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<h1 class="page-title">PEP 286 – Enhanced Argument Tuples</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">Martin von Löwis <martin at v.loewis.de></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="Inactive draft that may be taken up again at a later time">Deferred</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">03-Mar-2002</dd>
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<dt class="field-odd">Python-Version<span class="colon">:</span></dt>
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<dd class="field-odd">2.3</dd>
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<dt class="field-even">Post-History<span class="colon">:</span></dt>
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<dd class="field-even"><p></p></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-deferral">PEP Deferral</a></li>
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<li><a class="reference internal" href="#problem-description">Problem description</a></li>
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<li><a class="reference internal" href="#proposed-solution">Proposed solution</a></li>
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<li><a class="reference internal" href="#affected-converters">Affected converters</a></li>
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<li><a class="reference internal" href="#new-converters">New converters</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><code class="docutils literal notranslate"><span class="pre">PyArg_ParseTuple</span></code> is confronted with difficult memory management if
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an argument converter creates new memory. To deal with these
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cases, a specialized argument type is proposed.</p>
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</section>
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<section id="pep-deferral">
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<h2><a class="toc-backref" href="#pep-deferral" role="doc-backlink">PEP Deferral</a></h2>
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<p>Further exploration of the concepts covered in this PEP has been deferred
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for lack of a current champion interested in promoting the goals of the
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||
PEP and collecting and incorporating feedback, and with sufficient
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||
available time to do so effectively.</p>
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<p>The resolution of this PEP may also be affected by the resolution of
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||
<a class="pep reference internal" href="../pep-0426/" title="PEP 426 – Metadata for Python Software Packages 2.0">PEP 426</a>, which proposes the use of a preprocessing step to generate
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some aspects of C API interface code.</p>
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</section>
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<section id="problem-description">
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<h2><a class="toc-backref" href="#problem-description" role="doc-backlink">Problem description</a></h2>
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<p>Today, argument tuples keep references to the function arguments,
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which are guaranteed to live as long as the argument tuple exists
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which is at least as long as the function call is being executed.</p>
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<p>In some cases, parsing an argument will allocate new memory, which
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is then to be released by the caller. This has two problems:</p>
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<ol class="arabic simple">
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<li>In case of failure, the application cannot know what memory to
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release; most callers don’t even know that they have the
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responsibility to release that memory. Example for this are
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the <code class="docutils literal notranslate"><span class="pre">N</span></code> converter (bug #416288 <a class="footnote-reference brackets" href="#id3" id="id1">[1]</a>) and the <code class="docutils literal notranslate"><span class="pre">es#</span></code> converter (bug
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#501716 <a class="footnote-reference brackets" href="#id4" id="id2">[2]</a>).</li>
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<li>Even for successful argument parsing, it is still inconvenient
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for the caller to be responsible for releasing the memory. In
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some cases, this is unnecessarily inefficient. For example,
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the <code class="docutils literal notranslate"><span class="pre">es</span></code> converter copies the conversion result into memory, even
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though there already is a string object that has the right
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contents.</li>
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</ol>
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</section>
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<section id="proposed-solution">
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<h2><a class="toc-backref" href="#proposed-solution" role="doc-backlink">Proposed solution</a></h2>
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<p>A new type ‘argument tuple’ is introduced. This type derives from
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tuple, adding an <code class="docutils literal notranslate"><span class="pre">__dict__</span></code> member (at <code class="docutils literal notranslate"><span class="pre">tp_dictoffset</span></code> -4). Instances
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of this type might get the following attributes:</p>
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<ul class="simple">
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<li>‘failobjects’, a list of objects which need to be deallocated
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in case of success</li>
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<li>‘okobjects’, a list of object which will be released when the
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argument tuple is released</li>
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</ul>
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<p>To manage this type, the following functions will be added, and
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used appropriately in <code class="docutils literal notranslate"><span class="pre">ceval.c</span></code> and <code class="docutils literal notranslate"><span class="pre">getargs.c</span></code>:</p>
