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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 3132 – Extended Iterable Unpacking | peps.python.org</title> <link rel="shortcut icon" href="../_static/py.png"> <link rel="canonical" href="https://peps.python.org/pep-3132/"> <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 3132 – Extended Iterable Unpacking | peps.python.org'> <meta property="og:description" 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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 3132 – Extended Iterable Unpacking</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abbr title="Accepted and implementation complete, or no longer active">Final</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">30-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="#implementation">Implementation</a><ul> <li><a class="reference internal" href="#grammar-change">Grammar change</a></li> <li><a class="reference internal" href="#changes-to-the-compiler">Changes to the Compiler</a></li> <li><a class="reference internal" href="#changes-to-the-bytecode-interpreter">Changes to the Bytecode Interpreter</a></li> </ul> </li> <li><a class="reference internal" href="#acceptance">Acceptance</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>This PEP proposes a change to iterable unpacking syntax, allowing to specify a “catch-all” name which will be assigned a list of all items not assigned to a “regular” name.</p> <p>An example says more than a thousand words:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">a</span><span class="p">,</span> <span class="o">*</span><span class="n">b</span><span class="p">,</span> <span class="n">c</span> <span class="o">=</span> <span class="nb">range</span><span class="p">(</span><span class="mi">5</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">a</span> <span class="go">0</span> <span class="gp">>>> </span><span class="n">c</span> <span class="go">4</span> <span class="gp">>>> </span><span class="n">b</span> <span class="go">[1, 2, 3]</span> </pre></div> </div> </section> <section id="rationale"> <h2><a class="toc-backref" href="#rationale" role="doc-backlink">Rationale</a></h2> <p>Many algorithms require splitting a sequence in a “first, rest” pair. With the new syntax,</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">first</span><span class="p">,</span> <span class="n">rest</span> <span class="o">=</span> <span class="n">seq</span><span class="p">[</span><span class="mi">0</span><span class="p">],</span> <span class="n">seq</span><span class="p">[</span><span class="mi">1</span><span class="p">:]</span> </pre></div> </div> <p>is replaced by the cleaner and probably more efficient:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">first</span><span class="p">,</span> <span class="o">*</span><span class="n">rest</span> <span class="o">=</span> <span class="n">seq</span> </pre></div> </div> <p>For more complex unpacking patterns, the new syntax looks even cleaner, and the clumsy index handling is not necessary anymore.</p> <p>Also, if the right-hand value is not a list, but an iterable, it has to be converted to a list before being able to do slicing; to avoid creating this temporary list, one has to resort to</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">it</span> <span class="o">=</span> <span class="nb">iter</span><span class="p">(</span><span class="n">seq</span><span class="p">)</span> <span class="n">first</span> <span class="o">=</span> <span class="n">it</span><span class="o">.</span><span class="n">next</span><span class="p">()</span> <span class="n">rest</span> <span class="o">=</span> <span class="nb">list</span><span class="p">(</span><span class="n">it</span><span class="p">)</span> </pre></div> </div> </section> <section id="specification"> <h2><a class="toc-backref" href="#specification" role="doc-backlink">Specification</a></h2> <p>A tuple (or list) on the left side of a simple assignment (unpacking is not defined for augmented assignment) may contain at most one expression prepended with a single asterisk (which is henceforth called a “starred” expression, while the other expressions in the list are called “mandatory”). This designates a subexpression that will be assigned a list of all items from the iterable being unpacked that are not assigned to any of the mandatory expressions, or an empty list if there are no such items.