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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 467 – Minor API improvements for binary sequences</h1> <dl class="rfc2822 field-list simple"> <dt class="field-odd">Author<span class="colon">:</span></dt> <dd class="field-odd">Alyssa Coghlan <ncoghlan at gmail.com>, Ethan Furman <ethan at stoneleaf.us></dd> <dt class="field-even">Discussions-To<span class="colon">:</span></dt> <dd class="field-even"><a class="reference external" href="https://discuss.python.org/t/42001">Discourse thread</a></dd> <dt class="field-odd">Status<span class="colon">:</span></dt> <dd class="field-odd"><abbr title="Proposal under active discussion and revision">Draft</abbr></dd> <dt class="field-even">Type<span class="colon">:</span></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">Created<span class="colon">:</span></dt> <dd class="field-odd">30-Mar-2014</dd> <dt class="field-even">Python-Version<span class="colon">:</span></dt> <dd class="field-even">3.13</dd> <dt class="field-odd">Post-History<span class="colon">:</span></dt> <dd class="field-odd">30-Mar-2014, 15-Aug-2014, 16-Aug-2014, 07-Jun-2016, 01-Sep-2016, 13-Apr-2021, 03-Nov-2021, 27-Dec-2023</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="#motivation">Motivation</a></li> <li><a class="reference internal" href="#proposals">Proposals</a><ul> <li><a class="reference internal" href="#addition-of-explicit-count-and-byte-initialised-sequence-constructors">Addition of explicit “count and byte initialised sequence” constructors</a></li> <li><a class="reference internal" href="#addition-of-explicit-single-byte-constructors">Addition of explicit “single byte” constructors</a></li> <li><a class="reference internal" href="#addition-of-getbyte-method-to-retrieve-a-single-byte">Addition of “getbyte” method to retrieve a single byte</a></li> <li><a class="reference internal" href="#addition-of-optimised-iterator-methods-that-produce-bytes-objects">Addition of optimised iterator methods that produce <code class="docutils literal notranslate"><span class="pre">bytes</span></code> objects</a></li> </ul> </li> <li><a class="reference internal" href="#design-discussion">Design discussion</a><ul> <li><a class="reference internal" href="#why-not-rely-on-sequence-repetition-to-create-zero-initialised-sequences">Why not rely on sequence repetition to create zero-initialised sequences?</a></li> <li><a class="reference internal" href="#current-workarounds">Current Workarounds</a></li> <li><a class="reference internal" href="#omitting-the-originally-proposed-builtin-function">Omitting the originally proposed builtin function</a></li> <li><a class="reference internal" href="#scope-limitation-memoryview">Scope limitation: memoryview</a></li> </ul> </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 small adjustments to the APIs of the <code class="docutils literal notranslate"><span class="pre">bytes</span></code> and <code class="docutils literal notranslate"><span class="pre">bytearray</span></code> types to make it easier to operate entirely in the binary domain:</p> <ul class="simple"> <li>Add <code class="docutils literal notranslate"><span class="pre">fromsize</span></code> alternative constructor</li> <li>Add <code class="docutils literal notranslate"><span class="pre">fromint</span></code> alternative constructor</li> <li>Add <code class="docutils literal notranslate"><span class="pre">getbyte</span></code> byte retrieval method</li> <li>Add <code class="docutils literal notranslate"><span class="pre">iterbytes</span></code> alternative iterator</li> </ul> </section> <section id="rationale"> <h2><a class="toc-backref" href="#rationale" role="doc-backlink">Rationale</a></h2> <p>During the initial development of the Python 3 language specification, the core <code class="docutils literal notranslate"><span class="pre">bytes</span></code> type for arbitrary binary data started as the mutable type that is now referred to as <code class="docutils literal notranslate"><span class="pre">bytearray</span></code>. Other aspects of operating in the binary domain in Python have also evolved over the course of the Python 3 series, for example with <a class="pep reference internal" href="../pep-0461/" title="PEP 461 – Adding % formatting to bytes and bytearray">PEP 461</a>.</p> </section> <section id="motivation"> <h2><a class="toc-backref" href="#motivation" role="doc-backlink">Motivation</a></h2> <p>With Python 3 and the split between <code class="docutils literal notranslate"><span class="pre">str</span></code> and <code class="docutils literal notranslate"><span class="pre">bytes</span></code>, one small but important area of programming became slightly more difficult, and much more painful – wire format protocols.