CINXE.COM

<!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 604 – Allow writing union types as X | Y | peps.python.org</title> <link rel="shortcut icon" href="../_static/py.png"> <link rel="canonical" href="https://peps.python.org/pep-0604/"> <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 604 – Allow writing union types as X | Y | peps.python.org'> <meta property="og:description" content="This PEP proposes overloading the | operator on types to allow writing Union[X, Y] as X | Y, and allows it to appear in isinstance and issubclass calls."> <meta property="og:type" content="website"> <meta property="og:url" content="https://peps.python.org/pep-0604/"> <meta property="og:site_name" content="Python Enhancement Proposals (PEPs)"> <meta property="og:image" content="https://peps.python.org/_static/og-image.png"> <meta property="og:image:alt" content="Python PEPs"> <meta property="og:image:width" content="200"> <meta property="og:image:height" content="200"> <meta name="description" content="This PEP proposes overloading the | operator on types to allow writing Union[X, Y] as X | Y, and allows it to appear in isinstance and issubclass calls."> <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 604</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> Toggle light / dark / auto colour theme </button> </header> <article> <section id="pep-content"> <h1 class="page-title">PEP 604 – Allow writing union types as <code class="docutils literal notranslate">X | Y</code></h1> <dl class="rfc2822 field-list simple"> <dt class="field-odd">Author:</dt> <dd class="field-odd">Philippe PRADOS <python at prados.fr>, Maggie Moss <maggiebmoss at gmail.com></dd> <dt class="field-even">Sponsor:</dt> <dd class="field-even">Chris Angelico <rosuav at gmail.com></dd> <dt class="field-odd">BDFL-Delegate:</dt> <dd class="field-odd">Guido van Rossum <guido at python.org></dd> <dt class="field-even">Discussions-To:</dt> <dd class="field-even"><a class="reference external" href="https://mail.python.org/archives/list/typing-sig@python.org/">Typing-SIG list</a></dd> <dt class="field-odd">Status:</dt> <dd class="field-odd"><abbr title="Accepted and implementation complete, or no longer active">Final</abbr></dd> <dt class="field-even">Type:</dt> <dd class="field-even"><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-odd">Topic:</dt> <dd class="field-odd"><a class="reference external" href="../topic/typing/">Typing</a></dd> <dt class="field-even">Created:</dt> <dd class="field-even">28-Aug-2019</dd> <dt class="field-odd">Python-Version:</dt> <dd class="field-odd">3.10</dd> <dt class="field-even">Post-History:</dt> <dd class="field-even">28-Aug-2019, 05-Aug-2020</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="#motivation">Motivation</a></li> <li><a class="reference internal" href="#proposal">Proposal</a></li> <li><a class="reference internal" href="#specification">Specification</a><ul> <li><a class="reference internal" href="#simplified-syntax">Simplified Syntax</a></li> <li><a class="reference internal" href="#isinstance-and-issubclass">isinstance and issubclass</a></li> </ul> </li> <li><a class="reference internal" href="#incompatible-changes">Incompatible changes</a></li> <li><a class="reference internal" href="#objections-and-responses">Objections and responses</a><ul> <li><a class="reference internal" href="#add-a-new-operator-for-union-type1-type2">1. Add a new operator for <code class="docutils literal notranslate">Union[type1, type2]</code>?</a></li> <li><a class="reference internal" href="#change-only-pep-484-type-hints-to-accept-the-syntax-type1-type2">2. Change only PEP 484 (Type hints) to accept the syntax <code class="docutils literal notranslate">type1 | type2</code> ?</a></li> <li><a class="reference internal" href="#extend-isinstance-and-issubclass-to-accept-union">3. Extend <code class="docutils literal notranslate">isinstance()</code> and <code class="docutils literal notranslate">issubclass()</code> to accept <code class="docutils literal notranslate">Union</code> ?</a></li> </ul> </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> <div class="pep-banner canonical-doc sticky-banner admonition important"> Important This PEP is a historical document. The up-to-date, canonical documentation can now be found at <a class="reference external" href="https://docs.python.org/3/library/stdtypes.html#types-union" title="(in Python v3.13)">Union Type</a>. × See <a class="pep reference internal" href="../pep-0001/" title="PEP 1 – PEP Purpose and Guidelines">PEP 1</a> for how to propose changes. </div> <section id="abstract"> <h2><a class="toc-backref" href="#abstract" role="doc-backlink">Abstract</a></h2> This PEP proposes overloading the <code class="docutils literal notranslate">|</code> operator