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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 409 – Suppressing exception context | peps.python.org</title> <link rel="shortcut icon" href="../_static/py.png"> <link rel="canonical" href="https://peps.python.org/pep-0409/"> <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 409 – Suppressing exception context | peps.python.org'> <meta property="og:description" content="One of the open issues from PEP 3134 is suppressing context: currently there is no way to do it. 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title="The Python Programming Language">Python</a> » </li> <li><a href="../pep-0000/">PEP Index</a> » </li> <li>PEP 409</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 409 – Suppressing exception context</h1> <dl class="rfc2822 field-list simple"> <dt class="field-odd">Author<span class="colon">:</span></dt> <dd class="field-odd">Ethan Furman <ethan at stoneleaf.us></dd> <dt class="field-even">Status<span class="colon">:</span></dt> <dd class="field-even"><abbr title="Accepted and implementation complete, or no longer active">Final</abbr></dd> <dt class="field-odd">Type<span class="colon">:</span></dt> <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> <dt class="field-even">Created<span class="colon">:</span></dt> <dd class="field-even">26-Jan-2012</dd> <dt class="field-odd">Python-Version<span class="colon">:</span></dt> <dd class="field-odd">3.3</dd> <dt class="field-even">Post-History<span class="colon">:</span></dt> <dd class="field-even">30-Aug-2002, 01-Feb-2012, 03-Feb-2012</dd> <dt class="field-odd">Superseded-By<span class="colon">:</span></dt> <dd class="field-odd"><a class="reference external" href="../pep-0415/">415</a></dd> <dt class="field-even">Resolution<span class="colon">:</span></dt> <dd class="field-even"><a class="reference external" href="https://mail.python.org/pipermail/python-dev/2012-February/116136.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="#rationale">Rationale</a></li> <li><a class="reference internal" href="#alternatives">Alternatives</a></li> <li><a class="reference internal" href="#proposal">Proposal</a></li> <li><a class="reference internal" href="#implementation-discussion">Implementation Discussion</a></li> <li><a class="reference internal" href="#language-details">Language Details</a></li> <li><a class="reference internal" href="#patches">Patches</a></li> <li><a class="reference internal" href="#references">References</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>One of the open issues from <a class="pep reference internal" href="../pep-3134/" title="PEP 3134 – Exception Chaining and Embedded Tracebacks">PEP 3134</a> is suppressing context: currently there is no way to do it. This PEP proposes one.</p> </section> <section id="rationale"> <h2><a class="toc-backref" href="#rationale" role="doc-backlink">Rationale</a></h2> <p>There are two basic ways to generate exceptions:</p> <ol class="arabic simple"> <li>Python does it (buggy code, missing resources, ending loops, etc.)</li> <li>manually (with a raise statement)</li> </ol> <p>When writing libraries, or even just custom classes, it can become necessary to raise exceptions; moreover it can be useful, even necessary, to change from one exception to another. To take an example from my dbf module:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">try</span><span class="p">:</span> <span class="n">value</span> <span class="o">=</span> <span class="nb">int</span><span class="p">(</span><span class="n">value</span><span class="p">)</span> <span class="k">except</span> <span class="ne">Exception</span><span class="p">:</span> <span class="k">raise</span> <span class="n">DbfError</span><span class="p">(</span><span class="o">...</span><span class="p">)</span> </pre></div> </div> <p>Whatever the original exception was (<code class="docutils literal notranslate"><span class="pre">ValueError</span></code>, <code class="docutils literal notranslate"><span class="pre">TypeError</span></code>, or something else) is irrelevant. The exception from this point on is a <code class="docutils literal notranslate"><span class="pre">DbfError</span></code>, and the original exception is of no value. However, if this exception is printed, we would currently see both.</p> </section> <section id="alternatives"> <h2><a class="toc-backref" href="#alternatives" role="doc-backlink">Alternatives</a></h2> <p>Several possibilities have been put forth:</p> <ul> <li><code class="docutils literal notranslate"><span class="pre">raise</span> <span class="pre">as</span> <span class="pre">NewException()</span></code><p>Reuses the <code class="docutils literal notranslate"><span class="pre">as</span></code> keyword; can be confusing since we are not really reraising the originating exception</p> </li> <li><code class="docutils literal notranslate"><span class="pre">raise</span> <span class="pre">NewException()</span> <span class="pre">from</span> <span class="pre">None</span></code><p>Follows existing syntax of explicitly declaring the originating exception</p> </li> <li><code class="docutils literal notranslate"><span class="pre">exc</span> <span class="pre">=</span> <span class="pre">NewException();</span> <span class="pre">exc.