</p> <p>This area of programming is characterized by a mixture of binary data and ASCII compatible segments of text (aka ASCII-encoded text). The addition of the new constructors, methods, and iterators will aid both in writing new wire format code, and in porting any remaining Python 2 wire format code.</p> <p>Common use-cases include <code class="docutils literal notranslate"><span class="pre">dbf</span></code> and <code class="docutils literal notranslate"><span class="pre">pdf</span></code> file formats, <code class="docutils literal notranslate"><span class="pre">email</span></code> formats, and <code class="docutils literal notranslate"><span class="pre">FTP</span></code> and <code class="docutils literal notranslate"><span class="pre">HTTP</span></code> communications, among many others.</p> </section> <section id="proposals"> <h2><a class="toc-backref" href="#proposals" role="doc-backlink">Proposals</a></h2> <section id="addition-of-explicit-count-and-byte-initialised-sequence-constructors"> <h3><a class="toc-backref" href="#addition-of-explicit-count-and-byte-initialised-sequence-constructors" role="doc-backlink">Addition of explicit “count and byte initialised sequence” constructors</a></h3> <p>To replace the discouraged behavior of creating zero-filled <code class="docutils literal notranslate"><span class="pre">bytes</span></code>-like objects from the basic constructors (i.e. <code class="docutils literal notranslate"><span class="pre">bytes(1)</span></code> –> <code class="docutils literal notranslate"><span class="pre">b'\x00'</span></code>), this PEP proposes the addition of an explicit <code class="docutils literal notranslate"><span class="pre">fromsize</span></code> alternative constructor as a class method on both <code class="docutils literal notranslate"><span class="pre">bytes</span></code> and <code class="docutils literal notranslate"><span class="pre">bytearray</span></code> whose first argument is the count, and whose second argument is the fill byte to use (defaults to <code class="docutils literal notranslate"><span class="pre">\x00</span></code>):</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">bytes</span><span class="o">.</span><span class="n">fromsize</span><span class="p">(</span><span class="mi">3</span><span class="p">)</span> <span class="go">b'\x00\x00\x00'</span> <span class="gp">>>> </span><span class="nb">bytearray</span><span class="o">.</span><span class="n">fromsize</span><span class="p">(</span><span class="mi">3</span><span class="p">)</span> <span class="go">bytearray(b'\x00\x00\x00')</span> <span class="gp">>>> </span><span class="nb">bytes</span><span class="o">.</span><span class="n">fromsize</span><span class="p">(</span><span class="mi">5</span><span class="p">,</span> <span class="sa">b</span><span class="s1">'</span><span class="se">\x0a</span><span class="s1">'</span><span class="p">)</span> <span class="go">b'\x0a\x0a\x0a\x0a\x0a'</span> <span class="gp">>>> </span><span class="nb">bytearray</span><span class="o">.</span><span class="n">fromsize</span><span class="p">(</span><span class="mi">5</span><span class="p">,</span> <span class="n">fill</span><span class="o">=</span><span class="sa">b</span><span class="s1">'</span><span class="se">\x0a</span><span class="s1">'</span><span class="p">)</span> <span class="go">bytearray(b'\x0a\x0a\x0a\x0a\x0a')</span> </pre></div> </div> <p><code class="docutils literal notranslate"><span class="pre">fromsize</span></code> will behave just as the current constructors behave when passed a single integer, while allowing for non-zero fill values when needed.</p> </section> <section id="addition-of-explicit-single-byte-constructors"> <h3><a class="toc-backref" href="#addition-of-explicit-single-byte-constructors" role="doc-backlink">Addition of explicit “single byte” constructors</a></h3> <p>As binary counterparts to the text <code class="docutils literal notranslate"><span class="pre">chr</span></code> function, this PEP proposes the addition of an explicit <code class="docutils literal notranslate"><span class="pre">fromint</span></code> alternative constructor as a class method on both <code class="docutils literal notranslate"><span class="pre">bytes</span></code> and <code class="docutils literal notranslate"><span class="pre">bytearray</span></code>:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">bytes</span><span class="o">.</span><span class="n">fromint</span><span class="p">(</span><span class="mi">65</span><span class="p">)</span> <span class="go">b'A'</span> <span class="gp">>>> </span><span class="nb">bytearray</span><span class="o">.