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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 3117 – Postfix type declarations | peps.python.org</title> <link rel="shortcut icon" href="../_static/py.png"> <link rel="canonical" href="https://peps.python.org/pep-3117/"> <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 3117 – Postfix type declarations | peps.python.org'> <meta property="og:description" content="This PEP proposes the addition of a postfix type declaration syntax to Python. 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It also specifies a new typedef statement which is used to create new mappings between types and declarators."> <meta name="theme-color" content="#3776ab"> </head> <body> <svg xmlns="http://www.w3.org/2000/svg" style="display: none;"> <symbol id="svg-sun-half" viewBox="0 0 24 24" pointer-events="all"> <title>Following system colour scheme</title> <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"> <circle cx="12" cy="12" r="9"></circle> <path d="M12 3v18m0-12l4.65-4.65M12 14.3l7.37-7.37M12 19.6l8.85-8.85"></path> </svg> </symbol> <symbol id="svg-moon" viewBox="0 0 24 24" pointer-events="all"> <title>Selected dark colour scheme</title> <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"> <path stroke="none" d="M0 0h24v24H0z" fill="none"></path> <path d="M12 3c.132 0 .263 0 .393 0a7.5 7.5 0 0 0 7.92 12.446a9 9 0 1 1 -8.313 -12.454z"></path> </svg> </symbol> <symbol id="svg-sun" viewBox="0 0 24 24" pointer-events="all"> <title>Selected light colour scheme</title> <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"> <circle cx="12" cy="12" r="5"></circle> <line x1="12" y1="1" x2="12" y2="3"></line> <line x1="12" y1="21" x2="12" y2="23"></line> <line x1="4.22" y1="4.22" x2="5.64" y2="5.64"></line> <line x1="18.36" y1="18.36" x2="19.78" y2="19.78"></line> <line x1="1" y1="12" x2="3" y2="12"></line> <line x1="21" y1="12" x2="23" y2="12"></line> <line x1="4.22" y1="19.78" x2="5.64" y2="18.36"></line> <line x1="18.36" y1="5.64" x2="19.78" y2="4.22"></line> </svg> </symbol> </svg> <script> document.documentElement.dataset.colour_scheme = localStorage.getItem("colour_scheme") || "auto" </script> <section id="pep-page-section"> <header> <h1>Python Enhancement Proposals</h1> <ul class="breadcrumbs"> <li><a href="https://www.python.org/" title="The Python Programming Language">Python</a> » </li> <li><a href="../pep-0000/">PEP Index</a> » </li> <li>PEP 3117</li> </ul> <button id="colour-scheme-cycler" onClick="setColourScheme(nextColourScheme())"> <svg aria-hidden="true" class="colour-scheme-icon-when-auto"><use href="#svg-sun-half"></use></svg> <svg aria-hidden="true" class="colour-scheme-icon-when-dark"><use 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 3117 – Postfix type declarations</h1> <dl class="rfc2822 field-list simple"> <dt class="field-odd">Author<span class="colon">:</span></dt> <dd class="field-odd">Georg Brandl <georg at python.org></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">01-Apr-2007</dd> <dt class="field-odd">Python-Version<span class="colon">:</span></dt> <dd class="field-odd">3.0</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="#abstract">Abstract</a></li> <li><a class="reference internal" href="#rationale">Rationale</a></li> <li><a class="reference internal" href="#specification">Specification</a></li> <li><a class="reference internal" href="#unicode-replacement-units">Unicode replacement units</a></li> <li><a class="reference internal" href="#the-typedef-statement">The <code class="docutils literal notranslate"><span class="pre">typedef</span></code> statement</a></li> <li><a class="reference internal" href="#example">Example</a></li> <li><a class="reference internal" href="#compatibility-issues">Compatibility issues</a></li> <li><a class="reference internal" href="#rejection">Rejection</a></li> <li><a class="reference internal" href="#references">References</a></li> <li><a class="reference internal" href="#acknowledgements">Acknowledgements</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>This PEP proposes the addition of a postfix type declaration syntax to Python. It also specifies a new <code class="docutils literal notranslate"><span class="pre">typedef</span></code> statement which is used to create new mappings between types and declarators.</p> <p>Its acceptance will greatly enhance the Python user experience as well as eliminate one of the warts that deter users of other programming languages from switching to Python.