on types to allow writing <code class="docutils literal notranslate">Union[X, Y]</code> as <code class="docutils literal notranslate">X | Y</code>, and allows it to appear in <code class="docutils literal notranslate">isinstance</code> and <code class="docutils literal notranslate">issubclass</code> calls. </section> <section id="motivation"> <h2><a class="toc-backref" href="#motivation" role="doc-backlink">Motivation</a></h2> <a class="pep reference internal" href="../pep-0484/" title="PEP 484 – Type Hints">PEP 484</a> and <a class="pep reference internal" href="../pep-0526/" title="PEP 526 – Syntax for Variable Annotations">PEP 526</a> propose a generic syntax to add typing to variables, parameters and function returns. <a class="pep reference internal" href="../pep-0585/" title="PEP 585 – Type Hinting Generics In Standard Collections">PEP 585</a> proposes to <a class="pep reference internal" href="../pep-0585/#parameters-to-generics-are-available-at-runtime" title="PEP 585 – Type Hinting Generics In Standard Collections § Parameters to generics are available at runtime">expose parameters to generics at runtime</a>. Mypy <a class="footnote-reference brackets" href="#id5" id="id1">[1]</a> accepts a syntax which looks like: <div class="highlight-default notranslate"><div class="highlight"><pre>annotation: name_type name_type: NAME (args)? args: '[' paramslist ']' paramslist: annotation (',' annotation)* [','] </pre></div> </div> <ul class="simple"> <li>To describe a disjunction (union type), the user must use <code class="docutils literal notranslate">Union[X, Y]</code>.</li> </ul> The verbosity of this syntax does not help with type adoption. </section> <section id="proposal"> <h2><a class="toc-backref" href="#proposal" role="doc-backlink">Proposal</a></h2> Inspired by Scala <a class="footnote-reference brackets" href="#id6" id="id2">[2]</a> and Pike <a class="footnote-reference brackets" href="#id7" id="id3">[3]</a>, this proposal adds operator <code class="docutils literal notranslate">type.__or__()</code>. With this new operator, it is possible to write <code class="docutils literal notranslate">int | str</code> instead of <code class="docutils literal notranslate">Union[int, str]</code>. In addition to annotations, the result of this expression would then be valid in <code class="docutils literal notranslate">isinstance()</code> and <code class="docutils literal notranslate">issubclass()</code>: <div class="highlight-default notranslate"><div class="highlight"><pre>isinstance(5, int | str) issubclass(bool, int | float) </pre></div> </div> We will also be able to write <code class="docutils literal notranslate">t | None</code> or <code class="docutils literal notranslate">None | t</code> instead of <code class="docutils literal notranslate">Optional[t]</code>: <div class="highlight-default notranslate"><div class="highlight"><pre>isinstance(None, int | None) isinstance(42, None | int) </pre></div> </div> </section> <section id="specification"> <h2><a class="toc-backref" href="#specification" role="doc-backlink">Specification</a></h2> The new union syntax should be accepted for function, variable and parameter annotations. <section id="simplified-syntax"> <h3><a class="toc-backref" href="#simplified-syntax" role="doc-backlink">Simplified Syntax</a></h3> <div class="highlight-default notranslate"><div class="highlight"><pre># Instead of # def f(list: List[Union[int, str]], param: Optional[int]) -> Union[float, str] def f(list: List[int | str], param: int | None) -> float | str: pass f([1, "abc"], None) # Instead of typing.List[typing.Union[str, int]] typing.List[str | int] list[str | int] # Instead of typing.Dict[str, typing.Union[int, float]] typing.Dict[str, int | float] dict[str, int | float] </pre></div> </div> The existing <code class="docutils literal notranslate">typing.Union</code> and <code class="docutils literal notranslate">|</code> syntax should be equivalent. <div class="highlight-default notranslate"><div class="highlight"><pre>int | str == typing.Union[int, str] typing.Union[int, int] == int int | int == int </pre></div> </div> The order of the items in the Union should not matter for equality. <div class="highlight-default notranslate"><div class="highlight"><pre>(int | str) == (str | int) (int | str | float) == typing.Union[str, float, int] </pre></div> </div> Optional values should be equivalent to the new union syntax <div class="highlight-default notranslate"><div class="highlight"><pre>None | t == typing.Optional[t] </pre></div> </div> A new Union.