mirror of https://github.com/python/peps
304 lines
20 KiB
HTML
304 lines
20 KiB
HTML
|
||
<!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 279 – The enumerate() built-in function | peps.python.org</title>
|
||
<link rel="shortcut icon" href="../_static/py.png">
|
||
<link rel="canonical" href="https://peps.python.org/pep-0279/">
|
||
<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 279 – The enumerate() built-in function | peps.python.org'>
|
||
<meta property="og:type" content="website">
|
||
<meta property="og:url" content="https://peps.python.org/pep-0279/">
|
||
<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="Python Enhancement Proposals (PEPs)">
|
||
<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 279</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 279 – The enumerate() built-in function</h1>
|
||
<dl class="rfc2822 field-list simple">
|
||
<dt class="field-odd">Author<span class="colon">:</span></dt>
|
||
<dd class="field-odd">Raymond Hettinger <python at rcn.com></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-Jan-2002</dd>
|
||
<dt class="field-odd">Python-Version<span class="colon">:</span></dt>
|
||
<dd class="field-odd">2.3</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="#bdfl-pronouncements">BDFL Pronouncements</a></li>
|
||
<li><a class="reference internal" href="#specification-for-a-new-built-in">Specification for a new built-in</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 introduces a new built-in function, <code class="docutils literal notranslate"><span class="pre">enumerate()</span></code> to
|
||
simplify a commonly used looping idiom. It provides all iterable
|
||
collections with the same advantage that <code class="docutils literal notranslate"><span class="pre">iteritems()</span></code> affords to
|
||
dictionaries – a compact, readable, reliable index notation.</p>
|
||
</section>
|
||
<section id="rationale">
|
||
<h2><a class="toc-backref" href="#rationale" role="doc-backlink">Rationale</a></h2>
|
||
<p>Python 2.2 introduced the concept of an iterable interface as
|
||
proposed in <a class="pep reference internal" href="../pep-0234/" title="PEP 234 – Iterators">PEP 234</a>. The <code class="docutils literal notranslate"><span class="pre">iter()</span></code> factory function was provided
|
||
as common calling convention and deep changes were made to use
|
||
iterators as a unifying theme throughout Python. The unification
|
||
came in the form of establishing a common iterable interface for
|
||
mappings, sequences, and file objects.</p>
|
||
<p>Generators, as proposed in <a class="pep reference internal" href="../pep-0255/" title="PEP 255 – Simple Generators">PEP 255</a>, were introduced as a means
|
||
for making it easier to create iterators, especially ones with
|
||
complex internal execution or variable states. The availability
|
||
of generators makes it possible to improve on the loop counter
|
||
ideas in <a class="pep reference internal" href="../pep-0212/" title="PEP 212 – Loop Counter Iteration">PEP 212</a>. Those ideas provided a clean syntax for
|
||
iteration with indices and values, but did not apply to all
|
||
iterable objects. Also, that approach did not have the memory
|
||
friendly benefit provided by generators which do not evaluate the
|
||
entire sequence all at once.</p>
|
||
<p>The new proposal is to add a built-in function, <code class="docutils literal notranslate"><span class="pre">enumerate()</span></code> which
|
||
was made possible once iterators and generators became available.
|
||
It provides all iterables with the same advantage that <code class="docutils literal notranslate"><span class="pre">iteritems()</span></code>
|
||
affords to dictionaries – a compact, readable, reliable index
|
||
notation. Like <code class="docutils literal notranslate"><span class="pre">zip()</span></code>, it is expected to become a commonly used
|
||
looping idiom.</p>
|
||
<p>This suggestion is designed to take advantage of the existing
|
||
implementation and require little additional effort to
|
||
incorporate. It is backwards compatible and requires no new
|
||
keywords. The proposal will go into Python 2.3 when generators
|
||
become final and are not imported from <code class="docutils literal notranslate"><span class="pre">__future__</span></code>.</p>
|
||
</section>
|
||
<section id="bdfl-pronouncements">
|
||
<h2><a class="toc-backref" href="#bdfl-pronouncements" role="doc-backlink">BDFL Pronouncements</a></h2>
|
||
<p>The new built-in function is ACCEPTED.</p>
|
||
</section>
|
||
<section id="specification-for-a-new-built-in">
|
||
<h2><a class="toc-backref" href="#specification-for-a-new-built-in" role="doc-backlink">Specification for a new built-in</a></h2>
|
||
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">enumerate</span><span class="p">(</span><span class="n">collection</span><span class="p">):</span>
|
||
<span class="s1">'Generates an indexed series: (0,coll[0]), (1,coll[1]) ...'</span>
|
||
<span class="n">i</span> <span class="o">=</span> <span class="mi">0</span>
|
||
<span class="n">it</span> <span class="o">=</span> <span class="nb">iter</span><span class="p">(</span><span class="n">collection</span><span class="p">)</span>
|
||
<span class="k">while</span> <span class="mi">1</span><span class="p">:</span>
|
||
<span class="k">yield</span> <span class="p">(</span><span class="n">i</span><span class="p">,</span> <span class="n">it</span><span class="o">.</span><span class="n">next</span><span class="p">())</span>
|
||
