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<h1 class="page-title">PEP 279 – The enumerate() built-in function</h1>
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<dl class="rfc2822 field-list simple">
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<dt class="field-odd">Author<span class="colon">:</span></dt>
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<dd class="field-odd">Raymond Hettinger <python at rcn.com></dd>
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<dt class="field-even">Status<span class="colon">:</span></dt>
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<dd class="field-even"><abbr title="Accepted and implementation complete, or no longer active">Final</abbr></dd>
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<dt class="field-odd">Type<span class="colon">:</span></dt>
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<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>
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<dt class="field-even">Created<span class="colon">:</span></dt>
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<dd class="field-even">30-Jan-2002</dd>
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<dt class="field-odd">Python-Version<span class="colon">:</span></dt>
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<dd class="field-odd">2.3</dd>
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<dt class="field-even">Post-History<span class="colon">:</span></dt>
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<dd class="field-even"><p></p></dd>
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<section id="contents">
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<details><summary>Table of Contents</summary><ul class="simple">
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<li><a class="reference internal" href="#abstract">Abstract</a></li>
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<li><a class="reference internal" href="#rationale">Rationale</a></li>
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<li><a class="reference internal" href="#bdfl-pronouncements">BDFL Pronouncements</a></li>
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<li><a class="reference internal" href="#specification-for-a-new-built-in">Specification for a new built-in</a></li>
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<li><a class="reference internal" href="#copyright">Copyright</a></li>
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</ul>
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</details></section>
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<section id="abstract">
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<h2><a class="toc-backref" href="#abstract" role="doc-backlink">Abstract</a></h2>
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<p>This PEP introduces a new built-in function, <code class="docutils literal notranslate"><span class="pre">enumerate()</span></code> to
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simplify a commonly used looping idiom. It provides all iterable
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collections with the same advantage that <code class="docutils literal notranslate"><span class="pre">iteritems()</span></code> affords to
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dictionaries – a compact, readable, reliable index notation.</p>
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</section>
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<section id="rationale">
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<h2><a class="toc-backref" href="#rationale" role="doc-backlink">Rationale</a></h2>
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<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
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||
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>
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||
<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
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||
iterable objects. Also, that approach did not have the memory
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||
friendly benefit provided by generators which do not evaluate the
|
||
entire sequence all at once.</p>
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||
<p>The new proposal is to add a built-in function, <code class="docutils literal notranslate"><span class="pre">enumerate()</span></code> which
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was made possible once iterators and generators became available.
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||
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
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||
notation. Like <code class="docutils literal notranslate"><span class="pre">zip()</span></code>, it is expected to become a commonly used
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looping idiom.</p>
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<p>This suggestion is designed to take advantage of the existing
|
||
implementation and require little additional effort to
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||
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>
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||
<section id="bdfl-pronouncements">
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||
<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>
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||
<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="w"> </span><span class="nf">enumerate</span><span class="p">(</span><span class="n">collection</span><span class="p">):</span>
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||
<span class="s1">'Generates an indexed series: (0,coll[0]), (1,coll[1]) ...'</span>
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||
<span class="n">i</span> <span class="o">=</span> <span class="mi">0</span>
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||
<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>
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||
<span class="k">while</span> <span class="mi">1</span><span class="p">:</span>
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||
<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>
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||
<span class="n">i</span> <span class="o">+=</span> <span class="mi">1</span>
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||
</pre></div>
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||
</div>
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||
<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">
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||
<tbody>
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||
<tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">iterindexed()</span></code></td>
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||
<td>five syllables is a mouthful</td>
|
||
</tr>
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||
<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>
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||
<tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">iteritems()</span></code></td>
|
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<td>conflicts with key:value concept for dictionaries</td>
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||
</tr>
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||
<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>
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||
</tr>
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||
<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>
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||
</tr>
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||
</tbody>
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||
</table>
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||
<p>All of the names involving ‘count’ had the further disadvantage of
|
||
implying that the count would begin from one instead of zero.</p>
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||
<p>All of the names involving ‘index’ clashed with usage in database
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languages where indexing implies a sorting operation rather than
|
||
linear sequencing.</p>
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<p>Note D: This function was originally proposed with optional start
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||
and stop arguments. GvR pointed out that the function call
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||
<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.
|
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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>
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||
<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>
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||
<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>
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||
<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>
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||
</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">2025-02-01 08:55:40 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>
|
||
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