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<li><a href="../pep-0000/">PEP Index</a> » </li>
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<li>PEP 244</li>
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<section id="pep-content">
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<h1 class="page-title">PEP 244 – The <code class="docutils literal notranslate"><span class="pre">directive</span></code> statement</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="Formally declined and will not be accepted">Rejected</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">20-Mar-2001</dd>
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<dt class="field-odd">Python-Version<span class="colon">:</span></dt>
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<dd class="field-odd">2.1</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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</dl>
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<hr class="docutils" />
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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="#motivation">Motivation</a></li>
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<li><a class="reference internal" href="#syntax">Syntax</a></li>
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<li><a class="reference internal" href="#semantics">Semantics</a></li>
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<li><a class="reference internal" href="#specific-directives-transitional">Specific Directives: transitional</a></li>
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<li><a class="reference internal" href="#backwards-compatibility">Backwards Compatibility</a></li>
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<li><a class="reference internal" href="#unresolved-problems-directive-as-the-first-identifier">Unresolved Problems: directive as the first identifier</a></li>
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<li><a class="reference internal" href="#questions-and-answers">Questions and Answers</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="motivation">
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<h2><a class="toc-backref" href="#motivation" role="doc-backlink">Motivation</a></h2>
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<p>From time to time, Python makes an incompatible change to the
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advertised semantics of core language constructs, or changes their
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accidental (implementation-dependent) behavior in some way. While
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this is never done capriciously, and is always done with the aim
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of improving the language over the long term, over the short term
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it’s contentious and disrupting.</p>
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<p><a class="pep reference internal" href="../pep-0005/" title="PEP 5 – Guidelines for Language Evolution">PEP 5</a>, Guidelines for Language Evolution suggests ways to ease
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the pain, and this PEP introduces some machinery in support of
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that.</p>
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<p><a class="pep reference internal" href="../pep-0227/" title="PEP 227 – Statically Nested Scopes">PEP 227</a>, Statically Nested Scopes is the first application, and
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will be used as an example here.</p>
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<p>When a new, potentially incompatible language feature is added,
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some modules and libraries may chose to use it, while others may
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not. This specification introduces a syntax where a module author
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can denote whether a certain language feature is used in the
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module or not.</p>
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<p>In discussion of this PEP, readers commented that there are two
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kinds of “settable” language features:</p>
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<ul class="simple">
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<li>those that are designed to eventually become the only option, at
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which time specifying use of them is not necessary anymore. The
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features for which the syntax of the “Back to the <code class="docutils literal notranslate"><span class="pre">__future__</span></code>”
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<a class="pep reference internal" href="../pep-0236/" title="PEP 236 – Back to the __future__">PEP 236</a>, Back to the <code class="docutils literal notranslate"><span class="pre">__future__</span></code> was proposed fall into this
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category. This PEP supports declaring such features, and
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supports phasing out the “old” meaning of constructs whose
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semantics has changed under the new feature. However, it
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defines no policy as to what features must be phased out
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eventually.</li>
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<li>those which are designed to stay optional forever, e.g. if they
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change some default setting in the interpreter. An example for
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such settings might be the request to always emit line-number
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instructions for a certain module; no specific flags of that
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kind are proposed in this specification.</li>
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</ul>
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<p>Since a primary goal of this PEP is to support new language
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constructs without immediately breaking old libraries, special
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care was taken not to break old libraries by introducing the new
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syntax.</p>
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</section>
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<section id="syntax">
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<h2><a class="toc-backref" href="#syntax" role="doc-backlink">Syntax</a></h2>
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<p>A directive_statement is a statement of the form:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>directive_statement: 'directive' ``NAME`` [atom] [';'] NEWLINE
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</pre></div>
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</div>
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<p>The name in the directive indicates the kind of the directive; it
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defines whether the optional atom can be present, and whether
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there are further syntactical or semantical restrictions to the
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atom. In addition, depending on the name of the directive,
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certain additional syntactical or semantical restrictions may be
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placed on the directive (e.g. placement of the directive in the
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module may be restricted to the top of the module).</p>
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<p>In the directive_statement, <code class="docutils literal notranslate"><span class="pre">directive</span></code> is a new
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keyword. According to <a class="pep reference internal" href="../pep-0005/" title="PEP 5 – Guidelines for Language Evolution">PEP 5</a>, this keyword is initially considered as
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a keyword only when used in a directive statement, see “Backwards
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Compatibility” below.</p>
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</section>
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<section id="semantics">
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<h2><a class="toc-backref" href="#semantics" role="doc-backlink">Semantics</a></h2>
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<p>A directive statement instructs the Python interpreter to process
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a source file in a different way; the specific details of that
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processing depend on the directive name. The optional atom is