<span class="n">i</span> <span class="o">+=</span> <span class="mi">1</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>Note A: <a class="pep reference internal" href="../pep-0212/" title="PEP 212 – Loop Counter Iteration">PEP 212</a> Loop Counter Iteration discussed several
|
||
proposals for achieving indexing. Some of the proposals only work
|
||
for lists unlike the above function which works for any generator,
|
||
xrange, sequence, or iterable object. Also, those proposals were
|
||
presented and evaluated in the world prior to Python 2.2 which did
|
||
not include generators. As a result, the non-generator version in
|
||
<a class="pep reference internal" href="../pep-0212/" title="PEP 212 – Loop Counter Iteration">PEP 212</a> had the disadvantage of consuming memory with a giant list
|
||
of tuples. The generator version presented here is fast and
|
||
light, works with all iterables, and allows users to abandon the
|
||
sequence in mid-stream with no loss of computation effort.</p>
|
||
<p>There are other PEPs which touch on related issues: integer
|
||
iterators, integer for-loops, and one for modifying the arguments
|
||
to <code class="docutils literal notranslate"><span class="pre">range</span></code> and <code class="docutils literal notranslate"><span class="pre">xrange</span></code>. The <code class="docutils literal notranslate"><span class="pre">enumerate()</span></code> proposal does not preclude
|
||
the other proposals and it still meets an important need even if
|
||
those are adopted – the need to count items in any iterable. The
|
||
other proposals give a means of producing an index but not the
|
||
corresponding value. This is especially problematic if a sequence
|
||
is given which doesn’t support random access such as a file
|
||
object, generator, or sequence defined with <code class="docutils literal notranslate"><span class="pre">__getitem__</span></code>.</p>
|
||
<p>Note B: Almost all of the PEP reviewers welcomed the function but
|
||
were divided as to whether there should be any built-ins. The
|
||
main argument for a separate module was to slow the rate of
|
||
language inflation. The main argument for a built-in was that the
|
||
function is destined to be part of a core programming style,
|
||
applicable to any object with an iterable interface. Just as
|
||
<code class="docutils literal notranslate"><span class="pre">zip()</span></code> solves the problem of looping over multiple sequences, the
|
||
<code class="docutils literal notranslate"><span class="pre">enumerate()</span></code> function solves the loop counter problem.</p>
|
||
<p>If only one built-in is allowed, then <code class="docutils literal notranslate"><span class="pre">enumerate()</span></code> is the most
|
||
important general purpose tool, solving the broadest class of
|
||
problems while improving program brevity, clarity and reliability.</p>
|
||
<p>Note C: Various alternative names were discussed:</p>
|
||
<table class="docutils align-default">
|
||
<tbody>
|
||
<tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">iterindexed()</span></code></td>
|
||
<td>five syllables is a mouthful</td>
|
||
</tr>
|
||
<tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">index()</span></code></td>
|
||
<td>nice verb but could be confused the <code class="docutils literal notranslate"><span class="pre">.index()</span></code> method</td>
|
||
</tr>
|
||
<tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">indexed()</span></code></td>
|
||
<td>widely liked however adjectives should be avoided</td>
|
||
</tr>
|
||
<tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">indexer()</span></code></td>
|
||
<td>noun did not read well in a for-loop</td>
|
||
</tr>
|
||
<tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">count()</span></code></td>
|
||
<td>direct and explicit but often used in other contexts</td>
|
||
</tr>
|
||
<tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">itercount()</span></code></td>
|
||
<td>direct, explicit and hated by more than one person</td>
|
||
</tr>
|
||
<tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">iteritems()</span></code></td>
|
||
<td>conflicts with key:value concept for dictionaries</td>
|
||
</tr>
|
||
<tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">itemize()</span></code></td>
|
||
<td>confusing because <code class="docutils literal notranslate"><span class="pre">amap.items()</span></code> != <code class="docutils literal notranslate"><span class="pre">list(itemize(amap))</span></code></td>
|
||
</tr>
|
||
<tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">enum()</span></code></td>
|
||
<td>pithy; less clear than enumerate; too similar to enum
|
||
in other languages where it has a different meaning</td>
|
||
</tr>
|
||
</tbody>
|
||
</table>
|
||
<p>All of the names involving ‘count’ had the further disadvantage of
|
||
implying that the count would begin from one instead of zero.</p>
|
||
<p>All of the names involving ‘index’ clashed with usage in database
|
||
languages where indexing implies a sorting operation rather than
|
||
linear sequencing.</p>
|
||
<p>Note D: This function was originally proposed with optional start
|
||
and stop arguments. GvR pointed out that the function call
|
||
<code class="docutils literal notranslate"><span class="pre">enumerate(seqn,4,6)</span></code> had an alternate, plausible interpretation as
|
||
a slice that would return the fourth and fifth elements of the
|
||
sequence. To avoid the ambiguity, the optional arguments were
|
||
dropped even though it meant losing flexibility as a loop counter.
