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<h1 class="page-title">PEP 536 – Final Grammar for Literal String Interpolation</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">Philipp Angerer <phil.angerer at gmail.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="Removed from consideration by sponsor or authors">Withdrawn</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">11-Dec-2016</dd>
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<dt class="field-odd">Python-Version<span class="colon">:</span></dt>
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<dd class="field-odd">3.7</dd>
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<dt class="field-even">Post-History<span class="colon">:</span></dt>
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<dd class="field-even"><a class="reference external" href="https://mail.python.org/archives/list/python-ideas@python.org/thread/FOYKXOFWEINPVQSK2XGEHKXSTEVO5WWA/" title="Python-Ideas thread">18-Aug-2016</a>,
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||
<a class="reference external" href="https://mail.python.org/archives/list/python-ideas@python.org/thread/YKKEA5NIMMKHZTMRE5UFHST4WQ4NN3XJ/" title="Python-Ideas thread">23-Dec-2016</a>,
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||
<a class="reference external" href="https://mail.python.org/archives/list/python-dev@python.org/thread/N43O4KNLZW4U7YZC4NVPCETZIVRDUVU2/" title="Python-Dev thread">15-Mar-2019</a></dd>
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||
<dt class="field-odd">Resolution<span class="colon">:</span></dt>
|
||
<dd class="field-odd"><a class="reference external" href="https://discuss.python.org/t/pep-536-should-be-marked-as-rejected/35226/4">Discourse message</a></dd>
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</dl>
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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="#pep-withdrawal">PEP Withdrawal</a></li>
|
||
<li><a class="reference internal" href="#terminology">Terminology</a></li>
|
||
<li><a class="reference internal" href="#motivation">Motivation</a></li>
|
||
<li><a class="reference internal" href="#rationale">Rationale</a></li>
|
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<li><a class="reference internal" href="#specification">Specification</a></li>
|
||
<li><a class="reference internal" href="#backwards-compatibility">Backwards Compatibility</a></li>
|
||
<li><a class="reference internal" href="#reference-implementation">Reference Implementation</a></li>
|
||
<li><a class="reference internal" href="#references">References</a></li>
|
||
<li><a class="reference internal" href="#copyright">Copyright</a></li>
|
||
</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><a class="pep reference internal" href="../pep-0498/" title="PEP 498 – Literal String Interpolation">PEP 498</a> introduced Literal String Interpolation (or “f-strings”).
|
||
The expression portions of those literals however are subject to
|
||
certain restrictions. This PEP proposes a formal grammar lifting
|
||
those restrictions, promoting “f-strings” to “f expressions” or f-literals.</p>
|
||
<p>This PEP expands upon the f-strings introduced by <a class="pep reference internal" href="../pep-0498/" title="PEP 498 – Literal String Interpolation">PEP 498</a>,
|
||
so this text requires familiarity with <a class="pep reference internal" href="../pep-0498/" title="PEP 498 – Literal String Interpolation">PEP 498</a>.</p>
|
||
</section>
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||
<section id="pep-withdrawal">
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<h2><a class="toc-backref" href="#pep-withdrawal" role="doc-backlink">PEP Withdrawal</a></h2>
|
||
<p>This PEP has been withdrawn in favour of <a class="pep reference internal" href="../pep-0701/" title="PEP 701 – Syntactic formalization of f-strings">PEP 701</a>.
|
||
<a class="pep reference internal" href="../pep-0701/" title="PEP 701 – Syntactic formalization of f-strings">PEP 701</a> addresses all important points of this PEP.</p>
|
||
</section>
|
||
<section id="terminology">
|
||
<h2><a class="toc-backref" href="#terminology" role="doc-backlink">Terminology</a></h2>
|
||
<p>This text will refer to the existing grammar as “f-strings”,
|
||
and the proposed one as “f-literals”.</p>
|
||
<p>Furthermore, it will refer to the <code class="docutils literal notranslate"><span class="pre">{}</span></code>-delimited expressions in
|
||
f-literals/f-strings as “expression portions” and the static string content
|
||
around them as “string portions”.</p>
|
||
</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>
|
||
<p>The current implementation of f-strings in CPython relies on the existing
|
||
string parsing machinery and a post processing of its tokens. This results in
|
||
several restrictions to the possible expressions usable within f-strings:</p>
|
||
<ol class="arabic">
|
||
<li>It is impossible to use the quote character delimiting the f-string
|
||
within the expression portion:<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="sa">f</span><span class="s1">'Magic wand: </span><span class="si">{</span><span class="w"> </span><span class="n">bag</span><span class="p">[</span><span class="s1">'wand'</span><span class="p">]</span><span class="w"> </span><span class="si">}</span><span class="s1">'</span>
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||
<span class="go"> ^</span>
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||
<span class="go">SyntaxError: invalid syntax</span>
|
||
</pre></div>
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||
</div>
|
||
</li>
|
||
<li>A previously considered way around it would lead to escape sequences
|
||
in executed code and is prohibited in f-strings:<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>>>> f'Magic wand { bag[\'wand\'] } string'
|
||
SyntaxError: f-string expression portion cannot include a backslash
|
||
</pre></div>
|
||
</div>
|
||
</li>
|
||
<li>Comments are forbidden even in multi-line f-strings:<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>>>> f'''A complex trick: {
|
||
... bag['bag'] # recursive bags!