</p> </section> <section id="rationale"> <h2><a class="toc-backref" href="#rationale" role="doc-backlink">Rationale</a></h2> <p>Python has long suffered from the lack of explicit type declarations. Being one of the few aspects in which the language deviates from its Zen, this wart has sparked many a discussion between Python heretics and members of the PSU (for a few examples, see <a class="reference internal" href="#ex1" id="id1"><span>[EX1]</span></a>, <a class="reference internal" href="#ex2" id="id2"><span>[EX2]</span></a> or <a class="reference internal" href="#ex3" id="id3"><span>[EX3]</span></a>), and it also made it a large-scale enterprise success unlikely.</p> <p>However, if one wants to put an end to this misery, a decent Pythonic syntax must be found. In almost all languages that have them, type declarations lack this quality: they are verbose, often needing <em>multiple words</em> for a single type, or they are hard to comprehend (e.g., a certain language uses completely unrelated <a class="footnote-reference brackets" href="#id8" id="id4">[1]</a> adjectives like <code class="docutils literal notranslate"><span class="pre">dim</span></code> for type declaration).</p> <p>Therefore, this PEP combines the move to type declarations with another bold move that will once again prove that Python is not only future-proof but future-embracing: the introduction of Unicode characters as an integral constituent of source code.</p> <p>Unicode makes it possible to express much more with much less characters, which is in accordance with the <a class="pep reference internal" href="../pep-0020/" title="PEP 20 – The Zen of Python">Zen</a> (“Readability counts.”). Additionally, it eliminates the need for a separate type declaration statement, and last but not least, it makes Python measure up to Perl 6, which already uses Unicode for its operators. <a class="footnote-reference brackets" href="#id9" id="id5">[2]</a></p> </section> <section id="specification"> <h2><a class="toc-backref" href="#specification" role="doc-backlink">Specification</a></h2> <p>When the type declaration mode is in operation, the grammar is changed so that each <code class="docutils literal notranslate"><span class="pre">NAME</span></code> must consist of two parts: a name and a type declarator, which is exactly one Unicode character.</p> <p>The declarator uniquely specifies the type of the name, and if it occurs on the left hand side of an expression, this type is enforced: an <code class="docutils literal notranslate"><span class="pre">InquisitionError</span></code> exception is raised if the returned type doesn’t match the declared type. <a class="footnote-reference brackets" href="#id10" id="id6">[3]</a></p> <p>Also, function call result types have to be specified. If the result of the call does not have the declared type, an <code class="docutils literal notranslate"><span class="pre">InquisitionError</span></code> is raised. Caution: the declarator for the result should not be confused with the declarator for the function object (see the example below).</p> <p>Type declarators after names that are only read, not assigned to, are not strictly necessary but enforced anyway (see the Python Zen: “Explicit is better than implicit.”).</p> <p>The mapping between types and declarators is not static. It can be completely customized by the programmer, but for convenience there are some predefined mappings for some built-in types:</p> <table class="docutils align-default"> <thead> <tr class="row-odd"><th class="head">Type</th> <th class="head">Declarator</th> </tr> </thead> <tbody> <tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">object</span></code></td> <td>� (REPLACEMENT CHARACTER)</td> </tr> <tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">int</span></code></td> <td>ℕ (DOUBLE-STRUCK CAPITAL N)</td> </tr> <tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">float</span></code></td> <td>℮ (ESTIMATED SYMBOL)</td> </tr> <tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">bool</span></code></td> <td>✓ (CHECK MARK)</td> </tr> <tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">complex</span></code></td> <td>ℂ (DOUBLE-STRUCK CAPITAL C)</td> </tr> <tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">str</span></code></td> <td>✎ (LOWER RIGHT PENCIL)</td> </tr> <tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">unicode</span></code></td> <td>✒ (BLACK NIB)</td> </tr> <tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">tuple</span></code></td> <td>⒯ (PARENTHESIZED LATIN SMALL LETTER T)</td> </tr> <tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">list</span></code></td> <td>♨ (HOT SPRINGS)</td> </tr> <tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">dict</span></code></td> <td>⧟ (DOUBLE-ENDED MULTIMAP)</td> </tr> <tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">set</span></code></td> <td>∅ (EMPTY SET) (<em>Note:</em> this is also for full