__context__</span> <span class="pre">=</span> <span class="pre">None;</span> <span class="pre">raise</span> <span class="pre">exc</span></code><p>Very verbose way of the previous method</p> </li> <li><code class="docutils literal notranslate"><span class="pre">raise</span> <span class="pre">NewException.no_context(...)</span></code><p>Make context suppression a class method.</p> </li> </ul> <p>All of the above options will require changes to the core.</p> </section> <section id="proposal"> <h2><a class="toc-backref" href="#proposal" role="doc-backlink">Proposal</a></h2> <p>I propose going with the second option:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">raise</span> <span class="n">NewException</span> <span class="kn">from</span><span class="w"> </span><span class="kc">None</span> </pre></div> </div> <p>It has the advantage of using the existing pattern of explicitly setting the cause:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">raise</span> <span class="ne">KeyError</span><span class="p">()</span> <span class="kn">from</span><span class="w"> </span><span class="nn">NameError</span><span class="p">()</span> </pre></div> </div> <p>but because the cause is <code class="docutils literal notranslate"><span class="pre">None</span></code> the previous context is not displayed by the default exception printing routines.</p> </section> <section id="implementation-discussion"> <h2><a class="toc-backref" href="#implementation-discussion" role="doc-backlink">Implementation Discussion</a></h2> <p>Note: after acceptance of this PEP, a cleaner implementation mechanism was proposed and accepted in <a class="pep reference internal" href="../pep-0415/" title="PEP 415 – Implement context suppression with exception attributes">PEP 415</a>. Refer to that PEP for more details on the implementation actually used in Python 3.3.</p> <p>Currently, <code class="docutils literal notranslate"><span class="pre">None</span></code> is the default for both <code class="docutils literal notranslate"><span class="pre">__context__</span></code> and <code class="docutils literal notranslate"><span class="pre">__cause__</span></code>. In order to support <code class="docutils literal notranslate"><span class="pre">raise</span> <span class="pre">...</span> <span class="pre">from</span> <span class="pre">None</span></code> (which would set <code class="docutils literal notranslate"><span class="pre">__cause__</span></code> to <code class="docutils literal notranslate"><span class="pre">None</span></code>) we need a different default value for <code class="docutils literal notranslate"><span class="pre">__cause__</span></code>. Several ideas were put forth on how to implement this at the language level:</p> <ul> <li>Overwrite the previous exception information (side-stepping the issue and leaving <code class="docutils literal notranslate"><span class="pre">__cause__</span></code> at <code class="docutils literal notranslate"><span class="pre">None</span></code>).<p>Rejected as this can seriously hinder debugging due to <a class="reference external" href="http://bugs.python.org/msg152294">poor error messages</a>.</p> </li> <li>Use one of the boolean values in <code class="docutils literal notranslate"><span class="pre">__cause__</span></code>: <code class="docutils literal notranslate"><span class="pre">False</span></code> would be the default value, and would be replaced when <code class="docutils literal notranslate"><span class="pre">from</span> <span class="pre">...</span></code> was used with the explicitly chained exception or <code class="docutils literal notranslate"><span class="pre">None</span></code>.<p>Rejected as this encourages the use of two different objects types for <code class="docutils literal notranslate"><span class="pre">__cause__</span></code> with one of them (boolean) not allowed to have the full range of possible values (<code class="docutils literal notranslate"><span class="pre">True</span></code> would never be used).</p> </li> <li>Create a special exception class, <code class="docutils literal notranslate"><span class="pre">__NoException__</span></code>.<p>Rejected as possibly confusing, possibly being mistakenly raised by users, and not being a truly unique value as <code class="docutils literal notranslate"><span class="pre">None</span></code>, <code class="docutils literal notranslate"><span class="pre">True</span></code>, and <code class="docutils literal notranslate"><span class="pre">False</span></code> are.</p> </li> <li>Use <code class="docutils literal notranslate"><span class="pre">Ellipsis</span></code> as the default value (the <code class="docutils literal notranslate"><span class="pre">...</span></code> singleton).<p>Accepted.</p> <p>Ellipses are commonly used in English as place holders when words are omitted. This works in our favor here as a signal that <code class="docutils literal notranslate"><span class="pre">__cause__</span></code> is omitted, so look in <code class="docutils literal notranslate"><span class="pre">__context__</span></code> for more details.</p> <p>Ellipsis is not an exception, so cannot be raised.</p> <p>There is only one Ellipsis, so no unused values.</p> <p>Error information is not thrown away, so custom code can trace the entire exception chain even if the default code does not.