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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 240 – Adding a Rational Literal to Python | peps.python.org</title> <link rel="shortcut icon" href="../_static/py.png"> <link rel="canonical" href="https://peps.python.org/pep-0240/"> <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 240 – Adding a Rational Literal to Python | peps.python.org'> <meta property="og:description" content="A different PEP suggests adding a builtin rational type to Python. 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Craig <python-pep at ccraig.org>, Moshe Zadka <moshez at zadka.site.co.il></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">11-Mar-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">16-Mar-2001</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="#bdfl-pronouncement">BDFL Pronouncement</a></li> <li><a class="reference internal" href="#rationale">Rationale</a></li> <li><a class="reference internal" href="#proposal">Proposal</a></li> <li><a class="reference internal" href="#backwards-compatibility">Backwards Compatibility</a></li> <li><a class="reference internal" href="#common-objections">Common Objections</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>A <a class="pep reference internal" href="../pep-0239/" title="PEP 239 – Adding a Rational Type to Python">different PEP</a> suggests adding a builtin rational type to Python. This PEP suggests changing the ddd.ddd float literal to a rational in Python, and modifying non-integer division to return it.</p> </section> <section id="bdfl-pronouncement"> <h2><a class="toc-backref" href="#bdfl-pronouncement" role="doc-backlink">BDFL Pronouncement</a></h2> <p>This PEP is rejected. The needs outlined in the rationale section have been addressed to some extent by the acceptance of <a class="pep reference internal" href="../pep-0327/" title="PEP 327 – Decimal Data Type">PEP 327</a> for decimal arithmetic. Guido also noted, “Rational arithmetic was the default ‘exact’ arithmetic in ABC and it did not work out as expected”. See the python-dev discussion on 17 June 2005 <a class="footnote-reference brackets" href="#id2" id="id1">[1]</a>.</p> </section> <section id="rationale"> <h2><a class="toc-backref" href="#rationale" role="doc-backlink">Rationale</a></h2> <p>Rational numbers are useful for exact and unsurprising arithmetic. They give the correct results people have been taught in various math classes. Making the “obvious” non-integer type one with more predictable semantics will surprise new programmers less than using floating point numbers. As quite a few posts on c.l.py and on <a class="reference external" href="mailto:tutor%40python.org">tutor<span>@</span>python<span>.</span>org</a> have shown, people often get bit by strange semantics of floating point numbers: for example, <code class="docutils literal notranslate"><span class="pre">round(0.98,</span> <span class="pre">2)</span></code> still gives 0.97999999999999998.</p> </section> <section id="proposal"> <h2><a class="toc-backref" href="#proposal" role="doc-backlink">Proposal</a></h2> <p>Literals conforming to the regular expression <code class="docutils literal notranslate"><span class="pre">'\d*.\d*'</span></code> will be rational numbers.</p> </section> <section id="backwards-compatibility"> <h2><a class="toc-backref" href="#backwards-compatibility" role="doc-backlink">Backwards Compatibility</a></h2> <p>The only backwards compatible issue is the type of literals mentioned above. The following migration is suggested:</p> <ol class="arabic simple"> <li>The next Python after approval will allow <code class="docutils literal notranslate"><span class="pre">from</span> <span class="pre">__future__</span> <span class="pre">import</span> <span class="pre">rational_literals</span></code> to cause all such literals to be treated as rational numbers.</li> <li>Python 3.0 will have a warning, turned on by default, about such literals in the absence of a <code class="docutils literal notranslate"><span class="pre">__future__</span></code> statement. The warning message will contain information about the <code class="docutils literal notranslate"><span class="pre">__future__</span></code> statement, and indicate that to get floating point literals, they should be suffixed with “e0”.</li> <li>Python 3.1 will have the warning turned off by default. This warning will stay in place for 24 months, at which time the literals will be rationals and the warning will be removed.</li> </ol> </section> <section id="common-objections"> <h2><a class="toc-backref" href="#common-objections" role="doc-backlink">Common Objections</a></h2> <p>Rationals are slow and memory intensive! (Relax, I’m not taking floats away, I’m just adding two more characters. <code class="docutils literal notranslate"><span class="pre">1e0</span></code> will still be a float)</p> <p>Rationals must present themselves as a decimal float or they will be horrible for users expecting decimals (i.e. <code class="docutils literal notranslate"><span class="pre">str(.5)</span></code> should return <code class="docutils literal notranslate"><span class="pre">'.5'</span></code> and not <code class="docutils literal notranslate"><span class="pre">'1/2'</span></code>). This means that many rationals must be truncated at some point, which gives us a new loss of precision.</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="id2" role="doc-footnote"> <dt class="label" id="id2">[<a href="#id1">1</a>]</dt> <dd>Raymond Hettinger, Propose rejection of PEPs 239 and 240 – a builtin rational type and rational literals <a class="reference external" href="https://mail.python.org/pipermail/python-dev/2005-June/054281.html">https://mail.python.org/pipermail/python-dev/2005-June/054281.html</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-0240.rst">https://github.com/python/peps/blob/main/peps/pep-0240.rst</a></p> <p>Last modified: <a class="reference external" href="https://github.com/python/peps/commits/main/peps/pep-0240.rst">2025-02-01 08:55:40 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="#bdfl-pronouncement">BDFL Pronouncement</a></li> <li><a class="reference internal" href="#rationale">Rationale</a></li> <li><a class="reference internal" href="#proposal">Proposal</a></li> <li><a class="reference internal" href="#backwards-compatibility">Backwards Compatibility</a></li> <li><a class="reference internal" href="#common-objections">Common Objections</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-0240.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>