</p> <p>For example, if <code class="docutils literal notranslate"><span class="pre">seq</span></code> is a sliceable sequence, all the following assignments are equivalent if <code class="docutils literal notranslate"><span class="pre">seq</span></code> has at least two elements:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">a</span><span class="p">,</span> <span class="n">b</span><span class="p">,</span> <span class="n">c</span> <span class="o">=</span> <span class="n">seq</span><span class="p">[</span><span class="mi">0</span><span class="p">],</span> <span class="nb">list</span><span class="p">(</span><span class="n">seq</span><span class="p">[</span><span class="mi">1</span><span class="p">:</span><span class="o">-</span><span class="mi">1</span><span class="p">]),</span> <span class="n">seq</span><span class="p">[</span><span class="o">-</span><span class="mi">1</span><span class="p">]</span> <span class="n">a</span><span class="p">,</span> <span class="o">*</span><span class="n">b</span><span class="p">,</span> <span class="n">c</span> <span class="o">=</span> <span class="n">seq</span> <span class="p">[</span><span class="n">a</span><span class="p">,</span> <span class="o">*</span><span class="n">b</span><span class="p">,</span> <span class="n">c</span><span class="p">]</span> <span class="o">=</span> <span class="n">seq</span> </pre></div> </div> <p>It is an error (as it is currently) if the iterable doesn’t contain enough items to assign to all the mandatory expressions.</p> <p>It is also an error to use the starred expression as a lone assignment target, as in</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o">*</span><span class="n">a</span> <span class="o">=</span> <span class="nb">range</span><span class="p">(</span><span class="mi">5</span><span class="p">)</span> </pre></div> </div> <p>This, however, is valid syntax:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o">*</span><span class="n">a</span><span class="p">,</span> <span class="o">=</span> <span class="nb">range</span><span class="p">(</span><span class="mi">5</span><span class="p">)</span> </pre></div> </div> <p>Note that this proposal also applies to tuples in implicit assignment context, such as in a <code class="docutils literal notranslate"><span class="pre">for</span></code> statement:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">for</span> <span class="n">a</span><span class="p">,</span> <span class="o">*</span><span class="n">b</span> <span class="ow">in</span> <span class="p">[(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">),</span> <span class="p">(</span><span class="mi">4</span><span class="p">,</span> <span class="mi">5</span><span class="p">,</span> <span class="mi">6</span><span class="p">,</span> <span class="mi">7</span><span class="p">)]:</span> <span class="nb">print</span><span class="p">(</span><span class="n">b</span><span class="p">)</span> </pre></div> </div> <p>would print out</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">]</span> <span class="p">[</span><span class="mi">5</span><span class="p">,</span> <span class="mi">6</span><span class="p">,</span> <span class="mi">7</span><span class="p">]</span> </pre></div> </div> <p>Starred expressions are only allowed as assignment targets, using them anywhere else (except for star-args in function calls, of course) is an error.</p> </section> <section id="implementation"> <h2><a class="toc-backref" href="#implementation" role="doc-backlink">Implementation</a></h2> <section id="grammar-change"> <h3><a class="toc-backref" href="#grammar-change" role="doc-backlink">Grammar change</a></h3> <p>This feature requires a new grammar rule:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">star_expr</span><span class="p">:</span> <span class="p">[</span><span class="s1">'*'</span><span class="p">]</span> <span class="n">expr</span> </pre></div> </div> <p>In these two rules, <code class="docutils literal notranslate"><span class="pre">expr</span></code> is changed to <code class="docutils literal notranslate"><span class="pre">star_expr</span></code>:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">comparison</span><span class="p">:</span> <span class="n">star_expr</span> <span class="p">(</span><span class="n">comp_op</span> <span class="n">star_expr</span><span class="p">)</span><span class="o">*</span> <span class="n">exprlist</span><span class="p">:</span> <span class="n">star_expr</span> <span class="p">(</span><span class="s1">','</span> <span class="n">star_expr</span><span class="p">)</span><span class="o">*</span> <span class="p">[</span><span class="s1">','</span><span class="p">]</span> </pre></div> </div> </section> <section id="changes-to-the-compiler"> <h3><a class="toc-backref" href="#changes-to-the-compiler" role="doc-backlink">Changes to the Compiler</a></h3> <p>A new ASDL expression type <code class="docutils literal notranslate"><span class="pre">Starred</span></code> is added which represents a starred expression. Note that the starred expression element introduced here is universal and could later be used for other purposes in non-assignment context, such as the <code class="docutils literal notranslate"><span class="pre">yield</span> <span class="pre">*iterable</span></code> proposal.