</p> </li> </ul> </section> <section id="language-details"> <h2><a class="toc-backref" href="#language-details" role="doc-backlink">Language Details</a></h2> <p>To support <code class="docutils literal notranslate"><span class="pre">raise</span> <span class="pre">Exception</span> <span class="pre">from</span> <span class="pre">None</span></code>, <code class="docutils literal notranslate"><span class="pre">__context__</span></code> will stay as it is, but <code class="docutils literal notranslate"><span class="pre">__cause__</span></code> will start out as <code class="docutils literal notranslate"><span class="pre">Ellipsis</span></code> and will change to <code class="docutils literal notranslate"><span class="pre">None</span></code> when the <code class="docutils literal notranslate"><span class="pre">raise</span> <span class="pre">Exception</span> <span class="pre">from</span> <span class="pre">None</span></code> method is used.</p> <table class="docutils align-default"> <thead> <tr class="row-odd"><th class="head">form</th> <th class="head">__context__</th> <th class="head">__cause__</th> </tr> </thead> <tbody> <tr class="row-even"><td>raise</td> <td><code class="docutils literal notranslate"><span class="pre">None</span></code></td> <td><code class="docutils literal notranslate"><span class="pre">Ellipsis</span></code></td> </tr> <tr class="row-odd"><td>reraise</td> <td>previous exception</td> <td><code class="docutils literal notranslate"><span class="pre">Ellipsis</span></code></td> </tr> <tr class="row-even"><td>reraise from <code class="docutils literal notranslate"><span class="pre">None</span></code> | <code class="docutils literal notranslate"><span class="pre">ChainedException</span></code></td> <td>previous exception</td> <td><code class="docutils literal notranslate"><span class="pre">None</span></code> | explicitly chained exception</td> </tr> </tbody> </table> <p>The default exception printing routine will then:</p> <ul class="simple"> <li>If <code class="docutils literal notranslate"><span class="pre">__cause__</span></code> is <code class="docutils literal notranslate"><span class="pre">Ellipsis</span></code> the <code class="docutils literal notranslate"><span class="pre">__context__</span></code> (if any) will be printed.</li> <li>If <code class="docutils literal notranslate"><span class="pre">__cause__</span></code> is <code class="docutils literal notranslate"><span class="pre">None</span></code> the <code class="docutils literal notranslate"><span class="pre">__context__</span></code> will not be printed.</li> <li>if <code class="docutils literal notranslate"><span class="pre">__cause__</span></code> is anything else, <code class="docutils literal notranslate"><span class="pre">__cause__</span></code> will be printed.</li> </ul> <p>In both of the latter cases the exception chain will stop being followed.</p> <p>Because the default value for <code class="docutils literal notranslate"><span class="pre">__cause__</span></code> is now <code class="docutils literal notranslate"><span class="pre">Ellipsis</span></code> and <code class="docutils literal notranslate"><span class="pre">raise</span> <span class="pre">Exception</span> <span class="pre">from</span> <span class="pre">Cause</span></code> is simply syntactic sugar for:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">_exc</span> <span class="o">=</span> <span class="n">NewException</span><span class="p">()</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="p">()</span> <span class="k">raise</span> <span class="n">_exc</span> </pre></div> </div> <p><code class="docutils literal notranslate"><span class="pre">Ellipsis</span></code>, as well as <code class="docutils literal notranslate"><span class="pre">None</span></code>, is now allowed as a cause:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">raise</span> <span class="ne">Exception</span> <span class="kn">from</span><span class="w"> </span><span class="nn">Ellipsis</span> </pre></div> </div> </section> <section id="patches"> <h2><a class="toc-backref" href="#patches" role="doc-backlink">Patches</a></h2> <p>There is a patch for CPython implementing this attached to <a class="reference external" href="http://bugs.python.org/issue6210">Issue 6210</a>.</p> </section> <section id="references"> <h2><a class="toc-backref" href="#references" role="doc-backlink">References</a></h2> <p>Discussion and refinements in this <a class="reference external" href="https://mail.python.org/pipermail/python-dev/2012-January/115838.html">thread on python-dev</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-0409.rst">https://github.com/python/peps/blob/main/peps/pep-0409.rst</a></p> <p>Last modified: <a class="reference external" href="https://github.com/python/peps/commits/main/peps/pep-0409.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">Rationale</a></li> <li><a class="reference internal" href="#alternatives">Alternatives</a></li> <li><a class="reference internal" href="#proposal">Proposal</a></li> <li><a class="reference internal" href="#implementation-discussion">Implementation Discussion</a></li> <li><a class="reference internal" href="#language-details">Language Details</a></li> <li><a class="reference internal" href="#patches">Patches</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-0409.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>