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<h1 class="page-title">PEP 415 – Implement context suppression with exception attributes</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">Benjamin Peterson <benjamin at python.org></dd>
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<dt class="field-even">BDFL-Delegate<span class="colon">:</span></dt>
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<dd class="field-even">Alyssa Coghlan</dd>
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<dt class="field-odd">Status<span class="colon">:</span></dt>
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<dd class="field-odd"><abbr title="Accepted and implementation complete, or no longer active">Final</abbr></dd>
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<dt class="field-even">Type<span class="colon">:</span></dt>
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<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>
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<dt class="field-odd">Created<span class="colon">:</span></dt>
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<dd class="field-odd">26-Feb-2012</dd>
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<dt class="field-even">Python-Version<span class="colon">:</span></dt>
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<dd class="field-even">3.3</dd>
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<dt class="field-odd">Post-History<span class="colon">:</span></dt>
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<dd class="field-odd">26-Feb-2012</dd>
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<dt class="field-even">Replaces<span class="colon">:</span></dt>
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<dd class="field-even"><a class="reference external" href="../pep-0409/">409</a></dd>
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<dt class="field-odd">Resolution<span class="colon">:</span></dt>
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<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>
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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="#pep-acceptance">PEP Acceptance</a></li>
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<li><a class="reference internal" href="#rationale">Rationale</a></li>
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<li><a class="reference internal" href="#proposal">Proposal</a></li>
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<li><a class="reference internal" href="#patches">Patches</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><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
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allow the display of the exception context to be explicitly suppressed.
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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>,
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but replaces the underlying implementation mechanism with a simpler approach
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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>
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instances.</p>
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</section>
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<section id="pep-acceptance">
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<h2><a class="toc-backref" href="#pep-acceptance" role="doc-backlink">PEP Acceptance</a></h2>
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<p>This PEP was accepted by Alyssa Coghlan on the 14th of May, 2012.</p>
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</section>
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<section id="rationale">
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<h2><a class="toc-backref" href="#rationale" role="doc-backlink">Rationale</a></h2>
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<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
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<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
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will be printed should the exception be uncaught.</p>
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<p>The main problem with this scheme is it complicates the role of
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<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
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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
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also not easily extended in the future. For example, we may someday want to
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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
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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>
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<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
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exclusively in extended slicing. Extended slicing has nothing to do with
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exceptions, so it’s not clear to someone inspecting an exception object why
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<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
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<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>
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</section>
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<section id="proposal">
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<h2><a class="toc-backref" href="#proposal" role="doc-backlink">Proposal</a></h2>
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<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
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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>
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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>
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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
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printing code will check for that attribute to determine whether
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context and cause will be printed. <code class="docutils literal notranslate"><span class="pre">__cause__</span></code> will return to its
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original purpose and values.</p>
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||
<p>There is precedence for <code class="docutils literal notranslate"><span class="pre">__suppress_context__</span></code> with the
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<code class="docutils literal notranslate"><span class="pre">print_line_and_file</span></code> exception attribute.</p>
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||
<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>
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<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>
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<span class="k">raise</span> <span class="n">exc</span>
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</pre></div>
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</div>
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<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
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<code class="docutils literal notranslate"><span class="pre">exc.__suppress_context__</span></code>.</p>
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</section>
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<section id="patches">
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<h2><a class="toc-backref" href="#patches" role="doc-backlink">Patches</a></h2>
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<p>There is a patch on <a class="reference external" href="http://bugs.python.org/issue14133">Issue 14133</a>.</p>
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</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>
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<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-0415.rst">https://github.com/python/peps/blob/main/peps/pep-0415.rst</a></p>
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<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>
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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>
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<li><a class="reference internal" href="#pep-acceptance">PEP Acceptance</a></li>
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<li><a class="reference internal" href="#rationale">Rationale</a></li>
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<li><a class="reference internal" href="#proposal">Proposal</a></li>
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<li><a class="reference internal" href="#patches">Patches</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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