</span><span class="n">fromint</span><span class="p">(</span><span class="mi">65</span><span class="p">)</span> <span class="go">bytearray(b'A')</span> </pre></div> </div> <p>These methods will only accept integers in the range 0 to 255 (inclusive):</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">bytes</span><span class="o">.</span><span class="n">fromint</span><span class="p">(</span><span class="mi">512</span><span class="p">)</span> <span class="gt">Traceback (most recent call last):</span> File <span class="nb">"<stdin>"</span>, line <span class="m">1</span>, in <span class="n"><module></span> <span class="gr">ValueError</span>: <span class="n">integer must be in range(0, 256)</span> <span class="gp">>>> </span><span class="nb">bytes</span><span class="o">.</span><span class="n">fromint</span><span class="p">(</span><span class="mf">1.0</span><span class="p">)</span> <span class="gt">Traceback (most recent call last):</span> File <span class="nb">"<stdin>"</span>, line <span class="m">1</span>, in <span class="n"><module></span> <span class="gr">TypeError</span>: <span class="n">'float' object cannot be interpreted as an integer</span> </pre></div> </div> <p>The documentation of the <code class="docutils literal notranslate"><span class="pre">ord</span></code> builtin will be updated to explicitly note that <code class="docutils literal notranslate"><span class="pre">bytes.fromint</span></code> is the primary inverse operation for binary data, while <code class="docutils literal notranslate"><span class="pre">chr</span></code> is the inverse operation for text data, and that <code class="docutils literal notranslate"><span class="pre">bytearray.fromint</span></code> also exists.</p> <p>Behaviorally, <code class="docutils literal notranslate"><span class="pre">bytes.fromint(x)</span></code> will be equivalent to the current <code class="docutils literal notranslate"><span class="pre">bytes([x])</span></code> (and similarly for <code class="docutils literal notranslate"><span class="pre">bytearray</span></code>). The new spelling is expected to be easier to discover and easier to read (especially when used in conjunction with indexing operations on binary sequence types).</p> <p>As a separate method, the new spelling will also work better with higher order functions like <code class="docutils literal notranslate"><span class="pre">map</span></code>.</p> <p>These new methods intentionally do NOT offer the same level of general integer support as the existing <code class="docutils literal notranslate"><span class="pre">int.to_bytes</span></code> conversion method, which allows arbitrarily large integers to be converted to arbitrarily long bytes objects. The restriction to only accept positive integers that fit in a single byte means that no byte order information is needed, and there is no need to handle negative numbers. The documentation of the new methods will refer readers to <code class="docutils literal notranslate"><span class="pre">int.to_bytes</span></code> for use cases where handling of arbitrary integers is needed.</p> </section> <section id="addition-of-getbyte-method-to-retrieve-a-single-byte"> <h3><a class="toc-backref" href="#addition-of-getbyte-method-to-retrieve-a-single-byte" role="doc-backlink">Addition of “getbyte” method to retrieve a single byte</a></h3> <p>This PEP proposes that <code class="docutils literal notranslate"><span class="pre">bytes</span></code> and <code class="docutils literal notranslate"><span class="pre">bytearray</span></code> gain the method <code class="docutils literal notranslate"><span class="pre">getbyte</span></code> which will always return <code class="docutils literal notranslate"><span class="pre">bytes</span></code>:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="sa">b</span><span class="s1">'abc'</span><span class="o">.</span><span class="n">getbyte</span><span class="p">(</span><span class="mi">0</span><span class="p">)</span> <span class="go">b'a'</span> </pre></div> </div> <p>If an index is asked for that doesn’t exist, <code class="docutils literal notranslate"><span class="pre">IndexError</span></code> is raised:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="sa">b</span><span class="s1">'abc'</span><span class="o">.