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||
... }'''
|
||
SyntaxError: f-string expression part cannot include '#'
|
||
</pre></div>
|
||
</div>
|
||
</li>
|
||
<li>Expression portions need to wrap <code class="docutils literal notranslate"><span class="pre">':'</span></code> and <code class="docutils literal notranslate"><span class="pre">'!'</span></code> in braces:<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="sa">f</span><span class="s1">'Useless use of lambdas: </span><span class="si">{</span><span class="w"> </span><span class="k">lambda</span><span class="w"> </span><span class="n">x</span><span class="si">:</span><span class="s1"> x*2 </span><span class="si">}</span><span class="s1">'</span>
|
||
<span class="go">SyntaxError: unexpected EOF while parsing</span>
|
||
</pre></div>
|
||
</div>
|
||
</li>
|
||
</ol>
|
||
<p>These limitations serve no purpose from a language user perspective and
|
||
can be lifted by giving f-literals a regular grammar without exceptions
|
||
and implementing it using dedicated parse code.</p>
|
||
</section>
|
||
<section id="rationale">
|
||
<h2><a class="toc-backref" href="#rationale" role="doc-backlink">Rationale</a></h2>
|
||
<p>The restrictions mentioned in <a class="reference internal" href="#motivation">Motivation</a> are non-obvious and counter-intuitive
|
||
unless the user is familiar with the f-literals’ implementation details.</p>
|
||
<p>As mentioned, a previous version of <a class="pep reference internal" href="../pep-0498/" title="PEP 498 – Literal String Interpolation">PEP 498</a> allowed escape sequences
|
||
anywhere in f-strings, including as ways to encode the braces delimiting
|
||
the expression portions and in their code. They would be expanded before
|
||
the code is parsed, which would have had several important ramifications:</p>
|
||
<p>#. It would not be clear to human readers which portions are Expressions
|
||
and which are strings. Great material for an “obfuscated/underhanded
|
||
Python challenge”
|
||
#. Syntax highlighters are good in parsing nested grammar, but not
|
||
in recognizing escape sequences. ECMAScript 2016 (JavaScript) allows
|
||
escape sequences in its identifiers <a class="footnote-reference brackets" href="#id3" id="id1">[1]</a> and the author knows of no
|
||
syntax highlighter able to correctly highlight code making use of this.</p>
|
||
<p>As a consequence, the expression portions would be harder to recognize
|
||
with and without the aid of syntax highlighting. With the new grammar,
|
||
it is easy to extend syntax highlighters to correctly parse
|
||
and display f-literals:</p>
|
||
<pre><span style=color:#ff5500>f'Magic wand: </span><span style=color:#3daee9>{</span>bag[<span style=color:#bf0303>'wand'</span>]<span style=color:#3daee9>:^10}</span><span style=color:#ff5500>'</span></pre><p>Highlighting expression portions with possible escape sequences would
|
||
mean to create a modified copy of all rules of the complete expression
|
||
grammar, accounting for the possibility of escape sequences in key words,
|
||
delimiters, and all other language syntax. One such duplication would
|
||
yield one level of escaping depth and have to be repeated for a deeper
|
||
escaping in a recursive f-literal. This is the case since no highlighting
|
||
engine known to the author supports expanding escape sequences before
|
||
applying rules to a certain context. Nesting contexts however is a
|
||
standard feature of all highlighting engines.</p>
|
||
<p>Familiarity also plays a role: Arbitrary nesting of expressions
|
||
without expansion of escape sequences is available in every single
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other language employing a string interpolation method that uses
|
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expressions instead of just variable names. <a class="footnote-reference brackets" href="#id4" id="id2">[2]</a></p>
|
||
</section>
|
||
<section id="specification">
|
||
<h2><a class="toc-backref" href="#specification" role="doc-backlink">Specification</a></h2>
|
||
<p><a class="pep reference internal" href="../pep-0498/" title="PEP 498 – Literal String Interpolation">PEP 498</a> specified f-strings as the following, but places restrictions on it:</p>
|
||
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">f</span> <span class="s1">' <text> { <expression> <optional !s, !r, or !a> <optional : format specifier> } <text> ... '</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>All restrictions mentioned in the PEP are lifted from f-literals,
|
||
as explained below:</p>
|
||
<ol class="arabic simple">
|
||
<li>Expression portions may now contain strings delimited with the same
|
||
kind of quote that is used to delimit the f-literal.</li>
