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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 201 – Lockstep Iteration | peps.python.org</title> <link rel="shortcut icon" href="../_static/py.png"> <link rel="canonical" href="https://peps.python.org/pep-0201/"> <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 201 – Lockstep Iteration | peps.python.org'> <meta property="og:description" content="This PEP describes the ‘lockstep iteration’ proposal. 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This..."> <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 201</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 201 – Lockstep Iteration</h1> <dl class="rfc2822 field-list simple"> <dt class="field-odd">Author:</dt> <dd class="field-odd">Barry Warsaw <barry at python.org></dd> <dt class="field-even">Status:</dt> <dd class="field-even"><abbr title="Accepted and implementation complete, or no longer active">Final</abbr></dd> <dt class="field-odd">Type:</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:</dt> <dd class="field-even">13-Jul-2000</dd> <dt class="field-odd">Python-Version:</dt> <dd class="field-odd">2.0</dd> <dt class="field-even">Post-History:</dt> <dd class="field-even">27-Jul-2000</dd> </dl> <hr class="docutils" /> <section id="contents"> <details><summary>Table of Contents</summary><ul class="simple"> <li><a class="reference internal" href="#introduction">Introduction</a></li> <li><a class="reference internal" href="#motivation">Motivation</a></li> <li><a class="reference internal" href="#lockstep-for-loops">Lockstep For-Loops</a></li> <li><a class="reference internal" href="#the-proposed-solution">The Proposed Solution</a></li> <li><a class="reference internal" href="#return-value">Return Value</a></li> <li><a class="reference internal" href="#examples">Examples</a></li> <li><a class="reference internal" href="#reference-implementation">Reference Implementation</a></li> <li><a class="reference internal" href="#bdfl-pronouncements">BDFL Pronouncements</a></li> <li><a class="reference internal" href="#subsequent-change-to-zip">Subsequent Change to <code class="docutils literal notranslate">zip()</code></a></li> <li><a class="reference internal" href="#other-changes">Other Changes</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="introduction"> <h2><a class="toc-backref" href="#introduction" role="doc-backlink">Introduction</a></h2> This PEP describes the ‘lockstep iteration’ proposal. This PEP tracks the status and ownership of this feature, slated for introduction in Python 2.0. It contains a description of the feature and outlines changes necessary to support the feature. This PEP summarizes discussions held in mailing list forums, and provides URLs for further information, where appropriate. The CVS revision history of this file contains the definitive historical record. </section> <section id="motivation"> <h2><a class="toc-backref" href="#motivation" role="doc-backlink">Motivation</a></h2> Standard for-loops in Python iterate over every element in a sequence until the sequence is exhausted <a class="footnote-reference brackets" href="#id3" id="id1">[1]</a>. However, for-loops iterate over only a single sequence, and it is often desirable to loop over more than one sequence in a lock-step fashion. In other words, in a way such that the i-th iteration through the loop returns an object containing the i-th element from each sequence. The common idioms used to accomplish this are unintuitive. This PEP proposes a standard way of performing such iterations by introducing a new builtin function called <code class="docutils literal notranslate">zip</code>. While the primary motivation for zip() comes from lock-step iteration, by implementing zip() as a built-in function, it has additional utility in contexts other than for-loops. </section> <section id="lockstep-for-loops"> <h2><a class="toc-backref" href="#lockstep-for-loops" role="doc-backlink">Lockstep For-Loops</a></h2> Lockstep for-loops are non-nested iterations over two or more sequences, such that at each pass through the loop, one element from each sequence is taken to compose the target. This behavior can already be accomplished in Python through the use of the map() built-in function: <div class="highlight-default notranslate"><div class="highlight"><pre>>>> a = (1, 2, 3) >>> b = (4, 5, 6) >>> for i in map(None, a, b): print i ... (1, 4) (2, 5) (3, 6) >>> map(None, a, b) [(1, 4), (2, 5), (3, 6)] </pre></div> </div> The for-loop simply iterates over this list as normal. While the map() idiom is a common one in Python, it has several disadvantages: <ul> <li>It is non-obvious to programmers without a functional programming background.