</p> <p>The compiler is changed to recognize all cases where a starred expression is invalid and flag them with syntax errors.</p> <p>A new bytecode instruction, <code class="docutils literal notranslate"><span class="pre">UNPACK_EX</span></code>, is added, whose argument has the number of mandatory targets before the starred target in the lower 8 bits and the number of mandatory targets after the starred target in the upper 8 bits. For unpacking sequences without starred expressions, the old <code class="docutils literal notranslate"><span class="pre">UNPACK_ITERABLE</span></code> opcode is kept.</p> </section> <section id="changes-to-the-bytecode-interpreter"> <h3><a class="toc-backref" href="#changes-to-the-bytecode-interpreter" role="doc-backlink">Changes to the Bytecode Interpreter</a></h3> <p>The function <code class="docutils literal notranslate"><span class="pre">unpack_iterable()</span></code> in ceval.c is changed to handle the extended unpacking, via an <code class="docutils literal notranslate"><span class="pre">argcntafter</span></code> parameter. In the <code class="docutils literal notranslate"><span class="pre">UNPACK_EX</span></code> case, the function will do the following:</p> <ul class="simple"> <li>collect all items for mandatory targets before the starred one</li> <li>collect all remaining items from the iterable in a list</li> <li>pop items for mandatory targets after the starred one from the list</li> <li>push the single items and the resized list on the stack</li> </ul> <p>Shortcuts for unpacking iterables of known types, such as lists or tuples, can be added.</p> <p>The current implementation can be found at the SourceForge Patch tracker <a class="reference internal" href="#sfpatch" id="id1"><span>[SFPATCH]</span></a>. It now includes a minimal test case.</p> </section> </section> <section id="acceptance"> <h2><a class="toc-backref" href="#acceptance" role="doc-backlink">Acceptance</a></h2> <p>After a short discussion on the python-3000 list <a class="footnote-reference brackets" href="#id3" id="id2">[1]</a>, the PEP was accepted by Guido in its current form. Possible changes discussed were:</p> <ul class="simple"> <li>Only allow a starred expression as the last item in the exprlist. This would simplify the unpacking code a bit and allow for the starred expression to be assigned an iterator. This behavior was rejected because it would be too surprising.</li> <li>Try to give the starred target the same type as the source iterable, for example, <code class="docutils literal notranslate"><span class="pre">b</span></code> in <code class="docutils literal notranslate"><span class="pre">a,</span> <span class="pre">*b</span> <span class="pre">=</span> <span class="pre">'hello'</span></code> would be assigned the string <code class="docutils literal notranslate"><span class="pre">'ello'</span></code>. This may seem nice, but is impossible to get right consistently with all iterables.</li> <li>Make the starred target a tuple instead of a list. This would be consistent with a function’s <code class="docutils literal notranslate"><span class="pre">*args</span></code>, but make further processing of the result harder.</li> </ul> </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="sfpatch" role="doc-biblioentry"> <dt class="label" id="sfpatch">[<a href="#id1">SFPATCH</a>]</dt> <dd><a class="reference external" href="https://bugs.python.org/issue1711529">https://bugs.python.org/issue1711529</a></div> </div> <aside class="footnote-list brackets"> <aside class="footnote brackets" id="id3" role="doc-footnote"> <dt class="label" id="id3">[<a href="#id2">1</a>]</dt> <dd><a class="reference external" href="https://mail.python.org/pipermail/python-3000/2007-May/007198.html">https://mail.python.org/pipermail/python-3000/2007-May/007198.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-3132.rst">https://github.com/python/peps/blob/main/peps/pep-3132.rst</a></p> <p>Last modified: <a class="reference external" href="https://github.com/python/peps/commits/main/peps/pep-3132.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="#implementation">Implementation</a><ul> <li><a class="reference internal" href="#grammar-change">Grammar change</a></li> <li><a class="reference internal" href="#changes-to-the-compiler">Changes to the Compiler</a></li> <li><a class="reference internal" href="#changes-to-the-bytecode-interpreter">Changes to the Bytecode Interpreter</a></li> </ul> </li> <li><a class="reference internal" href="#acceptance">Acceptance</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-3132.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>