sets)</td> </tr> <tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">frozenset</span></code></td> <td>☃ (SNOWMAN)</td> </tr> <tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">datetime</span></code></td> <td>⌚ (WATCH)</td> </tr> <tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">function</span></code></td> <td>ƛ (LATIN SMALL LETTER LAMBDA WITH STROKE)</td> </tr> <tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">generator</span></code></td> <td>⚛ (ATOM SYMBOL)</td> </tr> <tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">Exception</span></code></td> <td>⌁ (ELECTRIC ARROW)</td> </tr> </tbody> </table> <p>The declarator for the <code class="docutils literal notranslate"><span class="pre">None</span></code> type is a zero-width space.</p> <p>These characters should be obvious and easy to remember and type for every programmer.</p> </section> <section id="unicode-replacement-units"> <h2><a class="toc-backref" href="#unicode-replacement-units" role="doc-backlink">Unicode replacement units</a></h2> <p>Since even in our modern, globalized world there are still some old-fashioned rebels who can’t or don’t want to use Unicode in their source code, and since Python is a forgiving language, a fallback is provided for those:</p> <p>Instead of the single Unicode character, they can type <code class="docutils literal notranslate"><span class="pre">name${UNICODE</span> <span class="pre">NAME</span> <span class="pre">OF</span> <span class="pre">THE</span> <span class="pre">DECLARATOR}$</span></code>. For example, these two function definitions are equivalent:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">def</span><span class="w"> </span><span class="nf">fooƛ</span><span class="p">(</span><span class="n">xℂ</span><span class="p">):</span> <span class="k">return</span> <span class="kc">None</span> </pre></div> </div> <p>and</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span>def foo${LATIN SMALL LETTER LAMBDA WITH STROKE}$(x${DOUBLE-STRUCK CAPITAL C}$): return None${ZERO WIDTH NO-BREAK SPACE}$ </pre></div> </div> <p>This is still easy to read and makes the full power of type-annotated Python available to ASCII believers.</p> </section> <section id="the-typedef-statement"> <h2><a class="toc-backref" href="#the-typedef-statement" role="doc-backlink">The <code class="docutils literal notranslate"><span class="pre">typedef</span></code> statement</a></h2> <p>The mapping between types and declarators can be extended with this new statement.</p> <p>The syntax is as follows:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">typedef_stmt</span> <span class="p">:</span><span class="o">:=</span> <span class="s2">"typedef"</span> <span class="n">expr</span> <span class="n">DECLARATOR</span> </pre></div> </div> <p>where <code class="docutils literal notranslate"><span class="pre">expr</span></code> resolves to a type object. For convenience, the <code class="docutils literal notranslate"><span class="pre">typedef</span></code> statement can also be mixed with the <code class="docutils literal notranslate"><span class="pre">class</span></code> statement for new classes, like so:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span>typedef class Foo☺(object�): pass </pre></div> </div> </section> <section id="example"> <h2><a class="toc-backref" href="#example" role="doc-backlink">Example</a></h2> <p>This is the standard <code class="docutils literal notranslate"><span class="pre">os.path.normpath</span></code> function, converted to type declaration syntax:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span>def normpathƛ(path✎)✎: """Normalize path, eliminating double slashes, etc.""" if path✎ == '': return '.' initial_slashes✓ = path✎.startswithƛ('/')✓ # POSIX allows one or two initial slashes, but treats three or more # as single slash. if (initial_slashes✓ and path✎.startswithƛ('//')✓ and not path✎.startswithƛ('///')✓)✓: initial_slashesℕ = 2 comps♨ = path✎.splitƛ('/')♨ new_comps♨ = []♨ for comp✎ in comps♨: if comp✎ in ('', '.')⒯: continue if (comp✎ != '..' or (not initial_slashesℕ and not new_comps♨)✓ or (new_comps♨ and new_comps♨[-1]✎ == '..')✓)✓: new_comps♨.appendƛ(comp✎) elif new_comps♨: new_comps♨.popƛ()✎ comps♨ = new_comps♨ path✎ = '/'.join(comps♨)✎ if initial_slashesℕ: path✎ = '/'*initial_slashesℕ + path✎ return path✎ or '.' </pre></div> </div> <p>As you can clearly see, the type declarations add expressiveness, while at the same time they make the code look much more professional.