|
||
<li>Backslashes may now appear within expressions just like anywhere else
|
||
in Python code. In case of strings nested within f-literals,
|
||
escape sequences are expanded when the innermost string is evaluated.</li>
|
||
<li>Comments, using the <code class="docutils literal notranslate"><span class="pre">'#'</span></code> character, are possible only in multi-line
|
||
f-literals, since comments are terminated by the end of the line
|
||
(which makes closing a single-line f-literal impossible).</li>
|
||
<li>Expression portions may contain <code class="docutils literal notranslate"><span class="pre">':'</span></code> or <code class="docutils literal notranslate"><span class="pre">'!'</span></code> wherever
|
||
syntactically valid. The first <code class="docutils literal notranslate"><span class="pre">':'</span></code> or <code class="docutils literal notranslate"><span class="pre">'!'</span></code> that is not part
|
||
of an expression has to be followed a valid coercion or format specifier.</li>
|
||
</ol>
|
||
<p>A remaining restriction not explicitly mentioned by <a class="pep reference internal" href="../pep-0498/" title="PEP 498 – Literal String Interpolation">PEP 498</a> is line breaks
|
||
in expression portions. Since strings delimited by single <code class="docutils literal notranslate"><span class="pre">'</span></code> or <code class="docutils literal notranslate"><span class="pre">"</span></code>
|
||
characters are expected to be single line, line breaks remain illegal
|
||
in expression portions of single line strings.</p>
|
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<div class="admonition note">
|
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<p class="admonition-title">Note</p>
|
||
<p>Is lifting of the restrictions sufficient,
|
||
or should we specify a more complete grammar?</p>
|
||
</div>
|
||
</section>
|
||
<section id="backwards-compatibility">
|
||
<h2><a class="toc-backref" href="#backwards-compatibility" role="doc-backlink">Backwards Compatibility</a></h2>
|
||
<p>f-literals are fully backwards compatible to f-strings,
|
||
and expands the syntax considered legal.</p>
|
||
</section>
|
||
<section id="reference-implementation">
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||
<h2><a class="toc-backref" href="#reference-implementation" role="doc-backlink">Reference Implementation</a></h2>
|
||
<p>TBD</p>
|
||
</section>
|
||
<section id="references">
|
||
<h2><a class="toc-backref" href="#references" role="doc-backlink">References</a></h2>
|
||
<aside class="footnote-list brackets">
|
||
<aside class="footnote brackets" id="id3" role="doc-footnote">
|
||
<dt class="label" id="id3">[<a href="#id1">1</a>]</dt>
|
||
<dd>ECMAScript <code class="docutils literal notranslate"><span class="pre">IdentifierName</span></code> specification
|
||
( <a class="reference external" href="http://ecma-international.org/ecma-262/6.0/#sec-names-and-keywords">http://ecma-international.org/ecma-262/6.0/#sec-names-and-keywords</a> )<p>Yes, <code class="docutils literal notranslate"><span class="pre">const</span> <span class="pre">cthulhu</span> <span class="pre">=</span> <span class="pre">{</span> <span class="pre">H̹̙̦̮͉̩̗̗ͧ̇̏̊̾Eͨ͆͒̆ͮ̃͏̷̮̣̫̤̣Cͯ̂͐͏̨̛͔̦̟͈̻O̜͎͍͙͚̬̝̣̽ͮ͐͗̀ͤ̍̀͢M̴̡̲̭͍͇̼̟̯̦̉̒͠Ḛ̛̙̞̪̗ͥͤͩ̾͑̔͐ͅṮ̴̷̷̗̼͍̿̿̓̽͐H̙̙̔̄͜\u0042:</span> <span class="pre">42</span> <span class="pre">}</span></code> is valid ECMAScript 2016</p>
|
||
</aside>
|
||
<aside class="footnote brackets" id="id4" role="doc-footnote">
|
||
<dt class="label" id="id4">[<a href="#id2">2</a>]</dt>
|
||
<dd>Wikipedia article on string interpolation
|
||
( <a class="reference external" href="https://en.wikipedia.org/wiki/String_interpolation">https://en.wikipedia.org/wiki/String_interpolation</a> )</aside>
|
||
</aside>
|
||
</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-0536.rst">https://github.com/python/peps/blob/main/peps/pep-0536.rst</a></p>
|
||
<p>Last modified: <a class="reference external" href="https://github.com/python/peps/commits/main/peps/pep-0536.rst">2023-10-26 09:35:14 GMT</a></p>
|
||
|
||
</article>
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||
<nav id="pep-sidebar">
|
||
<h2>Contents</h2>
|
||
<ul>
|
||
<li><a class="reference internal" href="#abstract">Abstract</a></li>
|
||
<li><a class="reference internal" href="#pep-withdrawal">PEP Withdrawal</a></li>
|
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<li><a class="reference internal" href="#terminology">Terminology</a></li>
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<li><a class="reference internal" href="#motivation">Motivation</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="#reference-implementation">Reference Implementation</a></li>
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