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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 270 – uniq method for list objects | peps.python.org</title> <link rel="shortcut icon" href="../_static/py.png"> <link rel="canonical" href="https://peps.python.org/pep-0270/"> <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 270 – uniq method for list objects | peps.python.org'> <meta property="og:description" 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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 270 – uniq method for list objects</h1> <dl class="rfc2822 field-list simple"> <dt class="field-odd">Author<span class="colon">:</span></dt> <dd class="field-odd">Jason Petrone <jp at demonseed.net></dd> <dt class="field-even">Status<span class="colon">:</span></dt> <dd class="field-even"><abbr title="Formally declined and will not be accepted">Rejected</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">21-Aug-2001</dd> <dt class="field-odd">Python-Version<span class="colon">:</span></dt> <dd class="field-odd">2.2</dd> <dt class="field-even">Post-History<span class="colon">:</span></dt> <dd class="field-even"><p></p></dd> </dl> <hr class="docutils" /> <section id="contents"> <details><summary>Table of Contents</summary><ul class="simple"> <li><a class="reference internal" href="#notice">Notice</a></li> <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="#considerations">Considerations</a></li> <li><a class="reference internal" href="#reference-implementation">Reference Implementation</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="notice"> <h2><a class="toc-backref" href="#notice" role="doc-backlink">Notice</a></h2> <p>This PEP is withdrawn by the author. He writes:</p> <blockquote> <div>Removing duplicate elements from a list is a common task, but there are only two reasons I can see for making it a built-in. The first is if it could be done much faster, which isn’t the case. The second is if it makes it significantly easier to write code. The introduction of <code class="docutils literal notranslate"><span class="pre">sets.py</span></code> eliminates this situation since creating a sequence without duplicates is just a matter of choosing a different data structure: a set instead of a list.</div></blockquote> <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 library for Python 2.3.</p> </section> <section id="abstract"> <h2><a class="toc-backref" href="#abstract" role="doc-backlink">Abstract</a></h2> <p>This PEP proposes adding a method for removing duplicate elements to the list object.</p> </section> <section id="rationale"> <h2><a class="toc-backref" href="#rationale" role="doc-backlink">Rationale</a></h2> <p>Removing duplicates from a list is a common task. I think it is useful and general enough to belong as a method in list objects. It also has potential for faster execution when implemented in C, especially if optimization using hashing or sorted cannot be used.</p> <p>On comp.lang.python there are many, many, posts <a class="footnote-reference brackets" href="#id3" id="id1">[1]</a> asking about the best way to do this task. It’s a little tricky to implement optimally and it would be nice to save people the trouble of figuring it out themselves.</p> </section> <section id="considerations"> <h2><a class="toc-backref" href="#considerations" role="doc-backlink">Considerations</a></h2> <p>Tim Peters suggests trying to use a hash table, then trying to sort, and finally falling back on brute force <a class="footnote-reference brackets" href="#id4" id="id2">[2]</a>. Should uniq maintain list order at the expense of speed?</p> <p>Is it spelled ‘uniq’ or ‘unique’?</p> </section> <section id="reference-implementation"> <h2><a class="toc-backref" href="#reference-implementation" role="doc-backlink">Reference Implementation</a></h2> <p>I’ve written the brute force version. It’s about 20 lines of code in <code class="docutils literal notranslate"><span class="pre">listobject.c</span></code>. Adding support for hash table and sorted duplicate removal would only take another hour or so.</p> </section> <section id="references"> <h2><a class="toc-backref" href="#references" role="doc-backlink">References</a></h2> <aside class="footnote-list brackets"> <aside class="footnote brackets" id="id3" role="doc-footnote"> <dt class="label" id="id3">[<a href="#id1">1</a>]</dt> <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> <aside class="footnote brackets" id="id4" role="doc-footnote"> <dt class="label" id="id4">[<a href="#id2">2</a>]</dt> <dd>Tim Peters unique() entry in the Python cookbook: <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> </aside> </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-0270.rst">https://github.com/python/peps/blob/main/peps/pep-0270.rst</a></p> <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> </article> <nav id="pep-sidebar"> <h2>Contents</h2> <ul> <li><a class="reference internal" href="#notice">Notice</a></li> <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="#considerations">Considerations</a></li> <li><a class="reference internal" href="#reference-implementation">Reference Implementation</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-0270.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>