</li> <li>The use of the magic <code class="docutils literal notranslate">None</code> first argument is non-obvious.</li> <li>It has arbitrary, often unintended, and inflexible semantics when the lists are not of the same length: the shorter sequences are padded with <code class="docutils literal notranslate">None</code>:<div class="highlight-default notranslate"><div class="highlight"><pre>>>> c = (4, 5, 6, 7) >>> map(None, a, c) [(1, 4), (2, 5), (3, 6), (None, 7)] </pre></div> </div> </li> </ul> For these reasons, several proposals were floated in the Python 2.0 beta time frame for syntactic support of lockstep for-loops. Here are two suggestions: <div class="highlight-default notranslate"><div class="highlight"><pre>for x in seq1, y in seq2: # stuff </pre></div> </div> <div class="highlight-default notranslate"><div class="highlight"><pre>for x, y in seq1, seq2: # stuff </pre></div> </div> Neither of these forms would work, since they both already mean something in Python and changing the meanings would break existing code. All other suggestions for new syntax suffered the same problem, or were in conflict with other another proposed feature called ‘list comprehensions’ (see <a class="pep reference internal" href="../pep-0202/" title="PEP 202 – List Comprehensions">PEP 202</a>). </section> <section id="the-proposed-solution"> <h2><a class="toc-backref" href="#the-proposed-solution" role="doc-backlink">The Proposed Solution</a></h2> The proposed solution is to introduce a new built-in sequence generator function, available in the <code class="docutils literal notranslate">__builtin__</code> module. This function is to be called <code class="docutils literal notranslate">zip</code> and has the following signature: <div class="highlight-default notranslate"><div class="highlight"><pre>zip(seqa, [seqb, [...]]) </pre></div> </div> <code class="docutils literal notranslate">zip()</code> takes one or more sequences and weaves their elements together, just as <code class="docutils literal notranslate">map(None, ...)</code> does with sequences of equal length. The weaving stops when the shortest sequence is exhausted. </section> <section id="return-value"> <h2><a class="toc-backref" href="#return-value" role="doc-backlink">Return Value</a></h2> <code class="docutils literal notranslate">zip()</code> returns a real Python list, the same way <code class="docutils literal notranslate">map()</code> does. </section> <section id="examples"> <h2><a class="toc-backref" href="#examples" role="doc-backlink">Examples</a></h2> Here are some examples, based on the reference implementation below: <div class="highlight-default notranslate"><div class="highlight"><pre>>>> a = (1, 2, 3, 4) >>> b = (5, 6, 7, 8) >>> c = (9, 10, 11) >>> d = (12, 13) >>> zip(a, b) [(1, 5), (2, 6), (3, 7), (4, 8)] >>> zip(a, d) [(1, 12), (2, 13)] >>> zip(a, b, c, d) [(1, 5, 9, 12), (2, 6, 10, 13)] </pre></div> </div> Note that when the sequences are of the same length, <code class="docutils literal notranslate">zip()</code> is reversible: <div class="highlight-default notranslate"><div class="highlight"><pre>>>> a = (1, 2, 3) >>> b = (4, 5, 6) >>> x = zip(a, b) >>> y = zip(*x) # alternatively, apply(zip, x) >>> z = zip(*y) # alternatively, apply(zip, y) >>> x [(1, 4), (2, 5), (3, 6)] >>> y [(1, 2, 3), (4, 5, 6)] >>> z [(1, 4), (2, 5), (3, 6)] >>> x == z 1 </pre></div> </div> It is not possible to reverse zip this way when the sequences are not all the same length. </section> <section id="reference-implementation"> <h2><a class="toc-backref" href="#reference-implementation" role="doc-backlink">Reference Implementation</a></h2> Here is a reference implementation, in Python of the zip() built-in function. This will be replaced with a C implementation after final