</span><span class="n">getbyte</span><span class="p">(</span><span class="mi">9</span><span class="p">)</span> <span class="gt">Traceback (most recent call last):</span> File <span class="nb">"<stdin>"</span>, line <span class="m">1</span>, in <span class="n"><module></span> <span class="gr">IndexError</span>: <span class="n">index out of range</span> </pre></div> </div> </section> <section id="addition-of-optimised-iterator-methods-that-produce-bytes-objects"> <h3><a class="toc-backref" href="#addition-of-optimised-iterator-methods-that-produce-bytes-objects" role="doc-backlink">Addition of optimised iterator methods that produce <code class="docutils literal notranslate"><span class="pre">bytes</span></code> objects</a></h3> <p>This PEP proposes that <code class="docutils literal notranslate"><span class="pre">bytes</span></code> and <code class="docutils literal notranslate"><span class="pre">bytearray</span></code> gain an optimised <code class="docutils literal notranslate"><span class="pre">iterbytes</span></code> method that produces length 1 <code class="docutils literal notranslate"><span class="pre">bytes</span></code> objects rather than integers:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="n">data</span><span class="o">.</span><span class="n">iterbytes</span><span class="p">():</span> <span class="c1"># x is a length 1 ``bytes`` object, rather than an integer</span> </pre></div> </div> <p>For example:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">tuple</span><span class="p">(</span><span class="sa">b</span><span class="s2">"ABC"</span><span class="o">.</span><span class="n">iterbytes</span><span class="p">())</span> <span class="go">(b'A', b'B', b'C')</span> </pre></div> </div> </section> </section> <section id="design-discussion"> <h2><a class="toc-backref" href="#design-discussion" role="doc-backlink">Design discussion</a></h2> <section id="why-not-rely-on-sequence-repetition-to-create-zero-initialised-sequences"> <h3><a class="toc-backref" href="#why-not-rely-on-sequence-repetition-to-create-zero-initialised-sequences" role="doc-backlink">Why not rely on sequence repetition to create zero-initialised sequences?</a></h3> <p>Zero-initialised sequences can be created via sequence repetition:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="sa">b</span><span class="s1">'</span><span class="se">\x00</span><span class="s1">'</span> <span class="o">*</span> <span class="mi">3</span> <span class="go">b'\x00\x00\x00'</span> <span class="gp">>>> </span><span class="nb">bytearray</span><span class="p">(</span><span class="sa">b</span><span class="s1">'</span><span class="se">\x00</span><span class="s1">'</span><span class="p">)</span> <span class="o">*</span> <span class="mi">3</span> <span class="go">bytearray(b'\x00\x00\x00')</span> </pre></div> </div> <p>However, this was also the case when the <code class="docutils literal notranslate"><span class="pre">bytearray</span></code> type was originally designed, and the decision was made to add explicit support for it in the type constructor. The immutable <code class="docutils literal notranslate"><span class="pre">bytes</span></code> type then inherited that feature when it was introduced in <a class="pep reference internal" href="../pep-3137/" title="PEP 3137 – Immutable Bytes and Mutable Buffer">PEP 3137</a>.</p> <p>This PEP isn’t revisiting that original design decision, just changing the spelling as users sometimes find the current behavior of the binary sequence constructors surprising. In particular, there’s a reasonable case to be made that <code class="docutils literal notranslate"><span class="pre">bytes(x)</span></code> (where <code class="docutils literal notranslate"><span class="pre">x</span></code> is an integer) should behave like the <code class="docutils literal notranslate"><span class="pre">bytes.fromint(x)</span></code> proposal in this PEP. Providing both behaviors as separate class methods avoids that ambiguity.</p> </section> <section id="current-workarounds"> <h3><a class="toc-backref" href="#current-workarounds" role="doc-backlink">Current Workarounds</a></h3> <p>After nearly a decade, there’s seems to be no consensus on the best workarounds for byte iteration, as demonstrated by <a class="reference external" href="https://discuss.python.org/t/get-single-byte-bytes-objects-from-a-bytes-object/41709">Get single-byte bytes objects from bytes objects</a>.</p> </section> <section id="omitting-the-originally-proposed-builtin-function"> <h3><a class="toc-backref" href="#omitting-the-originally-proposed-builtin-function" role="doc-backlink">Omitting the originally proposed builtin function</a></h3> <p>When submitted to the Steering Council, this PEP proposed the introduction of a <code class="docutils literal notranslate"><span class="pre">bchr</span></code> builtin (with the same behaviour as <code class="docutils literal notranslate"><span class="pre">bytes.fromint</span></code>), recreating the <code class="docutils literal notranslate"><span class="pre">ord</span></code>/<code class="docutils literal notranslate"><span class="pre">chr</span></code>/<code class="docutils literal notranslate"><span class="pre">unichr</span></code> trio from Python 2 under a different naming scheme (<code class="docutils literal notranslate"><span class="pre">ord</span></code>/<code class="docutils literal notranslate"><span class="pre">bchr</span></code>/<code class="docutils literal notranslate"><span class="pre">chr</span></code>).