__repr__() method should be implemented. <div class="highlight-default notranslate"><div class="highlight"><pre>str(int | list[str]) # int | list[str] str(int | int) # int </pre></div> </div> </section> <section id="isinstance-and-issubclass"> <h3><a class="toc-backref" href="#isinstance-and-issubclass" role="doc-backlink">isinstance and issubclass</a></h3> The new syntax should be accepted for calls to <code class="docutils literal notranslate">isinstance</code> and <code class="docutils literal notranslate">issubclass</code> as long as the Union items are valid arguments to <code class="docutils literal notranslate">isinstance</code> and <code class="docutils literal notranslate">issubclass</code> themselves. <div class="highlight-default notranslate"><div class="highlight"><pre># valid isinstance("", int | str) # invalid isinstance(2, list[int]) # TypeError: isinstance() argument 2 cannot be a parameterized generic isinstance(1, int | list[int]) # valid issubclass(bool, int | float) # invalid issubclass(bool, bool | list[int]) </pre></div> </div> </section> </section> <section id="incompatible-changes"> <h2><a class="toc-backref" href="#incompatible-changes" role="doc-backlink">Incompatible changes</a></h2> In some situations, some exceptions will not be raised as expected. If a metaclass implements the <code class="docutils literal notranslate">__or__</code> operator, it will override this: <div class="highlight-default notranslate"><div class="highlight"><pre>>>> class M(type): ... def __or__(self, other): return "Hello" ... >>> class C(metaclass=M): pass ... >>> C | int 'Hello' >>> int | C typing.Union[int, __main__.C] >>> Union[C, int] typing.Union[__main__.C, int] </pre></div> </div> </section> <section id="objections-and-responses"> <h2><a class="toc-backref" href="#objections-and-responses" role="doc-backlink">Objections and responses</a></h2> For more details about discussions, see links below: <ul class="simple"> <li><a class="reference external" href="https://mail.python.org/archives/list/python-ideas@python.org/thread/FCTXGDT2NNKRJQ6CDEPWUXHVG2AAQZZY/">Discussion in python-ideas</a></li> <li><a class="reference external" href="https://mail.python.org/archives/list/typing-sig@python.org/thread/D5HCB4NT4S3WSK33WI26WZSFEXCEMNHN/">Discussion in typing-sig</a></li> </ul> <section id="add-a-new-operator-for-union-type1-type2"> <h3><a class="toc-backref" href="#add-a-new-operator-for-union-type1-type2" role="doc-backlink">1. Add a new operator for <code class="docutils literal notranslate">Union[type1, type2]</code>?</a></h3> PROS: <ul class="simple"> <li>This syntax can be more readable, and is similar to other languages (Scala, …)</li> <li>At runtime, <code class="docutils literal notranslate">int|str</code> might return a simple object in 3.10, rather than everything that you’d need to grab from importing <code class="docutils literal notranslate">typing</code></li> </ul> CONS: <ul class="simple"> <li>Adding this operator introduces a dependency between <code class="docutils literal notranslate">typing</code> and <code class="docutils literal notranslate">builtins</code></li> <li>Breaks the backport (in that <code class="docutils literal notranslate">typing</code> can easily be backported but core <code class="docutils literal notranslate">types</code> can’t)</li> <li>If Python itself doesn’t have to be changed, we’d still need to implement it in mypy, Pyre, PyCharm, Pytype, and who knows what else (it’s a minor change see “Reference Implementation”)</li> </ul> </section> <section id="change-only-pep-484-type-hints-to-accept-the-syntax-type1-type2"> <h3><a class="toc-backref" href="#change-only-pep-484-type-hints-to-accept-the-syntax-type1-type2" role="doc-backlink">2. Change only PEP 484 (Type hints) to accept the syntax <code class="docutils literal notranslate">type1 | type2</code> ?</a></h3> <a class="pep reference internal" href="../pep-0563/" title="PEP 563 – Postponed Evaluation of Annotations">PEP 563</a> (Postponed Evaluation of Annotations) is enough to accept this proposition, if we accept to not be compatible with the dynamic evaluation of annotations (<code class="docutils literal notranslate">eval()</code>). <div class="highlight-default notranslate"><div class="highlight"><pre>>>> from __future__ import annotations >>> def foo() -> int | str: pass ... >>> eval(foo.__annotations__['return']) Traceback (most recent call last): File "<stdin>", line 1, in <module> File "<string>", line 1, in <module> TypeError: unsupported operand type(s) for |: 'type' and 'type' </pre></div> </div> </section> <section id="extend-isinstance-and-issubclass-to-accept-union"> <h3><a class="toc-backref" href="#extend-isinstance-and-issubclass-to-accept-union" role="doc-backlink">3. Extend <code class="docutils literal notranslate">isinstance()</code> and <code class="docutils literal notranslate">issubclass()</code> to accept <code class="docutils literal notranslate">Union</code> ?