approval: <div class="highlight-default notranslate"><div class="highlight"><pre>def zip(*args): if not args: raise TypeError('zip() expects one or more sequence arguments') ret = [] i = 0 try: while 1: item = [] for s in args: item.append(s[i]) ret.append(tuple(item)) i = i + 1 except IndexError: return ret </pre></div> </div> </section> <section id="bdfl-pronouncements"> <h2><a class="toc-backref" href="#bdfl-pronouncements" role="doc-backlink">BDFL Pronouncements</a></h2> Note: the BDFL refers to Guido van Rossum, Python’s Benevolent Dictator For Life. <ul class="simple"> <li>The function’s name. An earlier version of this PEP included an open issue listing 20+ proposed alternative names to <code class="docutils literal notranslate">zip()</code>. In the face of no overwhelmingly better choice, the BDFL strongly prefers <code class="docutils literal notranslate">zip()</code> due to its Haskell <a class="footnote-reference brackets" href="#id4" id="id2">[2]</a> heritage. See version 1.7 of this PEP for the list of alternatives.</li> <li><code class="docutils literal notranslate">zip()</code> shall be a built-in function.</li> <li>Optional padding. An earlier version of this PEP proposed an optional <code class="docutils literal notranslate">pad</code> keyword argument, which would be used when the argument sequences were not the same length. This is similar behavior to the <code class="docutils literal notranslate">map(None, ...)</code> semantics except that the user would be able to specify pad object. This has been rejected by the BDFL in favor of always truncating to the shortest sequence, because of the KISS principle. If there’s a true need, it is easier to add later. If it is not needed, it would still be impossible to delete it in the future.</li> <li>Lazy evaluation. An earlier version of this PEP proposed that <code class="docutils literal notranslate">zip()</code> return a built-in object that performed lazy evaluation using <code class="docutils literal notranslate">__getitem__()</code> protocol. This has been strongly rejected by the BDFL in favor of returning a real Python list. If lazy evaluation is desired in the future, the BDFL suggests an <code class="docutils literal notranslate">xzip()</code> function be added.</li> <li><code class="docutils literal notranslate">zip()</code> with no arguments. the BDFL strongly prefers this raise a TypeError exception.</li> <li><code class="docutils literal notranslate">zip()</code> with one argument. the BDFL strongly prefers that this return a list of 1-tuples.</li> <li>Inner and outer container control. An earlier version of this PEP contains a rather lengthy discussion on a feature that some people wanted, namely the ability to control what the inner and outer container types were (they are tuples and list respectively in this version of the PEP). Given the simplified API and implementation, this elaboration is rejected. For a more detailed analysis, see version 1.7 of this PEP.</li> </ul> </section> <section id="subsequent-change-to-zip"> <h2><a class="toc-backref" href="#subsequent-change-to-zip" role="doc-backlink">Subsequent Change to <code class="docutils literal notranslate">zip()</code></a></h2> In Python 2.4, zip() with no arguments was modified to return an empty list rather than raising a TypeError exception. The rationale for the original behavior was that the absence of arguments was thought to indicate a programming error. However, that thinking did not anticipate the use of zip() with the <code class="docutils literal notranslate">*</code> operator for unpacking variable length argument lists. For example, the inverse of zip could be defined as: <code class="docutils literal notranslate">unzip = lambda s: zip(*s)</code>. That transformation also defines a matrix transpose or an equivalent row/column swap for tables defined as lists of tuples. The latter transformation is commonly used when reading data files with records as rows and fields as columns. For example, the code: <div class="highlight-default notranslate"><div class="highlight"><pre>date, rain, high, low = zip(*csv.reader(file("weather.csv"))) </pre></div> </div> rearranges columnar data so that each field is collected into individual tuples for