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<h1 class="page-title">PEP 3120 – Using UTF-8 as the default source encoding</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">Martin von Löwis <martin at v.loewis.de></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abbr title="Accepted and implementation complete, or no longer active"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">15-Apr-2007</dd>
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<dt class="field-odd">Python-Version<span class="colon">:</span></dt>
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<dd class="field-odd">3.0</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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<details><summary>Table of Contents</summary><ul class="simple">
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<li><a class="reference internal" href="#specification">Specification</a></li>
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<li><a class="reference internal" href="#a-bit-of-history">A Bit of History</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="#implementation">Implementation</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="specification">
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<h2><a class="toc-backref" href="#specification" role="doc-backlink">Specification</a></h2>
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<p>This PEP proposes to change the default source encoding from ASCII to
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UTF-8. Support for alternative source encodings (<a class="pep reference internal" href="../pep-0263/" title="PEP 263 – Defining Python Source Code Encodings">PEP 263</a>) continues to
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exist; an explicit encoding declaration takes precedence over the
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default.</p>
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</section>
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<section id="a-bit-of-history">
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<h2><a class="toc-backref" href="#a-bit-of-history" role="doc-backlink">A Bit of History</a></h2>
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<p>In Python 1, the source encoding was unspecified, except that the
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source encoding had to be a superset of the system’s basic execution
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character set (i.e. an ASCII superset, on most systems). The source
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encoding was only relevant for the lexis itself (bytes representing
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letters for keywords, identifiers, punctuation, line breaks, etc).
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The contents of a string literal was copied literally from the file
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on source.</p>
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<p>In Python 2.0, the source encoding changed to Latin-1 as a side effect
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of introducing Unicode. For Unicode string literals, the characters
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were still copied literally from the source file, but widened on a
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character-by-character basis. As Unicode gives a fixed interpretation
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to code points, this algorithm effectively fixed a source encoding, at
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least for files containing non-ASCII characters in Unicode literals.</p>
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<p><a class="pep reference internal" href="../pep-0263/" title="PEP 263 – Defining Python Source Code Encodings">PEP 263</a> identified the problem that you can use only those Unicode
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characters in a Unicode literal which are also in Latin-1, and
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introduced a syntax for declaring the source encoding. If no source
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encoding was given, the default should be ASCII. For compatibility
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with Python 2.0 and 2.1, files were interpreted as Latin-1 for a
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transitional period. This transition ended with Python 2.5, which
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gives an error if non-ASCII characters are encountered and no source
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encoding is declared.</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>With <a class="pep reference internal" href="../pep-0263/" title="PEP 263 – Defining Python Source Code Encodings">PEP 263</a>, using arbitrary non-ASCII characters in a Python file is
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possible, but tedious. One has to explicitly add an encoding
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declaration. Even though some editors (like IDLE and Emacs) support
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the declarations of <a class="pep reference internal" href="../pep-0263/" title="PEP 263 – Defining Python Source Code Encodings">PEP 263</a>, many editors still do not (and never
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will); users have to explicitly adjust the encoding which the editor
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assumes on a file-by-file basis.</p>
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<p>When the default encoding is changed to UTF-8, adding non-ASCII text
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to Python files becomes easier and more portable: On some systems,
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editors will automatically choose UTF-8 when saving text (e.g. on Unix
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systems where the locale uses UTF-8). On other systems, editors will
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guess the encoding when reading the file, and UTF-8 is easy to
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guess. Yet other editors support associating a default encoding with a
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file extension, allowing users to associate .py with UTF-8.</p>
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<p>For Python 2, an important reason for using non-UTF-8 encodings was
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that byte string literals would be in the source encoding at run-time,
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allowing then to output them to a file or render them to the user
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as-is. With Python 3, all strings will be Unicode strings, so the
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original encoding of the source will have no impact at run-time.</p>
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</section>
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<section id="implementation">
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<h2><a class="toc-backref" href="#implementation" role="doc-backlink">Implementation</a></h2>
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<p>The parser needs to be changed to accept bytes > 127 if no source
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encoding is specified; instead of giving an error, it needs to check
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that the bytes are well-formed UTF-8 (decoding is not necessary,
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as the parser converts all source code to UTF-8, anyway).</p>
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<p>IDLE needs to be changed to use UTF-8 as the default encoding.</p>
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</section>
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<section id="copyright">
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<h2><a class="toc-backref" href="#copyright" role="doc-backlink">Copyright</a></h2>
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<p>This document has been placed in the public domain.</p>
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</section>
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</section>
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<hr class="docutils" />
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<p>Source: <a class="reference external" href="https://github.com/python/peps/blob/main/peps/pep-3120.rst">https://github.com/python/peps/blob/main/peps/pep-3120.rst</a></p>
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<p>Last modified: <a class="reference external" href="https://github.com/python/peps/commits/main/peps/pep-3120.rst">2023-09-09 17:39:29 GMT</a></p>
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<h2>Contents</h2>
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<ul>
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<li><a class="reference internal" href="#specification">Specification</a></li>
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<li><a class="reference internal" href="#a-bit-of-history">A Bit of History</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="#implementation">Implementation</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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