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typically interpreted when the source code is processed; details
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of that interpretation depend on the directive.</p>
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</section>
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<section id="specific-directives-transitional">
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<h2><a class="toc-backref" href="#specific-directives-transitional" role="doc-backlink">Specific Directives: transitional</a></h2>
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<p>If a syntactical or semantical change is added to Python which is
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incompatible, <a class="pep reference internal" href="../pep-0005/" title="PEP 5 – Guidelines for Language Evolution">PEP 5</a> mandates a transitional evolution of the
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language, where the new feature is initially available alongside
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with the old one. Such a transition is possible by means of the
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transitional directive.</p>
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<p>In a transitional directive, the <code class="docutils literal notranslate"><span class="pre">NAME</span></code> is ‘transitional’. The atom
|
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MUST be present, and it MUST be a <code class="docutils literal notranslate"><span class="pre">NAME</span></code>. The possible values for
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that name are defined when the language change is defined. One
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example for such a directive is:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">directive</span> <span class="n">transitional</span> <span class="n">nested_scopes</span>
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</pre></div>
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</div>
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<p>The transitional directive MUST occur at before any other
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statement in a module, except for the documentation string
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(i.e. it may appear as the second statement of a module only if
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the first statement is a <code class="docutils literal notranslate"><span class="pre">STRING+</span></code>).</p>
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</section>
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<section id="backwards-compatibility">
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<h2><a class="toc-backref" href="#backwards-compatibility" role="doc-backlink">Backwards Compatibility</a></h2>
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<p>Introducing <code class="docutils literal notranslate"><span class="pre">directive</span></code> as a new keyword might cause
|
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incompatibilities with existing code. Following the guideline in
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<a class="pep reference internal" href="../pep-0005/" title="PEP 5 – Guidelines for Language Evolution">PEP 5</a>, in the initial implementation of this specification,
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directive is a new keyword only if it was used in a valid
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directive_statement (i.e. if it appeared as the first non-string
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token in a module).</p>
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</section>
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<section id="unresolved-problems-directive-as-the-first-identifier">
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<h2><a class="toc-backref" href="#unresolved-problems-directive-as-the-first-identifier" role="doc-backlink">Unresolved Problems: directive as the first identifier</a></h2>
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<p>Using directive in a module as:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">directive</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>(i.e. the name directive appears as the first thing in a module)
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will treat it as keyword, not as identifier. It would be possible
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to classify it as a <code class="docutils literal notranslate"><span class="pre">NAME</span></code> with an additional look-ahead token, but
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such look-ahead is not available in the Python tokenizer.</p>
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</section>
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<section id="questions-and-answers">
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<h2><a class="toc-backref" href="#questions-and-answers" role="doc-backlink">Questions and Answers</a></h2>
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<p><strong>Q:</strong> It looks like this PEP was written to allow definition of source
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code character sets. Is that true?</p>
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<p><strong>A:</strong> No. Even though the directive facility can be extended to
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allow source code encodings, no specific directive is proposed.</p>
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<p><strong>Q:</strong> Then why was this PEP written at all?</p>
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<p><strong>A:</strong> It acts as a counter-proposal to <a class="pep reference internal" href="../pep-0236/" title="PEP 236 – Back to the __future__">PEP 236</a>, which proposes to
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overload the import statement with a new meaning. This PEP
|
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allows to solve the problem in a more general way.</p>
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<p><strong>Q:</strong> But isn’t mixing source encodings and language changes like
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mixing apples and oranges?</p>
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<p><strong>A:</strong> Perhaps. To address the difference, the predefined
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“transitional” directive has been defined.</p>
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</section>
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<section id="copyright">
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|
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<h2><a class="toc-backref" href="#copyright" role="doc-backlink">Copyright</a></h2>
|
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|
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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-0244.rst">https://github.com/python/peps/blob/main/peps/pep-0244.rst</a></p>
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<p>Last modified: <a class="reference external" href="https://github.com/python/peps/commits/main/peps/pep-0244.rst">2023-09-09 17:39:29 GMT</a></p>
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</article>
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<nav id="pep-sidebar">
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<h2>Contents</h2>
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<ul>
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<li><a class="reference internal" href="#motivation">Motivation</a></li>
|
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|
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<li><a class="reference internal" href="#syntax">Syntax</a></li>
|
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|
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<li><a class="reference internal" href="#semantics">Semantics</a></li>
|
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|
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<li><a class="reference internal" href="#specific-directives-transitional">Specific Directives: transitional</a></li>
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<li><a class="reference internal" href="#backwards-compatibility">Backwards Compatibility</a></li>
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<li><a class="reference internal" href="#unresolved-problems-directive-as-the-first-identifier">Unresolved Problems: directive as the first identifier</a></li>
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<li><a class="reference internal" href="#questions-and-answers">Questions and Answers</a></li>
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<li><a class="reference internal" href="#copyright">Copyright</a></li>
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<br>
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<a id="source" href="https://github.com/python/peps/blob/main/peps/pep-0244.rst">Page Source (GitHub)</a>
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