straightforward looping and summarization: <div class="highlight-default notranslate"><div class="highlight"><pre>print "Total rainfall", sum(rain) </pre></div> </div> Using <code class="docutils literal notranslate">zip(*args)</code> is more easily coded if <code class="docutils literal notranslate">zip(*[])</code> is handled as an allowable case rather than an exception. This is especially helpful when data is either built up from or recursed down to a null case with no records. Seeing this possibility, the BDFL agreed (with some misgivings) to have the behavior changed for Py2.4. </section> <section id="other-changes"> <h2><a class="toc-backref" href="#other-changes" role="doc-backlink">Other Changes</a></h2> <ul> <li>The <code class="docutils literal notranslate">xzip()</code> function discussed above was implemented in Py2.3 in the <code class="docutils literal notranslate">itertools</code> module as <code class="docutils literal notranslate">itertools.izip()</code>. This function provides lazy behavior, consuming single elements and producing a single tuple on each pass. The “just-in-time” style saves memory and runs faster than its list based counterpart, <code class="docutils literal notranslate">zip()</code>.</li> <li>The <code class="docutils literal notranslate">itertools</code> module also added <code class="docutils literal notranslate">itertools.repeat()</code> and <code class="docutils literal notranslate">itertools.chain()</code>. These tools can be used together to pad sequences with <code class="docutils literal notranslate">None</code> (to match the behavior of <code class="docutils literal notranslate">map(None, seqn)</code>):<div class="highlight-default notranslate"><div class="highlight"><pre>zip(firstseq, chain(secondseq, repeat(None))) </pre></div> </div> </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="id3" role="doc-footnote"> <dt class="label" id="id3">[<a href="#id1">1</a>]</dt> <dd><a class="reference external" href="http://docs.python.org/reference/compound_stmts.html#for">http://docs.python.org/reference/compound_stmts.html#for</a></aside> <aside class="footnote brackets" id="id4" role="doc-footnote"> <dt class="label" id="id4">[<a href="#id2">2</a>]</dt> <dd><a class="reference external" href="http://www.haskell.org/onlinereport/standard-prelude.html#$vzip">http://www.haskell.org/onlinereport/standard-prelude.html#$vzip</a></aside> </aside> Greg Wilson’s questionnaire on proposed syntax to some CS grad students <a class="reference external" href="http://www.python.org/pipermail/python-dev/2000-July/013139.html">http://www.python.org/pipermail/python-dev/2000-July/013139.html</a> </section> <section id="copyright"> <h2><a class="toc-backref" href="#copyright" role="doc-backlink">Copyright</a></h2> This document has been placed in the public domain. </section> </section> <hr class="docutils" /> Source: <a class="reference external" href="https://github.com/python/peps/blob/main/peps/pep-0201.rst">https://github.com/python/peps/blob/main/peps/pep-0201.rst</a> Last modified: <a class="reference external" href="https://github.com/python/peps/commits/main/peps/pep-0201.rst">2025-02-01 08:55:40 GMT</a> </article> <nav id="pep-sidebar"> <h2>Contents</h2> <ul> <li><a class="reference internal" href="#introduction">Introduction</a></li> <li><a class="reference internal" href="#motivation">Motivation</a></li> <li><a class="reference internal" href="#lockstep-for-loops">Lockstep For-Loops</a></li> <li><a class="reference internal" href="#the-proposed-solution">The Proposed Solution</a></li> <li><a class="reference internal" href="#return-value">Return Value</a></li> <li><a class="reference internal" href="#examples">Examples</a></li> <li><a class="reference internal" href="#reference-implementation">Reference Implementation</a></li> <li><a class="reference internal" href="#bdfl-pronouncements">BDFL Pronouncements</a></li> <li><a class="reference internal" href="#subsequent-change-to-zip">Subsequent Change to <code class="docutils literal notranslate">zip()</code></a></li> <li><a class="reference internal" href="#other-changes">Other Changes</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-0201.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>