</a></h3> <div class="highlight-default notranslate"><div class="highlight"><pre>isinstance(x, str | int) ==> "is x an instance of str or int" </pre></div> </div> PROS: <ul class="simple"> <li>If they were permitted, then instance checking could use an extremely clean-looking notation</li> </ul> CONS: <ul class="simple"> <li>Must migrate all of the <code class="docutils literal notranslate">typing</code> module in <code class="docutils literal notranslate">builtin</code></li> </ul> </section> </section> <section id="reference-implementation"> <h2><a class="toc-backref" href="#reference-implementation" role="doc-backlink">Reference Implementation</a></h2> A new built-in <code class="docutils literal notranslate">Union</code> type must be implemented to hold the return value of <code class="docutils literal notranslate">t1 | t2</code>, and it must be supported by <code class="docutils literal notranslate">isinstance()</code> and <code class="docutils literal notranslate">issubclass()</code>. This type can be placed in the <code class="docutils literal notranslate">types</code> module. Interoperability between <code class="docutils literal notranslate">types.Union</code> and <code class="docutils literal notranslate">typing.Union</code> must be provided. Once the Python language is extended, mypy <a class="footnote-reference brackets" href="#id5" id="id4">[1]</a> and other type checkers will need to be updated to accept this new syntax. <ul class="simple"> <li>A proposed implementation for <a class="reference external" href="https://github.com/python/cpython/pull/21515">cpython is here</a>.</li> <li>A proposed implementation for <a class="reference external" href="https://github.com/pprados/mypy/tree/PEP604">mypy is here</a>.</li> </ul> </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="id5" role="doc-footnote"> <dt class="label" id="id5">[1] (<a href='#id1'>1</a>, <a href='#id4'>2</a>) </dt> <dd>mypy <a class="reference external" href="http://mypy-lang.org/">http://mypy-lang.org/</a></aside> <aside class="footnote brackets" id="id6" role="doc-footnote"> <dt class="label" id="id6">[<a href="#id2">2</a>]</dt> <dd>Scala Union Types <a class="reference external" href="https://dotty.epfl.ch/docs/reference/new-types/union-types.html">https://dotty.epfl.ch/docs/reference/new-types/union-types.html</a></aside> <aside class="footnote brackets" id="id7" role="doc-footnote"> <dt class="label" id="id7">[<a href="#id3">3</a>]</dt> <dd>Pike <a class="reference external" href="http://pike.lysator.liu.se/docs/man/chapter_3.html#3.5">http://pike.lysator.liu.se/docs/man/chapter_3.html#3.5</a></aside> </aside> </section> <section id="copyright"> <h2><a class="toc-backref" href="#copyright" role="doc-backlink">Copyright</a></h2> This document is placed in the public domain or under the CC0-1.0-Universal license, whichever is more permissive. </section> </section> <hr class="docutils" /> Source: <a class="reference external" href="https://github.com/python/peps/blob/main/peps/pep-0604.rst">https://github.com/python/peps/blob/main/peps/pep-0604.rst</a> Last modified: <a class="reference external" href="https://github.com/python/peps/commits/main/peps/pep-0604.rst">2024-02-16 17:06:07 GMT</a> </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="#motivation">Motivation</a></li> <li><a class="reference internal" href="#proposal">Proposal</a></li> <li><a class="reference internal" href="#specification">Specification</a><ul> <li><a class="reference internal" href="#simplified-syntax">Simplified Syntax</a></li> <li><a class="reference internal" href="#isinstance-and-issubclass">isinstance and issubclass</a></li> </ul> </li> <li><a class="reference internal" href="#incompatible-changes">Incompatible changes</a></li> <li><a class="reference internal" href="#objections-and-responses">Objections and responses</a><ul> <li><a class="reference internal" href="#add-a-new-operator-for-union-type1-type2">1. Add a new operator for <code class="docutils literal notranslate">Union[type1, type2]</code>?</a></li> <li><a class="reference internal" href="#change-only-pep-484-type-hints-to-accept-the-syntax-type1-type2">2. Change only PEP 484 (Type hints) to accept the syntax <code class="docutils literal notranslate">type1 | type2</code> ?</a></li> <li><a class="reference internal" href="#extend-isinstance-and-issubclass-to-accept-union">3. Extend <code class="docutils literal notranslate">isinstance()</code> and <code class="docutils literal notranslate">issubclass()</code> to accept <code class="docutils literal notranslate">Union</code> ?</a></li> </ul> </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> <a id="source" href="https://github.com/python/peps/blob/main/peps/pep-0604.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>