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<h1 class="page-title">PEP 270 – uniq method for list objects</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">Jason Petrone <jp at demonseed.net></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="Formally declined and will not be accepted">Rejected</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">21-Aug-2001</dd>
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<dt class="field-odd">Python-Version<span class="colon">:</span></dt>
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<dd class="field-odd">2.2</dd>
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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="#notice">Notice</a></li>
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<li><a class="reference internal" href="#abstract">Abstract</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="#considerations">Considerations</a></li>
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<li><a class="reference internal" href="#reference-implementation">Reference Implementation</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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<section id="notice">
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<h2><a class="toc-backref" href="#notice" role="doc-backlink">Notice</a></h2>
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<p>This PEP is withdrawn by the author. He writes:</p>
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<blockquote>
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<div>Removing duplicate elements from a list is a common task, but
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there are only two reasons I can see for making it a built-in.
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The first is if it could be done much faster, which isn’t the
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case. The second is if it makes it significantly easier to
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write code. The introduction of <code class="docutils literal notranslate"><span class="pre">sets.py</span></code> eliminates this
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situation since creating a sequence without duplicates is just
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a matter of choosing a different data structure: a set instead
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of a list.</div></blockquote>
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<p>As described in <a class="pep reference internal" href="../pep-0218/" title="PEP 218 – Adding a Built-In Set Object Type">PEP 218</a>, sets are being added to the standard
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library for Python 2.3.</p>
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</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>This PEP proposes adding a method for removing duplicate elements to
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the list object.</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>Removing duplicates from a list is a common task. I think it is
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useful and general enough to belong as a method in list objects.
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It also has potential for faster execution when implemented in C,
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especially if optimization using hashing or sorted cannot be used.</p>
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<p>On comp.lang.python there are many, many, posts <a class="footnote-reference brackets" href="#id3" id="id1">[1]</a> asking about
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the best way to do this task. It’s a little tricky to implement
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optimally and it would be nice to save people the trouble of
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figuring it out themselves.</p>
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</section>
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<section id="considerations">
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<h2><a class="toc-backref" href="#considerations" role="doc-backlink">Considerations</a></h2>
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<p>Tim Peters suggests trying to use a hash table, then trying to
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sort, and finally falling back on brute force <a class="footnote-reference brackets" href="#id4" id="id2">[2]</a>. Should uniq
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maintain list order at the expense of speed?</p>
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<p>Is it spelled ‘uniq’ or ‘unique’?</p>
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</section>
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<section id="reference-implementation">
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<h2><a class="toc-backref" href="#reference-implementation" role="doc-backlink">Reference Implementation</a></h2>
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<p>I’ve written the brute force version. It’s about 20 lines of code
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in <code class="docutils literal notranslate"><span class="pre">listobject.c</span></code>. Adding support for hash table and sorted
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duplicate removal would only take another hour or so.</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><a class="reference external" href="https://groups.google.com/forum/#!searchin/comp.lang.python/duplicates">https://groups.google.com/forum/#!searchin/comp.lang.python/duplicates</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>Tim Peters unique() entry in the Python cookbook:
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<a class="reference external" href="http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/52560/index_txt">http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/52560/index_txt</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-0270.rst">https://github.com/python/peps/blob/main/peps/pep-0270.rst</a></p>
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<p>Last modified: <a class="reference external" href="https://github.com/python/peps/commits/main/peps/pep-0270.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="#notice">Notice</a></li>
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<li><a class="reference internal" href="#abstract">Abstract</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="#considerations">Considerations</a></li>
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<li><a class="reference internal" href="#reference-implementation">Reference Implementation</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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