Python-Version<span class="colon">:</span></dt> <dd class="field-even">3.3</dd> <dt class="field-odd">Post-History<span class="colon">:</span></dt> <dd class="field-odd">26-Feb-2012</dd> <dt class="field-even">Replaces<span class="colon">:</span></dt> <dd class="field-even"><a class="reference external" href="../pep-0409/">409</a></dd> <dt class="field-odd">Resolution<span class="colon">:</span></dt> <dd class="field-odd"><a class="reference external" href="https://mail.python.org/pipermail/python-dev/2012-May/119467.html">Python-Dev message</a></dd> </dl> <hr class="docutils" /> <section id="contents"> <details><summary>Table of Contents</summary><ul class="simple"> <li><a class="reference internal" href="#abstract">Abstract</a></li> <li><a class="reference internal" href="#pep-acceptance">PEP Acceptance</a></li> <li><a class="reference internal" href="#rationale">Rationale</a></li> <li><a class="reference internal" href="#proposal">Proposal</a></li> <li><a class="reference internal" href="#patches">Patches</a></li> <li><a class="reference internal" href="#copyright">Copyright</a></li> </ul> </details></section> <section id="abstract"> <h2><a class="toc-backref" href="#abstract" role="doc-backlink">Abstract</a></h2> <p><a class="pep reference internal" href="../pep-0409/" title="PEP 409 – Suppressing exception context">PEP 409</a> introduced support for the <code class="docutils literal notranslate"><span class="pre">raise</span> <span class="pre">exc</span> <span class="pre">from</span> <span class="pre">None</span></code> construct to allow the display of the exception context to be explicitly suppressed. This PEP retains the language level changes already implemented in <a class="pep reference internal" href="../pep-0409/" title="PEP 409 – Suppressing exception context">PEP 409</a>, but replaces the underlying implementation mechanism with a simpler approach based on a new <code class="docutils literal notranslate"><span class="pre">__suppress_context__</span></code> attribute on all <code class="docutils literal notranslate"><span class="pre">BaseException</span></code> instances.</p> </section> <section id="pep-acceptance"> <h2><a class="toc-backref" href="#pep-acceptance" role="doc-backlink">PEP Acceptance</a></h2> <p>This PEP was accepted by Alyssa Coghlan on the 14th of May, 2012.</p> </section> <section id="rationale"> <h2><a class="toc-backref" href="#rationale" role="doc-backlink">Rationale</a></h2> <p><a class="pep reference internal" href="../pep-0409/" title="PEP 409 – Suppressing exception context">PEP 409</a> changes <code class="docutils literal notranslate"><span class="pre">__cause__</span></code> to be <code class="docutils literal notranslate"><span class="pre">Ellipsis</span></code> by default. Then if <code class="docutils literal notranslate"><span class="pre">__cause__</span></code> is set to <code class="docutils literal notranslate"><span class="pre">None</span></code> by <code class="docutils literal notranslate"><span class="pre">raise</span> <span class="pre">exc</span> <span class="pre">from</span> <span class="pre">None</span></code>, no context or cause will be printed should the exception be uncaught.</p> <p>The main problem with this scheme is it complicates the role of <code class="docutils literal notranslate"><span class="pre">__cause__</span></code>. <code class="docutils literal notranslate"><span class="pre">__cause__</span></code> should indicate the cause of the exception not whether <code class="docutils literal notranslate"><span class="pre">__context__</span></code> should be printed or not. This use of <code class="docutils literal notranslate"><span class="pre">__cause__</span></code> is also not easily extended in the future. For example, we may someday want to allow the programmer to select which of <code class="docutils literal notranslate"><span class="pre">__context__</span></code> and <code class="docutils literal notranslate"><span class="pre">__cause__</span></code> will be printed. The <a class="pep reference internal" href="../pep-0409/" title="PEP 409 – Suppressing exception context">PEP 409</a> implementation is not amenable to this.</p> <p>The use of <code class="docutils literal notranslate"><span class="pre">Ellipsis</span></code> is a hack. Before <a class="pep reference internal" href="../pep-0409/" title="PEP 409 – Suppressing exception context">PEP 409</a>, <code class="docutils literal notranslate"><span class="pre">Ellipsis</span></code> was used exclusively in extended slicing. Extended slicing has nothing to do with exceptions, so it’s not clear to someone inspecting an exception object why <code class="docutils literal notranslate"><span class="pre">__cause__</span></code> should be set to <code class="docutils literal notranslate"><span class="pre">Ellipsis</span></code>. Using <code class="docutils literal notranslate"><span class="pre">Ellipsis</span></code> by default for <code class="docutils literal notranslate"><span class="pre">__cause__</span></code> makes it asymmetrical with <code class="docutils literal notranslate"><span class="pre">__context__</span></code>.</p> </section> <section id="proposal"> <h2><a class="toc-backref" href="#proposal" role="doc-backlink">Proposal</a></h2> <p>A new attribute on <code class="docutils literal notranslate"><span class="pre">BaseException</span></code>, <code class="docutils literal notranslate"><span class="pre">__suppress_context__</span></code>, will be introduced. Whenever <code class="docutils literal notranslate"><span class="pre">__cause__</span></code> is set, <code class="docutils literal notranslate"><span class="pre">__suppress_context__</span></code> will be set to <code class="docutils literal notranslate"><span class="pre">True</span></code>. In particular, <code class="docutils literal notranslate"><span class="pre">raise</span> <span class="pre">exc</span> <span class="pre">from</span> <span class="pre">cause</span></code> syntax will set <code class="docutils literal notranslate"><span class="pre">exc.__suppress_context__</span></code> to <code class="docutils literal notranslate"><span class="pre">True</span></code>. Exception printing code will check for that attribute to determine whether context and cause will be printed. <code class="docutils literal notranslate"><span class="pre">__cause__</span></code> will return to its original purpose and values.</p> <p>There is precedence for <code class="docutils literal notranslate"><span class="pre">__suppress_context__</span></code> with the <code class="docutils literal notranslate"><span class="pre">print_line_and_file</span></code> exception attribute.</p> <p>To summarize, <code class="docutils literal notranslate"><span class="pre">raise</span> <span class="pre">exc</span> <span class="pre">from</span> <span class="pre">cause</span></code> will be equivalent to:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">exc</span><span class="o">.</span><span class="n">__cause__</span> <span class="o">=</span> <span class="n">cause</span> <span class="k">raise</span> <span class="n">exc</span> </pre></div> </div> <p>where <code class="docutils literal notranslate"><span class="pre">exc.__cause__</span> <span class="pre">=</span> <span class="pre">cause</span></code> implicitly sets <code class="docutils literal notranslate"><span class="pre">exc.__suppress_context__</span></code>.</p> </section> <section id="patches"> <h2><a class="toc-backref" href="#patches" role="doc-backlink">Patches</a></h2> <p>There is a patch on <a class="reference external" href="http://bugs.python.org/issue14133">Issue 14133</a>.</p> </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-0415.rst">https://github.com/python/peps/blob/main/peps/pep-0415.rst</a></p> <p>Last modified: <a class="reference external" href="https://github.com/python/peps/commits/main/peps/pep-0415.rst">2023-10-11 12:05:51 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="#pep-acceptance">PEP Acceptance</a></li> <li><a class="reference internal" href="#rationale">Rationale</a></li> <li><a class="reference internal" href="#proposal">Proposal</a></li> <li><a class="reference internal" href="#patches">Patches</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-0415.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>