</p> <p>The SC indicated they didn’t think this functionality was needed often enough to justify offering two ways of doing the same thing, especially when one of those ways was a new builtin function. That part of the proposal was therefore dropped as being redundant with the <code class="docutils literal notranslate"><span class="pre">bytes.fromint</span></code> alternate constructor.</p> <p>Developers that use this method frequently will instead have the option to define their own <code class="docutils literal notranslate"><span class="pre">bchr</span> <span class="pre">=</span> <span class="pre">bytes.fromint</span></code> aliases.</p> </section> <section id="scope-limitation-memoryview"> <h3><a class="toc-backref" href="#scope-limitation-memoryview" role="doc-backlink">Scope limitation: memoryview</a></h3> <p>Updating <code class="docutils literal notranslate"><span class="pre">memoryview</span></code> with the new item retrieval methods is outside the scope of this PEP.</p> </section> </section> <section id="references"> <h2><a class="toc-backref" href="#references" role="doc-backlink">References</a></h2> <ul class="simple"> <li><a class="reference external" href="https://mail.python.org/pipermail/python-ideas/2014-March/027295.html">Initial March 2014 discussion thread on python-ideas</a></li> <li><a class="reference external" href="https://mail.python.org/pipermail/python-ideas/2014-March/027376.html">Guido’s initial feedback in that thread</a></li> <li><a class="reference external" href="https://github.com/python/cpython/issues/65094">Issue proposing moving zero-initialised sequences to a dedicated API</a></li> <li><a class="reference external" href="https://github.com/python/cpython/issues/65843">Issue proposing to use calloc() for zero-initialised binary sequences</a></li> <li><a class="reference external" href="https://mail.python.org/pipermail/python-ideas/2014-March/027295.html">August 2014 discussion thread on python-dev</a></li> <li><a class="reference external" href="https://mail.python.org/pipermail/python-dev/2016-June/144875.html">June 2016 discussion thread on python-dev</a></li> <li><a class="reference external" href="https://discuss.python.org/t/get-single-byte-bytes-objects-from-a-bytes-object/41709">Get single-byte bytes objects from bytes objects</a></li> </ul> </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-0467.rst">https://github.com/python/peps/blob/main/peps/pep-0467.rst</a></p> <p>Last modified: <a class="reference external" href="https://github.com/python/peps/commits/main/peps/pep-0467.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="#motivation">Motivation</a></li> <li><a class="reference internal" href="#proposals">Proposals</a><ul> <li><a class="reference internal" href="#addition-of-explicit-count-and-byte-initialised-sequence-constructors">Addition of explicit “count and byte initialised sequence” constructors</a></li> <li><a class="reference internal" href="#addition-of-explicit-single-byte-constructors">Addition of explicit “single byte” constructors</a></li> <li><a class="reference internal" href="#addition-of-getbyte-method-to-retrieve-a-single-byte">Addition of “getbyte” method to retrieve a single byte</a></li> <li><a class="reference internal" href="#addition-of-optimised-iterator-methods-that-produce-bytes-objects">Addition of optimised iterator methods that produce <code class="docutils literal notranslate"><span class="pre">bytes</span></code> objects</a></li> </ul> </li> <li><a class="reference internal" href="#design-discussion">Design discussion</a><ul> <li><a class="reference internal" href="#why-not-rely-on-sequence-repetition-to-create-zero-initialised-sequences">Why not rely on sequence repetition to create zero-initialised sequences?</a></li> <li><a class="reference internal" href="#current-workarounds">Current Workarounds</a></li> <li><a class="reference internal" href="#omitting-the-originally-proposed-builtin-function">Omitting the originally proposed builtin function</a></li> <li><a class="reference internal" href="#scope-limitation-memoryview">Scope limitation: memoryview</a></li> </ul> </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-0467.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>