</p> </section> <section id="compatibility-issues"> <h2><a class="toc-backref" href="#compatibility-issues" role="doc-backlink">Compatibility issues</a></h2> <p>To enable type declaration mode, one has to write:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span><span class="w"> </span><span class="nn">__future__</span><span class="w"> </span><span class="kn">import</span> <span class="n">type_declarations</span> </pre></div> </div> <p>which enables Unicode parsing of the source <a class="footnote-reference brackets" href="#id11" id="id7">[4]</a>, makes <code class="docutils literal notranslate"><span class="pre">typedef</span></code> a keyword and enforces correct types for all assignments and function calls.</p> </section> <section id="rejection"> <h2><a class="toc-backref" href="#rejection" role="doc-backlink">Rejection</a></h2> <p>After careful considering, much soul-searching, gnashing of teeth and rending of garments, it has been decided to reject this PEP.</p> </section> <section id="references"> <h2><a class="toc-backref" href="#references" role="doc-backlink">References</a></h2> <div role="list" class="citation-list"> <div class="citation" id="ex1" role="doc-biblioentry"> <dt class="label" id="ex1">[<a href="#id1">EX1</a>]</dt> <dd><a class="reference external" href="https://mail.python.org/pipermail/python-list/2003-June/210588.html">https://mail.python.org/pipermail/python-list/2003-June/210588.html</a></div> <div class="citation" id="ex2" role="doc-biblioentry"> <dt class="label" id="ex2">[<a href="#id2">EX2</a>]</dt> <dd><a class="reference external" href="https://mail.python.org/pipermail/python-list/2000-May/034685.html">https://mail.python.org/pipermail/python-list/2000-May/034685.html</a></div> <div class="citation" id="ex3" role="doc-biblioentry"> <dt class="label" id="ex3">[<a href="#id3">EX3</a>]</dt> <dd><a class="reference external" href="http://groups.google.com/group/comp.lang.python/browse_frm/thread/6ae8c6add913635a/de40d4ffe9bd4304?lnk=gst&q=type+declarations&rnum=6">http://groups.google.com/group/comp.lang.python/browse_frm/thread/6ae8c6add913635a/de40d4ffe9bd4304?lnk=gst&q=type+declarations&rnum=6</a></div> </div> <aside class="footnote-list brackets"> <aside class="footnote brackets" id="id8" role="doc-footnote"> <dt class="label" id="id8">[<a href="#id4">1</a>]</dt> <dd>Though, if you know the language in question, it may not be <em>that</em> unrelated.</aside> <aside class="footnote brackets" id="id9" role="doc-footnote"> <dt class="label" id="id9">[<a href="#id5">2</a>]</dt> <dd>Well, it would, if there was a Perl 6.</aside> <aside class="footnote brackets" id="id10" role="doc-footnote"> <dt class="label" id="id10">[<a href="#id6">3</a>]</dt> <dd>Since the name <code class="docutils literal notranslate"><span class="pre">TypeError</span></code> is already in use, this name has been chosen for obvious reasons.</aside> <aside class="footnote brackets" id="id11" role="doc-footnote"> <dt class="label" id="id11">[<a href="#id7">4</a>]</dt> <dd>The encoding in which the code is written is read from a standard coding cookie. There will also be an autodetection mechanism, invoked by <code class="docutils literal notranslate"><span class="pre">from</span> <span class="pre">__future__</span> <span class="pre">import</span> <span class="pre">encoding_hell</span></code>.</aside> </aside> </section> <section id="acknowledgements"> <h2><a class="toc-backref" href="#acknowledgements" role="doc-backlink">Acknowledgements</a></h2> <p>Many thanks go to Armin Ronacher, Alexander Schremmer and Marek Kubica who helped find the most suitable and mnemonic declarator for built-in types.</p> <p>Thanks also to the Unicode Consortium for including all those useful characters in the Unicode standard.</p> </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-3117.rst">https://github.com/python/peps/blob/main/peps/pep-3117.rst</a></p> <p>Last modified: <a class="reference external" href="https://github.com/python/peps/commits/main/peps/pep-3117.rst">2025-02-01 08:59:27 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="#rationale">Rationale</a></li> <li><a class="reference internal" href="#specification">Specification</a></li> <li><a class="reference internal" href="#unicode-replacement-units">Unicode replacement units</a></li> <li><a class="reference internal" href="#the-typedef-statement">The <code class="docutils literal notranslate"><span class="pre">typedef</span></code> statement</a></li> <li><a class="reference internal" href="#example">Example</a></li> <li><a class="reference internal" href="#compatibility-issues">Compatibility issues</a></li> <li><a class="reference internal" href="#rejection">Rejection</a></li> <li><a class="reference internal" href="#references">References</a></li> <li><a class="reference internal" href="#acknowledgements">Acknowledgements</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-3117.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>