|
||
That flexibility was most important for the common case of
|
||
counting from one, as in:</p>
|
||
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">for</span> <span class="n">linenum</span><span class="p">,</span> <span class="n">line</span> <span class="ow">in</span> <span class="nb">enumerate</span><span class="p">(</span><span class="n">source</span><span class="p">,</span><span class="mi">1</span><span class="p">):</span> <span class="nb">print</span> <span class="n">linenum</span><span class="p">,</span> <span class="n">line</span>
|
||
</pre></div>
|
||
</div>
|
||
<dl>
|
||
<dt>Comments from GvR:</dt><dd><code class="docutils literal notranslate"><span class="pre">filter</span></code> and <code class="docutils literal notranslate"><span class="pre">map</span></code> should die and be subsumed into list
|
||
comprehensions, not grow more variants. I’d rather introduce
|
||
built-ins that do iterator algebra (e.g. the iterzip that I’ve
|
||
often used as an example).<p>I like the idea of having some way to iterate over a sequence
|
||
and its index set in parallel. It’s fine for this to be a
|
||
built-in.</p>
|
||
<p>I don’t like the name “indexed”; adjectives do not make good
|
||
function names. Maybe <code class="docutils literal notranslate"><span class="pre">iterindexed()</span></code>?</p>
|
||
</dd>
|
||
<dt>Comments from Ka-Ping Yee:</dt><dd>I’m also quite happy with everything you
|
||
proposed … and the extra built-ins (really ‘indexed’ in
|
||
particular) are things I have wanted for a long time.</dd>
|
||
<dt>Comments from Neil Schemenauer:</dt><dd>The new built-ins sound okay. Guido
|
||
may be concerned with increasing the number of built-ins too
|
||
much. You might be better off selling them as part of a
|
||
module. If you use a module then you can add lots of useful
|
||
functions (Haskell has lots of them that we could steal).</dd>
|
||
<dt>Comments for Magnus Lie Hetland:</dt><dd>I think indexed would be a useful and
|
||
natural built-in function. I would certainly use it a lot. I
|
||
like <code class="docutils literal notranslate"><span class="pre">indexed()</span></code> a lot; +1. I’m quite happy to have it make PEP
|
||
281 obsolete. Adding a separate module for iterator utilities
|
||
seems like a good idea.</dd>
|
||
<dt>Comments from the Community:</dt><dd>The response to the <code class="docutils literal notranslate"><span class="pre">enumerate()</span></code> proposal
|
||
has been close to 100% favorable. Almost everyone loves the
|
||
idea.</dd>
|
||
<dt>Author response:</dt><dd>Prior to these comments, four built-ins were proposed.
|
||
After the comments, <code class="docutils literal notranslate"><span class="pre">xmap</span></code>, <code class="docutils literal notranslate"><span class="pre">xfilter</span></code> and <code class="docutils literal notranslate"><span class="pre">xzip</span></code> were withdrawn. The
|
||
one that remains is vital for the language and is proposed by
|
||
itself. <code class="docutils literal notranslate"><span class="pre">Indexed()</span></code> is trivially easy to implement and can be
|
||
documented in minutes. More importantly, it is useful in
|
||
everyday programming which does not otherwise involve explicit
|
||
use of generators.<p>This proposal originally included another function <code class="docutils literal notranslate"><span class="pre">iterzip()</span></code>.
|
||
That was subsequently implemented as the <code class="docutils literal notranslate"><span class="pre">izip()</span></code> function in
|
||
the <code class="docutils literal notranslate"><span class="pre">itertools</span></code> module.</p>
|
||
</dd>
|
||
</dl>
|
||
</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-0279.rst">https://github.com/python/peps/blob/main/peps/pep-0279.rst</a></p>
|
||
<p>Last modified: <a class="reference external" href="https://github.com/python/peps/commits/main/peps/pep-0279.rst">2023-09-09 17:39:29 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="#bdfl-pronouncements">BDFL Pronouncements</a></li>
|
||
<li><a class="reference internal" href="#specification-for-a-new-built-in">Specification for a new built-in</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-0279.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> |