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<ul class="simple">
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<li><code class="docutils literal notranslate"><span class="pre">PyArgTuple_New(int);</span></code></li>
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<li><code class="docutils literal notranslate"><span class="pre">PyArgTuple_AddFailObject(PyObject*,</span> <span class="pre">PyObject*);</span></code></li>
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<li><code class="docutils literal notranslate"><span class="pre">PyArgTuple_AddFailMemory(PyObject*,</span> <span class="pre">void*);</span></code></li>
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<li><code class="docutils literal notranslate"><span class="pre">PyArgTuple_AddOkObject(PyObject*,</span> <span class="pre">PyObject*);</span></code></li>
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<li><code class="docutils literal notranslate"><span class="pre">PyArgTuple_AddOkMemory(PyObject*,</span> <span class="pre">void*);</span></code></li>
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<li><code class="docutils literal notranslate"><span class="pre">PyArgTuple_ClearFailed(PyObject*);</span></code></li>
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</ul>
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<p>When argument parsing fails, all fail objects will be released
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through <code class="docutils literal notranslate"><span class="pre">Py_DECREF</span></code>, and all fail memory will be released through
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<code class="docutils literal notranslate"><span class="pre">PyMem_Free</span></code>. If parsing succeeds, the references to the fail
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objects and fail memory are dropped, without releasing anything.</p>
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<p>When the argument tuple is released, all ok objects and memory
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will be released.</p>
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<p>If those functions are called with an object of a different type,
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a warning is issued and no further action is taken; usage of the
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affected converters without using argument tuples is deprecated.</p>
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</section>
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<section id="affected-converters">
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<h2><a class="toc-backref" href="#affected-converters" role="doc-backlink">Affected converters</a></h2>
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||
<p>The following converters will add fail memory and fail objects: <code class="docutils literal notranslate"><span class="pre">N</span></code>,
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||
<code class="docutils literal notranslate"><span class="pre">es</span></code>, <code class="docutils literal notranslate"><span class="pre">et</span></code>, <code class="docutils literal notranslate"><span class="pre">es#</span></code>, <code class="docutils literal notranslate"><span class="pre">et#</span></code> (unless memory is passed into the converter)</p>
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</section>
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<section id="new-converters">
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<h2><a class="toc-backref" href="#new-converters" role="doc-backlink">New converters</a></h2>
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<p>To simplify Unicode conversion, the <code class="docutils literal notranslate"><span class="pre">e*</span></code> converters are duplicated
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||
as <code class="docutils literal notranslate"><span class="pre">E*</span></code> converters (<code class="docutils literal notranslate"><span class="pre">Es</span></code>, <code class="docutils literal notranslate"><span class="pre">Et</span></code>, <code class="docutils literal notranslate"><span class="pre">Es#</span></code>, <code class="docutils literal notranslate"><span class="pre">Et#</span></code>). The usage of the <code class="docutils literal notranslate"><span class="pre">E*</span></code>
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converters is identical to that of the <code class="docutils literal notranslate"><span class="pre">e*</span></code> converters, except that
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the application will not need to manage the resulting memory.
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This will be implemented through registration of Ok objects with
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the argument tuple. The <code class="docutils literal notranslate"><span class="pre">e*</span></code> converters are deprecated.</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="id3" role="doc-footnote">
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<dt class="label" id="id3">[<a href="#id1">1</a>]</dt>
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<dd>infrequent memory leak in pyexpat
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(<a class="reference external" href="http://bugs.python.org/issue416288">http://bugs.python.org/issue416288</a>)</aside>
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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">2</a>]</dt>
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<dd>“es#” parser marker leaks memory
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(<a class="reference external" href="http://bugs.python.org/issue501716">http://bugs.python.org/issue501716</a>)</aside>
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</aside>
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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-0286.rst">https://github.com/python/peps/blob/main/peps/pep-0286.rst</a></p>
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<p>Last modified: <a class="reference external" href="https://github.com/python/peps/commits/main/peps/pep-0286.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>
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<li><a class="reference internal" href="#pep-deferral">PEP Deferral</a></li>
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<li><a class="reference internal" href="#problem-description">Problem description</a></li>
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<li><a class="reference internal" href="#proposed-solution">Proposed solution</a></li>
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<li><a class="reference internal" href="#affected-converters">Affected converters</a></li>
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<li><a class="reference internal" href="#new-converters">New converters</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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